Insects are declining – but talk of “insectageddon” is premature

I originally posted this blog on the Nature Ecology & Evolution community forum; you can read the original version here.

Recent research showing severe declines in biomass of flying insects has been much-discussed in the literature and the global media. In a new study of a long-term insect population dataset, we found that the biomass of moths increased before it declined, and remains higher now than in the 1960s.

Insects play a number of vital roles in our ecosystems, and as a consequence, several studies in recent years that have reported sharp declines in their “biomass” (combined weight) have been greeted with alarm by the media and public.

However, the conclusions of these papers have been met with scepticism by the scientific community. There is a widely-held opinion that studies on insect biomass have so far been based on too little data to be certain of their conclusions, variously having too few separate sampling sites, data from too short a time period, or from only the beginning and end of a sequence (rather than continuous data), or data collected with a non-standardized method over time.

To try and understand patterns in insect biomass change over time in a more robust way, overcoming some of these criticisms, my colleagues and I turned to the long-running network of moth-traps of the Rothamsted Insect Survey (RIS). Rothamsted currently operate around 80 identical moth-traps across the UK and Ireland, each of which collects moths (which are then identified and counted) on a nightly basis using a standardized methodology. From among these, we identified 34 traps that had operated continuously for at least 30 years in the 51-year period since the trap network commenced in 1967.

Peppered Moth (Biston betularia) has declined in abundance by 81% since 1967, according to the Rothamsted Insect Survey’s data.

To translate the abundance records collected by the RIS into estimates of biomass, we first needed to know how much each moth weighed. Rothamsted don’t retain every moth they capture, and catching and weighing a representative sample of every species in their database would be prohibitively labour-intensive. Existing dry body mass data weren’t available for most British moth species either, but field guides do list typical forewing lengths for every species as an identification tool, so we resolved to model the relationship between these two variables.

Thanks to the hard work of summer project student Becci Kinsella, we collected an empirical dataset on the forewing length and body mass of 600 individual moths from 94 species and, fitting a model to this data, estimated the body mass of over 1000 species of larger moths. Amazingly, we found this allowed us to predict around 90% of the total variation in the biomass of mixed-species samples of moths.

Next, Masters student Jonny Williams applied these estimates to data from the 34 long-running traps from the RIS, generating annual estimates of moth biomass from each trap. The results were astonishing. We found that the average biomass of moths sampled annually by each trap had approximately doubled over the 50-year duration of our dataset. This was not a simple increasing trend, either: biomass had increased steeply between 1967 and 1982, but gradually declined thereafter. The 10% per decade rate of these more recent declines actually matches up well to the findings of other recent studies of insect abundance and biomass (none of which commenced earlier than 1976), but the overall longer-term pattern of steep increase, then gradual decline does not support the widely-suggested scenario of “insect Armageddon”.

Abundance of the Large Emerald (Geometra papilionaria) reached a peak in 1983, matching the trend for moth biomass as a whole.

To understand drivers of these recent declines, we turned our attention to land-use, categorising sites into four groups: arable, grassland, woodland and urban. Two drivers of environmental change that are often mooted as potential causes of insect decline, especially for moths, are agricultural intensification (most relevant in arable sites) and light pollution (most relevant in urban sites); but we found that the steepest post-1982 biomass declines were in grassland and woodland.

The third of the “usual suspects” is climate change. A strong degree of synchronisation in both year-to-year biomass change and longer-term trends between sites, land-use types, and taxonomic groups of moths, pointed the finger of blame squarely towards factors that operate across all land-use types, such as temperature and rainfall. Yet surprisingly, we found no relationship between either of these variables and biomass change. However, two of the biggest periods of biomass change – a strong increase in the late 1970s and a decline in the late 1990s – directly followed the heatwave/drought years of 1976 and 1996. It seems possible, therefore, that extreme climatic events can perturb populations and communities, with resultant ecosystem feedbacks causing subsequent biomass change – an avenue for further research, perhaps.

