Another beautiful piece placed in the Journal of Chemical Ecology, this was our second dive into the chemistry of Alliaria petiolata (garlic mustard) and how it was affecting a rare native butterfly, Pieris virginiensis (the West Virginia White). We found that one particular compound, sinigrin, did not affect oviposition rates, but it did effect how well caterpillars did. We also tested another compound, alliarinoside, and found that it was likely responsible for larval death on the invasive novel host. We did not ultimately find why A. petiolata is so attractive to P. virginiensis adults, but we did find what kills the caterpillars. Much gratitude to our international team for isolating and synthesizing the test compounds!
As it pertains to insect herbivores, the preference-performance hypothesis posits that females will choose oviposition sites that maximize their offspring’s fitness. However, both genetic and environmental cues contribute to oviposition preference, and occasionally “oviposition mistakes” occur, where insects oviposit on hosts unsuitable for larval development.
Pieris virginiensis is a pierine butterfly native to North America that regularly oviposits on an invasive plant, Alliaria petiolata, but the caterpillars are unable to survive. Alliaria petiolata has high concentrations of the glucosinolate sinigrin in its tissues, as well as a hydroxynitrile glucoside, alliarinoside. We investigated sinigrin as a possible cause of mistake oviposition, and sinigrin and alliarinoside as possible causes of larval mortality. We found that sinigrin applied to leaves of Cardamine diphylla, a major host of P. virginiensis that does not produce sinigrin, had no effect on oviposition rates. We tested the effect of sinigrin on larval performance using two host plants, one lacking sinigrin (C. diphylla) and one with sinigrin naturally present (Brassica juncea). We found no effect of sinigrin application on survival of caterpillars fed C. diphylla, but sinigrin delayed pupation and decreased pupal weight.
On B. juncea, sinigrin decreased survival, consumption, and caterpillar growth. We also tested the response of P. virginiensis caterpillars to alliarinoside, a compound unique to A. petiolata, which was applied to B. oleracea. We found a significant reduction in survival, leaf consumption, and caterpillar size when alliarinoside was consumed. The ‘novel weapon’ alliarinoside likely is largely responsible for larval failure on the novel host A. petiolata. Sinigrin most likely contributes to the larval mortality observed, however, we did not observe any effect of sinigrin on oviposition by P. virginiensis females. Further research needs to be done on non-glucosinolate contact cues, and volatile signals that may induce P. virginiensis oviposition.