The parasitoids of Small Tortoishell (Aglais urticae)

Over the last 10 years there has been a lot of interest in the decline of Small Tortoishell butterfly (Aglais urticae) and the increase in a relatively new parasitoid of A.urticae called Sturmia bella. The issue of whether Sturmia caused the decline in Small Tortoishells was largely handled in a 2011 paper by Sofia Gripenberg, Nia Hamer, Tom Brereton, David B. Roy and Owen T. Lewis (see below), which concluded:

In the light of the available data, concluding that S. bella is the main driver of the recent declines of A. urticae would be premature. The fact that host numbers have also – though to a lesser extent – been declining in continental Europe (Van Dyck et al., 2009), where S. bella has been present for a long time, suggests that other explanations for the decline need to be considered.

Gripenberg S. et al. 2011. A novel parasitoid and a declining butterfly: cause or coincidence?– Ecol. Entomol. 36: 271– 281.

In this article though I just wanted to cover how you might identify any tachinid parasitoids reared from Small Tortoishell. The following key should work only for specimens reared from Small Tortoishell butterfly. These rules do not apply to specimens reared from other species.

  • Eyes bald?
    • Scutellum orange, wing bases clear & abdomen sections 2&3 without strong vertical median and dorsocentral bristles: Sturmia bella
    • Scutellum dark, wing bases orange/yellow & abdomen sections 2&3 with strong vertical median and marginal bristles: Pelatachina tibialis
  • Eyes hairy?
    • Scutellum dark & facial ridges with strong bristles up most of the distance to the base of the antennae; ocellar bristles absent or hairlike: Compsilura concinnata
    • Orange-tinged scutellum and facial ridges with bristles only up to less than half of the distance to the base of the antennae; ocellar bristles strong: Phryxe spp.
      • Dusting on the frons mustard yellow; gap between the highest facial ridge bristle and the lowest parafacial bristle wider than the width of antennal segment 3: Phryxe nemea
      • Dusting on the frons bluish grey; gap between the highest facial ridge bristle and the lowest parafrontal less than the width of antennal segment 3: Phryxe vulgaris

Phryxe nemea, P.vulgaris & Compsilura concinnata are generalists, attacking a very broad range of Lepidoptera larvae, while Pelatachina tibialis and Sturmia bella are both vanessid butterfly specialists. If you rear wild caught Small Tortoishells then you are almost certainly going to come across them at some point. In fact even if you rear captive bred larvae but you feed them wild picked nettles then you might bring the parasitoid eggs in on the foodplant, as many tachinids “seed” the foodplants of their hosts with tiny “microtype” eggs which are designed to be eaten by an unwitting caterpillar.

Sturmia bella is a very frequent parasitoid reared from Small Tortoishell but my personal opinion is that the decline of Small Tortoishell is likely to be due to a number of causes. Small Tortoishell had been in decline well before the arrival of Sturmia and so it seems most likely that the primary cause of the decline is the same as with many species – the negative impacts of humans on the countryside. Sturmia might have come along at a bad time and might be adding pressure to populations already in decline but mainland Europe shows us that where Sturmia has been living alongside Small Tortoishell for centuries there is no real problem. If Small Tortoishells are thriving then their parasitoids coexist with them and have no noticeable effect.