Finally, we resampled the full RIS dataset to investigate the influence of data structure on estimates of insect biomass change, taking every possible subset of data of over 5 years’ duration, both for single study sites and the full 34 sites combined. It’s well-known that insect populations can fluctuate wildly from year to year, so unsurprisingly, longer spans of data were less likely to estimate massive increases or declines. Likewise, biomass changes were larger at single study sites than across the full dataset of 34 sites. Many studies rely on single return visits to previously-sampled locations, generating an estimate of biomass change between two points in time. Unfortunately, when compared to the trend fitted by a linear model through annual estimates over the same time periods, we found the two-sample approach incorrectly estimates the direction of biomass change in a quarter of cases. Lastly, it’s unwise to hang your hat on biomass change since an arbitrary start date, since patterns change over time: within our study, biomass increased since 1967, decreased since 1982, but has been roughly stable since 1997.

These findings emphasize the vital importance of long-term, standardized data collection for understanding population change. Several UK initiatives, like the RIS and the UK Butterfly Monitoring Scheme, have been doing this for decades on home soil, but globally very few such datasets exist, particularly in the tropics. Establishing long-term monitoring at a much broader scale is a challenging prospect, but a worthy goal.

Pollinators: switch street lights off at midnight to help moths and nocturnal wildlife

This article is republished from The Conversation under a Creative Commons license. Read the original article.

File 20190121 100261 js5ch1.jpg?ixlib=rb 1.1

Jazzi/Shutterstock

Callum Macgregor, University of York

Conservation is often a conflict between the demands of development and a desire to do what is best for the environment. It’s rare that we get the chance to report a decision which was taken for the good of people that has also panned out well for nature’s ecosystems. However, that is just what our new research paper found.

Saving energy from street lighting is not just a green option, it also makes good financial sense. Two solutions in particular include replacing old High-Pressure Sodium (HPS) lightbulbs with new and energy-efficient Light-Emitting Diodes (LEDs) and turning the lights out entirely during the latter part of the night when fewer people are around. In the UK, these changes in lighting technology have been gradually taking effect over the last decade or so.

While these decisions were made for good reasons, we knew little about how they would influence nocturnal wildlife. Our team of experts at York and Newcastle universities were interested in finding out how moths might be affected by the switchoff and new LED lighting, as they play an important role as night-time pollinators of a wide range of flowers, and have declined in abundance by 40% in 40 years.

Moths are important pollinators but have endured significant population declines in recent years.
Safwan Abd Rahman/Shutterstock

Light pollution, from street lights and other sources, has been suggested as a possible cause of this decline, though there are other factors such as climate change and habitat loss.

Our previous research showed that under HPS street lighting left on all night, moths were distracted from visiting flowers and instead flew higher up, around street lights. This resulted in less pollen being carried by moths in lit areas, and a subsequent study by Swiss researchers demonstrated that this actually caused reduced fruit production.




Read more:
Fatal attraction: how street lights prevent moths from pollinating


In our study, we asked whether the disruption to nocturnal ecosystems from street lights might be eased or exacerbated by the introduction of new energy-efficient LED street lighting. Working on farmland in East Yorkshire in the UK, we set up a chain of mock street lights alongside hedgerows that would allow us to manipulate the type and duration of lighting.

Older and less energy-efficient HPS lighting near wildflowers.
Callum Macgregor, Author provided

We compared older HPS lights to LEDS, and standard full-night lighting to part-night lighting, in which lights were turned off at midnight. All lighting was compared to an unlit control that replicated natural darkness. Under each lit and unlit treatment we placed several plants of White Campion (Silene latifolia), a common wildflower known to be pollinated by both bees and moths. We left each plant in the field for four days and nights before measuring what proportion of flowers had been pollinated, and the weight and number of seeds in every fruit.

LEDs are rich in blue light which is attractive to moths but we found no difference in the rate of pollination between plants under LEDs and those under HPS lights. Our data did show that the differences between pollination under full-night lighting and in natural darkness were erased when lights were turned off at midnight.

Newer and more energy-efficient LED lighting near wildflowers.
Callum Macgregor, Author provided

Surprisingly, this wasn’t just a partial improvement. We found no significant difference between rates of pollination in part-night lighting treatments and in natural darkness, and this suggests that turning lights off at or after midnight may allow nocturnal ecosystems to function as normal in the second half of the night.

