Be quiet to get close to feeding Silver Ys

A still, balmy night, there are many Silver Y moths feeding on the garden flowers: lavender, perennial peas and buddleia. I always have trouble taking photos of the Silver Ys, they are so flighty! A step forward and they all disperse. A typical shot when I am outside is like the one above. My best shots have always been from behind the conservatory window. Today I understood why. If found out that you want to get close to these moths you need to be quiet. Why did this surprise me? I knew moths are able to hear bat ultrasounds and respond with erratic flight and even dropping to the ground, I even wrote a post about it. But there is nothing like first hand experience for learning. I tried to take shots from the cloud of silver Ys feeding in the buddleia tonight, they were visible in the dusk light as hovering silouettes by the profile of the large flowers. And then, as I approached, camera pointing up and triying to decide which one to shoot, I stepped on a crunchy leave inadvertently. The whole cloud of moths responded instantly by flying in different directions: of course they can hear! Many sounds we make while moving about are actually ultrasounds (click your fingers near a bat detector to test this). I bet walking on my pebbly path does this too, and this was what was keeping them away. I moved slowly, placing each foot carefully and slowly on the ground. Stood next to the buddleia and waited until the silver Ys came back and resumed their feeding. And this time managed some outdoor clear shots.

Night pollinators

Although daytime pollinators get a lot of press, night time pollinators are important too. Many moths feed on flowers at night, and some species of flowers appear to have evolved to be pollinated mainly by moths. If you go out to the garden on balmy summer nights, such as the ones we are enjoying this early September, you are likely to see Silver Y, Autographa gamma, feeding. This lively moth looks like a blur as it flutters continuously while it feeds - the flash creates a false 'frozen' effect. Like another summer visitor the Hummingbird Hawkmoth, the Silver Y has a very long tongue which allows it to feed on Honeysuckle, Lavender, buddleia, peas and wallflowers (above). All photos taken yesterday and today in the garden.

on honeysuckle

on lavender

...and on wallflower Erysimum Bowles Mauve

 And they must be careful on their comings and goings, while flying above them, Pipistrelle bats are hunting...

On insect migration and fairy tales

When it comes to invertebrates, very odd ideas can circulate even in scientific circles. One of such is called 'The Pied Piper' effect, and states that many insects migrating to northern latitudes in the spring do it hopelessly, as their offspring will be incapable of making the return trip to the region of origin, where they would be able to successfully overwinter: a one way migration trip. Of course this makes no evolutionary sense. If in a population some individuals were genetically predisposed to migrate north each spring - while the rest stayed put - and the offspring of such migrants had no chance to make the return trip successfully, natural selection would select against migratory behaviour: migrants will quickly be purged out of the population, while the offspring of sedentary individuals would be more successful and propagate their sedentary tendencies resulting in a non-migratory species.
  No matter how ridiculous this idea might seem to us, it has persisted for decades, in a similar way that Aristotle believed that swallows and other birds hibernated in the mud at the bottom of ponds or that Redstarts turned into Robins in the winter. An enormous body of research has confirmed that birds are capable of migrating back and forth to distant regions and have a magnetic compass that allows them to navigate even in adverse meteorological conditions. Bird migration is now widely accepted. Could it be that our prejudices with invertebrates made us find impossible to believe that some of them also could be able of regular back-and-forth seasonal migration? It appears so. But an online early paper by Jason Chapman and co-workers, from Rothamsted Research Station demonstrates that migration in an insect is an adaptive behaviour. Migrating insects use resources that are geographically and seasonally partitioned: in the summer they are available in more northern latitudes than in the winter and a two way migration is the most profitable way to use these resources. They collected data on the Silver Y, a common noctuid moth which is known to migrate from the Mediterranean to temperate latitudes in northern Europe. This species breeds continually in up to five generations a year and cannot survive the winters of northern Europe. The very active adults (above, adults often vibrate their wings when resting and when feeding) feed on nectar-rich plants and their caterpillars grow on a range of plants, including crops, where they can become a pest.
Building on a large body of previous work of the group on insect migration, some of it I already covered in BugBlog, these researchers analysed long term moth trap monitoring data at a national scale and entomological radar data. Both sets of data agree beautifully, complementing each other. The authors estimate that between 10 and 240 million Silver Y moths migrate annually to the UK, There strong inter annual variation in numbers and three years between 2000 and 2009 with exceptionally large immigration events in 2000, 2003 and 2006, likely due to benign ecological conditions in their wintering grounds. Their results provide solid evidence of the astounding population increases - and therefore potential reproductive benefits - experienced by migrating Silver Y moths in their northern breeding grounds. As the summer progresses, adult populations increase on average four-fold and for each moth that arrives in the spring, three will start the autumn return flight. There is a clear two-way movement: northern in the spring and southern in the autumn, with the autumn migration being of larger magnitude. But are they able to migrate all the way back? Simulations of expected moth return flights using flight and wind speeds and orientation of flying moths detected using the radar support the hypothesis that most of these moths could reach to their wintering grounds in the Mediterranean successfully. The Silver Y now joins the Painted Lady and the Monarch butterfly as models of insect migration, banning the 'Pied Piper effect' to the realm of fairy tales.

