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