These results are quite encouraging. Local authorities can save money and energy from street lighting and help nocturnal ecosystems recover from light pollution at the same time.

So there’s no evidence that the switch from HPS lights to LEDs increases the negative impacts of lighting on wildlife, and even better, switching to part-night lighting actually appears to reduce them. By switching lights off at midnight there is the potential to tackle two issues at once – reduce energy bills and the ecological impact of light pollution.The Conversation

Callum Macgregor, Postdoctoral research associate, University of York

Wallace, Darwin and the night shift

Last week I had the great honour of attending the Royal Entomological Society’s annual conference, this year hosted by Edge Hill University, to accept the 2017 Alfred Russel Wallace Award. The award is given for the PhD thesis “judged to make the most significant contribution to entomology in the year”, and after several rounds of selection my own thesis, “The role of moths as pollinators, and the effects of environmental change”, was chosen. At the conference, I said that I was particularly proud to receive an award in Wallace’s name, because of his own connections to the subject matter of my thesis – nocturnal pollination by moths. In this blog, I re-tell the story of that connection…

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It is 25th January, 1862, and Charles Darwin is writing to his friend and confidante Joseph Dalton Hooker. Earlier that same day, Darwin had taken delivery of a box of specimens of exotic orchids from James Bateman, a Staffordshire-based orchid specialist.1 Among these, Darwin found one in particular (Angraecum sesquipedale, a species from Madagascar) to be remarkable, due to the great length of the nectary, extending 11.5 inches below the flower. “Good Heavens”, wrote Darwin, “what insect can suck it”.

Angraecum_sesquipedale.png
(L-R) a drawing of Angraecum sesquipedale by W.H. Fitch, included in: James Bateman (1876) A second century of orchidaceous plants. Curtis’s Botanical Magazine; two flowers of A. sesquipedale (© Michael Wolf | Wikimedia Commons | CC BY-SA 3.0); a plate commissioned by A.R. Wallace depicting the hypothetical hawkmoth pollinator of A. sesquipedale, drawn by T.W. Wood and included in: Alfred Russel Wallace (1867) Creation by Law. The Quarterly Journal of Science, 477.

Darwin must have acted quickly. In the same letter to Hooker he had described his “infinite satisfaction” at the prospect of correcting the proofs of his next books, Fertilisation of Orchids, “in 2 or 3 weeks”. Yet when Orchids emerged less than four months later on 15th May, Darwin had revised the text, incorporating a discussion of A. sesquipedale and a rough hypothesis as to its insect pollinator. Reasoning that the longest tongues among English insects belonged to hawkmoths (family Sphingidae), he wrote:

in Madagascar there must be moths with probosces [sic] capable of extension to a length of between ten and eleven inches

Darwin’s prediction sparked debate; among those who sprung to his defence was Alfred Russel Wallace. Contrary to popular opinion, correspondence between Darwin and Wallace was friendly and reasonably frequent after they jointly proposed evolution by natural selection (van Wyhe & Rookmaaker 2015). Five years after the publication of Orchids, Wallace’s Creation by Law asserted not only that Darwin’s predicted moth could safely be assumed to exist, but further, that naturalists visiting Madagascar “should search for it with as much confidence as astronomers searched for the planet Neptune”. Wallace picked out one moth in particular to back up this statement – Xanthopan morganii, then known from tropical Africa (as Macrosila morganii), which had an exceptionally long proboscis.

Xanthopan_morgani.png
Xanthopan morgani; a specimen from continental Africa. Malagasy X. m. praedicta have proboscides several inches longer still!

Several decades later, Walter Rothschild (2nd Baron Rothschild) and Karl Jordan described a newly-discovered subspecies of X. morganii, from Madagascar. Remarkably, the subspecies had a tongue that was, by several inches, longer than that of its continental cousins. In honour of the predictions made by first Darwin and later Wallace, the new subspecies was named X. morganii praedicta.