More information 
Chapman JW, Bell JR, Burgin LE, Reynolds DR, Pettersson LB, Hill JK, Bonsall MB, & Thomas JA (2012). Seasonal migration to high latitudes results in major reproductive benefits in an insect. Proceedings of the National Academy of Sciences of the United States of America PMID: 22927392

Silver Ys feeding

I've got a lavender hedge growing next to my conservatory, which comes very handy to do a daily bumblebee count. Tonight, while inside, I noticed a few silver Y moths feeding on the lavender and everlasting pea flowers which grow amongst the lavender. I went outside and more moths were feeding on the buddleias. As migratory moths, taking advantage of warm winters in high latitudes, silver Y numbers fluctuate a lot from year to year. This has been a warm summer and there are many of them around. When not resting, these moths are a flurry of activity, almost never stopping on the flower, and constantly vibrating their wings. Without using a flash, they are often a blurr, with it, they look like they are frozen in space. I had no choice but to use a flash, as it was too cloudy and dark, and the photos were taken trough glass. So here are my first night photos of the silver Y.

 Inserting its large proboscis on a lavender flower.

Caught in flight between flowers. This is a migrating species, found mainly from July to September.

Atop a lavender spike.

Several were feeding on the buddleia high up.

A silver Y resting next to the tomatoes earlier in the day.

High flyers: Silver Y moths

Yesterday I came across some piece of fascinating research on the silver Y (Autographa gamma), a brownish-grey moth so called for its Y shaped mark on each forewing. This moth is relatively large and is often active by day, with a liking for lavender and other flowers, moving its wings non-stop while feeding.

This is another great summer migrant into the UK and other parts of northern europe, where their caterpillars are often an agricultural pest. Jason Chapman and collaborators used an entomological Vertical Looking Radar (VLR) to detect individual flying insects. This radar is very sensitive and able to assess the high at which each insect is flying, its body orientation and flight speed. They analyzed radar data for nights in August 2000 and 2003, peak season for silver Y returning to their Mediterranean wintering grounds. Using trap data they established that the dominant flying moth species during those nights were silver Ys. Their data shows that this moth selects those nights with favorable wind directions for their migration (SSW) and that they adjust the hight they are flying to where the wind is fastest, usually over 100 m high.

Figure 1 from Chapman et at. 2008. Circular Distributions of Directional Data Obtained during Return Migrations of Autographa gamma Mean directions from each event are plotted (small circles at periphery). The bearing of the solid black arrow indicates the mean direction of the dataset, and the length of the arrow is proportional to the clustering of the dataset about the mean. (A)The mean tracks of high-flying migrant A. gamma during the 42 mass-migration events detected by vertical-looking radar (mean direction = 202). (B) The wind direction at 300 m at both radar sites during the migration periods. (C) The mean flight headings of migrant A.gamma during the 37 events with significant common orientation (mean heading = 205).

More astonishingly, their data shows that these moths compensate for their flight direction when the wind doesn't blow exactly in their preferred heading. This last discovery indicates that silver Ys use a compass. Given that the moths fly during nights when the moon is not visible and in overcast conditions, and their visual acuity, they could exclude a moon compass and a star compass. They conclude that the moths must have a magnetic compass, maybe adjusted by the sun at sunset. This fantastic technology illustrates how complex and plastic insect behaviour is, and how relatively slow flyers can get to their winter grounds in a few nights of flight.

More informationCHAPMAN, J., REYNOLDS, D., MOURITSEN, H., HILL, J., RILEY, J., SIVELL, D., SMITH, A., & WOIWOD, I. (2008). Wind Selection and Drift Compensation Optimize Migratory Pathways in a High-Flying Moth Current Biology, 18 (7), 514-518 DOI: 10.1016/j.cub.2008.02.080

An unexpected humming visitor

Yesterday, while taking photos of the resting Wool-Carder bees, I was nicely surprised by the low humming noise coming from a Hummingbird Hawkmoth, Macroglossum stellatarum. It was feeding high up in the blossoming buddleia. Fortunately I had the camera with me (I don't let it out of my sight these days!) and managed a few shots. The one on top is the best. Like the Painted ladies and silver Y moths, they are migrating insects from southern latitudes, which tend to come in warm weather and tend to appear on the same summers.

Graph showing my local records for three migrant species as year totals. The scale is logarithmic and I have replaced 0 by 0.1 to allow for this.

Hummingbird Hawkmoths take their name from their hovering flight. As other hawkmoths, they have long proboscides and do not land on flowers to feed, but just hover in front of them, darting sideways to the next bloom. They are active during the day and at dusk, which makes them easy to observe, unlike many nocturnal moths, including some of the same Hawk-moth family (Sphingidae). Hummingbird hawk-moths have fast flight and small wings, so, due to their high energy demands they prefer nectar rich flowers. My records are in Buddleia, Caryopteris, lavender, Centrarthus ruber and Ceanothus.

Hummingbird hawk-moth feeding on buddleia at sunset (Sep 2003)

 Hummingbird hawkmoths have been the subject of learning experiments carried out by Almut Kelber regarding flower colour and design. They are as good learners as bumblebees, much better than the butterflies tested, which had strong innate preferences. Although the hawk-moths have a innate preference for blue flowers, they are eager to try new colors and easily learn to associate different colours with food rewards. Given that they are relatively long-lived (up to 4 months in the laboratory) and strong migrants, this flexible learning behaviour might give then the ability to sustain their high energy requirements from different, seasonally or locally changing flower resources.
 If you have any records, Butterfly Conservation is carrying out a survey: Migrant Watch - Hummingbird Hawk-Moth.