DarwinWallaceRothschild
(L-R) Charles Darwin, Alfred Russel Wallace, and Walter Rothschild

At this point, the trail went cold for the best part of the 20th century. We had the flower, and we had found the moth with a proboscis to match it, but the final proof of their association was lacking because nobody had ever seen the two interact. That state of affairs lasted until the early 1990s, when Professor L.T. Wasserthal (then of the Friedrich-Alexander University Erlangen-Nuremburg) finally photographed, beyond doubt, the pollinator of A. sesquipedale. It was, indeed, X. morgani.

BotanicaActa
Making the front cover, a photograph of Xanthopan morgani praedicta visiting the flower of Angraecum sesquipedale finally resolves the mystery!

As a final chapter, the video below is well worth a watch: both for its remarkable footage, shot in 2004 by Dr Philip DeVries of the University of New Orleans, of this pollination interaction in action, and for the sheer joy and excitement shown by Dr DeVries and his colleagues when their hard work finally paid off!

If you want to learn more about this story, I owe a debt of gratitude to the review of Arditti and colleagues (2012), on which I drew heavily whilst researching this blog.

1: The relationship between Darwin and Bateman must have been complex. Bateman was, at this time, one of Britain’s foremost experts on, and cultivators of, orchids, and it is hard not to assume that he and Darwin corresponded while the latter prepared Fertilisation of Orchids. However, this box of specimens, and a brief subsequent exchange of letters, appears to be the only surviving evidence of direct interaction between the two (Darwin Correspondence Project). Bateman lived at Biddulph Grange in Staffordshire, designing (with his wife Maria) its famous landscape gardens, which are now owned by the National Trust. The gardens featured a Geological Gallery, seen as a direct attempt to refute Darwin’s theory of evolution by natural selection by drawing connections between evidence from the fossil record and the account of creation given in the book of Genesis.

References

Arditti, J., Elliot, J., Kitching, I.J. & Wasserthal, L.T. (2012) ‘Good Heavens what insect can suck it’–Charles Darwin, Angraecum sesquipedale and Xanthopan morganii praedicta. Botanical Journal of the Linnean Society169, 403-432.

Darwin Correspondence Project, “Letter no. 3356,” accessed on 3 April 2018, http://www.darwinproject.ac.uk/DCP-LETT-3356. Also published in The Correspondence of Charles Darwin, vol. 10.

Van Wyhe, J. & Rookmaaker, K. (2015) Alfred Russel Wallace: Letters from the Malay Archipelago. Oxford University Press, Oxford.

I’m a moth lover but London’s ‘toxic caterpillars’ worry even me

Callum Macgregor, University of York

File 20180502 153866 1eowuy1.jpg?ixlib=rb 1.1Sarah2 / shutterstock

Moths are the insect we truly love to hate. The press report almost annually on the looming threat of clothes moths. I have previously written in defence of diamondback moths, a migratory pest of cabbage crops, and highlighted the quirks of biology that drove the spectacle of thousands of Silver Y moths gatecrashing the Euro 2016 final without tickets. I am absolutely unapologetic about my love for these diverse and intriguing cousins of the much better-loved butterflies. Moths get a bad press thanks to a few species which negatively affect our lives (this also applies to other insects, such as wasps), but most are harmless (or beneficial), fascinating, and often even beautiful.

And so we come to recent news reports of a plague of toxic caterpillars descending on London. The caterpillars in question are those of the oak processionary moth (Thaumetopoea processionea, or just OPM) – just about the only species for which I struggle to summon up much sympathy.

So what’s the issue?

Delicately sketched in pencil, the adult oak processionary moth.
Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org, CC BY-SA

It’s not to say that this is an unattractive moth. The grey-black colour scheme of the adults, active in late summer, lends them the look of having been delicately sketched in pencil. You are much more likely to encounter the caterpillars, which are covered in very long, white hairs. Colonies of OPM caterpillars form white silk nests on oak trees and can be spotted moving about in remarkable nose-to-tail processions. Other moth species form similar nests in the UK, including occasionally on oak: if such a nest is found outside London (especially in Essex or Cambridgeshire), it is more likely to be the Brown-tail moth.

Keep an eye out for OPM caterpillars moving in crocodile-formation between oak trees.
OlegD / shutterstock

Unlike the Brown-tail, however, the oak processionary moth is not native to the UK. It was first recorded in Britain in 1983, but the species established properly in around 2006, when it’s believed some eggs arrived on imported oak trees. This isn’t in itself a reason to dislike OPM, as conservationists (including myself) can sometimes be hypocritical about non-native species: for instance, we are vocally concerned about the arrival of the horse-chestnut leaf-miner moth because it harms horse chestnut trees – even though the trees themselves are non-native.

However, OPM is also a potentially a public health problem. Each of the caterpillar’s hairs contains a toxin called thaumetopoein. Touching an OPM caterpillar directly could bring you out in a rash – in fact, as a general rule it’s always best to avoid hairy caterpillars unless you know what you’re dealing with. The hairs of OPM caterpillars can also break off and drift on the air and, if there are sufficiently high densities of caterpillars, these hairs can cause rashes and respiratory problems even to bystanders.

Besides affecting people, OPM can also impact the oak trees on which it feeds. A particularly severe infestation could strip a tree completely bare of its leaves, though few cases of this taking place in the UK have been reported.

Tackling the problem

Most people agree that something needs to be done, although the NGO Butterfly Conservation argues that, rather than tackling the moth wherever it appears, control efforts should focus on areas where the threat to human health is high or large numbers of trees are at risk of death. Nevertheless, controlling OPM outbreaks is difficult. The nests are constructed in the crowns of oak trees when they are in full leaf, and even if they can be reached, removing them manually requires full protective equipment to ward off the toxic hairs. For that reason the preferred approach is currently to tackle nests remotely, spraying trees with insecticide when the moth is most vulnerable – as a young caterpillar, between April and June.

There is currently no insecticide that is specific to OPM, so a bacterium known as “Bt” is used. Unfortunately, Bt is toxic not just to OPM, but to the caterpillars of all moths and butterflies. The financial cost of these control efforts is astronomical – estimated at around £1.2m per year in 2016-17.

The uncounted, and incalculable, cost to the oak woodland ecosystem could be greater still. The loss of much of the insect biodiversity from our woodlands would be tragic in itself but is likely to have further implications for the bats, birds and other wildlife that rely on these insects for food during their breeding seasons – a study of an OPM control programme in woods near Pangbourne, Berkshire, suggested that blue and great tits were breeding less after spraying took place. It’s these losses that have put me off OPM.

The attractive green silver-lines moth inhabits oak woodland, and is among the species likely to lose out from attempts to control OPM.
Hildesvini

Looking to the future

But let’s not panic – there are plenty of reasons to feel hopeful about the future of Britain’s oak woodlands. It’s true that the moth has been recorded “across vast regions of the south-east”, but that mostly only refers to the highly-dispersive males. To spread the outbreak requires the egg-laying females to travel, and they don’t fly nearly as far. This means that, for now, the toxic caterpillars are mainly confined to London. The outbreak has crossed the M25 ringroad in just a few places, and is still only expanding at a slow pace.

Encouragingly, some of the more isolated sections of the outbreak also appear to be coming under control. New outbreaks in Watford, Barnet, and Pangbourne all appear to have been successfully removed. An outbreak at Bethlem Hospital, Croydon, estimated to contain 4,000 nests in 2012, was confined to just four trees by 2016. Vigilance is key, and this year the Forestry Commission is once again asking the public to report any potential sightings of OPM through its Tree Alert scheme.

Finally, we may have some unexpected allies on our side. In its native southern and central Europe, OPM is not especially problematic because it rarely reaches sufficiently large population densities. That’s partly because its numbers are kept in check by its natural enemies – parasitoids. This is a catch-all term for various insects with a rather gruesome life-cycle: eggs are laid inside caterpillars and other insects, before the larvae eat their victim from inside out (killing it in the process) and emerge as a fully-formed fly or wasp ready to seek out new prey.

The ConversationOften, when an insect expands its range by artificial means (as OPM did, entering the UK on imported trees), it can take some time for its parasitoids to catch up, and in this lag period the insect may do particularly well. However, a recent study found nearly half of the OPM caterpillars sampled from the Croydon outbreak in 2014 were infested by one such natural enemy, the Carcelia iliaca tachinid fly. This suggests the oak processionary moth may be reaching the end of its lag period in the UK, and as the flies attack more caterpillars, this could help the control efforts. My enemy’s enemy is truly my friend, and in this case, perhaps it is a tiny fly.

Callum Macgregor, Postdoctoral research associate, University of York

This article was originally published on The Conversation. Read the original article.

Moths expert: match report on Ronaldo insect encounter at Euro 2016 final

Callum Macgregor, Newcastle University

After a month of football, Euro 2016 drew to a close with Portugal defeating France 1-0 in extra time to lift the famous trophy for the first time. But the final game was a turgid affair, and perhaps the image most likely to endure involved a rather unexpected participant.

As Cristiano Ronaldo sat in tears on the turf, the reality of his game-ending injury sinking in only 25 minutes into the game, a moth fluttered around his eye and settled on his forehead. This moth was one of thousands in the stadium, pictured in clouds on the pitch as the players warmed up, and being swept with brooms from advertising hoardings.

What were they doing there? The bizarre spectacle appears to have been the result of a coming together of several exceptional circumstances.

A very brightly lit stadium

It is commonly known that moths display an extremely strong attraction to sources of artificial light, although we still don’t really know why. The stronger and brighter the light, the better – and this is our first exceptional circumstance.

Fully lit, the Stade de France shines like a beacon even amid the glow of La Ville Lumière. Worse yet, the format of the lighting – in a ring, facing inwards – means that once a moth finds itself inside the stadium, it would be near-impossible for it to escape the influence of the lights.

One large moth-trap.
Srdjan Suki/EPA

But many football games have been played under floodlights at big stadiums, and this game began while it was still daylight. This brings us to our second exceptional circumstance, then – the stadium lights were left on overnight on the eve of the game, presumably to aid the ultra-tight security effort that has surrounded the whole tournament.

The lights had a full night to attract as many moths as they could into the stadium. When fans and players began to arrive, those moths were nearing the end of their daytime slumber.

Migration spectacular

Still, though, brightly lit stadia have held events on consecutive nights before without experiencing mothy visitations. So to find the third exceptional circumstance, we need to look more closely at the moths themselves.

As the game unfolded, it became clear to those in the know that the vast majority of moths on display were representatives of a single species: Autographa gamma, the Silver Y – both names stemming from the γ-shaped marking on the wing. The Silver Y carries metallic patterns on its wing, like many other species in its sub-family, the Plusiinae, including the beautiful Gold Spot and my personal favourite species, the Burnished Brass.

When not bugging footballers, Silver-Ys enjoy feeding on flowers.
Arto Hakola/Shutterstock

Importantly for our investigation, these moths bear a remarkable similarity to another species recently making headlines, the cabbage-chomping Diamondback moth, which also migrate in huge numbers to UK shores.

Silver Ys make the journey annually from North Africa, travelling through France (including Paris) to breed in Britain. Scientists at Rothamsted Research found that in peak years they number almost a quarter of a billion, and up to four times that number return southwards in the autumn. By a twist of fate, the Euro 2016 final appears to have coincided with this year’s migrating Silver Ys reaching Paris on their northward journey.

Bad news for moths?

We know from several recent studies that the profusion of artificial light at night spells trouble, both for moths and the other organisms they interact with in the ecosystem. Moths have short lifespans – Silver Ys may spend as little as two weeks in their adult form after emerging from the cocoon – and in this time must complete their migration before finding a mate and a suitable location for their eggs. Time spent under the influence of a light is time wasted.

With luck, organisers at the Stade de France will have the sense to leave the stadium lights off on Monday night. Returned to (relative) darkness, most of the stadium’s temporary lodgers will be able to escape the arena and continue their journey merrily northwards – perhaps, for some, to land on British soil within the coming days much to the excitement of moth lovers who, like me, eagerly await their arrival.

The Conversation

Callum Macgregor, PhD Candidate, Newcastle University

This article was originally published on The Conversation. Read the original article.

Channel-hopping moth plague: our clothes are safe, but mind the cabbages

Callum Macgregor, Newcastle University

The next great plague has descended, and this time it’s moths. Diamondback moths have been crossing the English Channel over the past fortnight in far greater numbers than usual.

Those of you reaching for moth-balls and carpet sprays, stop. There are only two species of British moth (from a cast of thousands) that damage clothes, and the diamondback is not one of them. You, the reader, really don’t need to worry – unless you grow cruciferous vegetables like cabbages or broccoli.

Plutella xylostella – more commonly known as the diamondback – is one of the world’s most widespread and prevalent agricultural pests; so widespread, indeed, that scientists disagree as to where exactly its native range lies.

Their success belongs in part to their preference as caterpillars for eating plants in the Brassicaceae family. This amazing group of crops (notably the species Brassica oleracea) has been selectively bred over hundreds of years to make use of almost every part of the plant, and now contains many popular vegetables such as cabbages, broccoli, Brussels sprouts and turnips, as well as oil-seed rape. The moths lay eggs on brassica leaves; a week later, caterpillars hatch and start eating. An infested plant can quickly be reduced to little more than leaf stems.

Cross-Channel travellers

Despite being one of the smallest moth species to occur in the UK, with a wingspan of under 15mm, diamondbacks are capable of dispersing vast distances, using the wind to make up for their small size. Research has shown that without assistance, diamondbacks are only capable of flying 35 metres – yet when carried along by the wind, they may travel hundreds of miles in a single day.

It is this ability to disperse that leads it to British shores. Every summer, adults cross the Channel and attempt to breed in the UK. A number of butterfly and moth species employ this strategy. In some cases, such as the Painted Lady butterfly, the offspring will return south ahead of the winter. For others, including diamondbacks, the offspring will attempt to remain, but British winters are typically too cold for them to survive.

What’s for dinner? Cabbage, broccoli and Brussels sprouts.
Yuangeng Zhang / shutterstock

For the moths, these fatalities are an unfortunate side effect of a highly dispersive life history; by having offspring constantly travelling beyond the edge of their range and attempting to breed, the moths ensure that no available space is wasted. Should some of the dispersing moths find a new area that is suitable for breeding success, they will benefit enormously from a lack of competition with their peers and, potentially, the absence of specialised predators and parasites. Thus, evolution favours and preserves these intrepid explorers.

Supermoth?

And it is once the moths begin to breed that the problems begin for farmers and gardeners, because these are not your average pest. Diamondback has been referred to in some coverage as a “super-moth”. It’s a title it certainly deserves.

The species has proven itself to be extraordinarily adept at evolving resistance to pesticides. Wild populations have developed significant resistance to almost every significant synthetic insecticide in use, including one of the main classes, pyrethroids.

Perhaps more impressively, diamondback was the first insect recorded to have developed resistance in the field to the bacterial pesticide Bacillus thuringiensis; only one other species is known to have managed this since (the “cabbage looper”, a pest moth named after its favourite food). Interestingly, this has led scientists to target it with the same genetic tools as are employed against the mosquitoes that transmit dengue fever and Zika virus. Field trials are planned in the US for this summer.

Tough times on the veg patch

In the short-term, perhaps. The influx of moths will soon begin to produce larvae, and these may cause substantial damage to crops where they are able to escape death by insecticide. The damage will be most severe to farmers of leaf crops, such as cabbages or kale, as the holes left behind in leaves by caterpillars make the crop unsellable.

Diamondback caterpillars leave cabbage unappetising and unsellable.
Rothamsted Research, CC BY

Looking at the longer term, this year’s invasion will die out over the winter. Next year there will be another, but this is nothing new: these moths come to the UK every year in substantial numbers. I spent the summer of 2014 collecting moths on farms in Oxfordshire for a study on the effects of light pollution on moths. Out of the 203 species of moth I recorded, diamondback was the 26th most common, forming around 1% of my total catch.

Britain’s cabbage-growers have been dealing with diamondback for years and have managed to persist thus far. They may have a hard summer ahead of them but it is not without hope. Although globally diamondbacks have developed resistance to almost all pesticides, most populations are only resistant to a subset of these, and scientists at Rothamsted Research are already studying the current influx to identify their weaknesses. Armed with the results of that research, farmers may be capable of restricting the damage.

The Conversation

Callum Macgregor, PhD Candidate, Newcastle University

This article was originally published on The Conversation. Read the original article.

Fatal attraction: how street lights prevent moths from pollinating

Callum Macgregor, University of Hull/Newcastle University

For centuries, we have observed that artificial sources of light hold a strange fascination for moths. Despite decades of research, we still don’t know the cause of this attraction. Some theories put it down to the way moths navigate; others think it’s a mechanism to help them to escape from perceived danger. But the truth is, little evidence exists to support either of these ideas.

Whatever the cause is, research has shown that this deadly attraction may have even more sinister consequences than we first thought. In an open access paper in Global Change Biology, my colleagues and I describe the first evidence which shows that the effects of artificial light on moths may have serious implications for the wider ecosystem.

There has been plenty written about the danger posed by declining bee and butterfly populations, on the basis that some plants rely on these insects to carry pollen and fertilise flowers, in order to reproduce. But many people aren’t aware that moths also perform this task: our study of field sites across Oxfordshire found that one in four moths were carrying pollen, from at least 28 different plant species.

Danger zone.
from http://www.shutterstock.com

And like their cousins the butterflies, moths are in trouble: according to Butterfly Conservation’s Richard Fox: “the total abundance of moths in Britain has decreased by over a quarter since the 1960s”. Research indicates that artificial light, such as street lamps, has contributed to this decline by affecting moths’ development, reproduction and ability to escape predators.

Now, our data suggests that street lights are also directly thwarting night-time pollination, by attracting moths upwards, away from the fields and hedgerows. We found that the abundance of moths at ground level was halved in lit areas, while flight activity at the height of the street light was nearly doubled. The diversity of species was also reduced at ground level, with 25% fewer moth species in lit areas, compared to places without street lighting.

This change is likely to disrupt nighttime pollination by moths, and indeed we found some evidence that moths may carry less pollen, from fewer plant species, in lighted streets. This could mean that the impacts of street lights go beyond posing a health risk to moths. Plants that rely on moths for pollination would also suffer if their reproduction is impeded – and this might, in turn, affect organisms that eat those plants or drink their nectar.

In a best-case scenario, some of these so-called cascading effects might be mitigated where flowers can rely on other insects such as bees for pollination. But there are further factors driving declines in pollinator populations, such as climate change, pesticides and habitat loss. Now, our research suggests that artificial light can be added to the list.

Unnatural selection

So how can we protect these beautiful, under-appreciated insects and the important role they play in our environment?

The Spindle Ermine moth knows what’s good for it.
gailhampshire/flickr, CC BY

Another recent paper published by researchers in Switzerland suggests that moths may be evolving to be less strongly attracted to lights. Under controlled experimental conditions in a flight cage, they found that Spindle Ermine moths from urban populations were less likely to be captured in light-baited moth traps than their rural counterparts.

These findings suggest that moths which can resist the temptation of lights put themselves at a significant advantage over their peers. Over time this has led city-dwelling moth populations to become less attracted to lights, through natural selection. But this is all relative: urban moths are still far from immune to the deadly allure of urban street lights.

We can’t simply switch street lighting off: although the evidence for its actual benefits is questionable, it certainly contributes to many people’s feeling of safety and security when outside after dark and proposals to turn lights off are often unpopular.

But if we don’t wish to wait for the slow crawl of evolution, it may be that recent advances in street lighting technology can help to mitigate the impacts of artificial light. For example, developments born out of a desire for energy efficiency could also minimise the impact of street lighting on moths. Measures such as switching on street lights for part of the night, dimming them or introducing motion-activated lighting would reduce moths’ exposure to street lights. Similarly, the flexibility of LED lights might allow for the creation of street lights that are less attractive to moths, which respond most strongly to short-wavelength blue light.

Nevertheless, artificial light at night continues to increase as we seek to drive darkness from the streets. Our research is another warning that this may have far-reaching consequences for the organisms around us.

The Conversation

Callum Macgregor, PhD Candidate, University of Hull

This article was originally published on The Conversation. Read the original article.