Oviposition strategies in Pieridae butterflies and the role of an egg-killing plant trait therein.

Most herbivorous insects are host-plant specialists that evolved detoxification mechanisms to overcome their host plant's toxins. In the evolutionary arms-races between Pieridae butterflies and Brassicaceae plants, some plant species have evolved another defence against the pierids: egg-killing. Underneath the eggs, leaves develop a so-called hypersensitive response (HR)-like cell death. Whether some butterflies have evolved oviposition strategies to counter-adapt against egg-killing remains to be studied. In this study, we assessed the oviposition site location of Pieridae butterflies on their natural host plants. We described the plant tissue on which we located the eggs of the most common Pieridae in the Netherlands: and . Additionally, we assessed expression of HR-like cell death in response to the deposited butterfly eggs. We found that both and mainly oviposited on the floral stem and the branch, respectively, and oviposited on host plants from lineages not expected to kill pierid eggs. Accordingly, no HR-like cell death was seen. All eggs found were located on leaves of their host, the only tissue found to express HR-like cell death. Furthermore, each species was found to at least occasionally oviposit on . This was the only plant species in this survey that expressed HR-like cell death in response to the eggs of and . Our observations demonstrate that HR-like cell death remains an effective defence strategy against these species and as such did not find evidence for the hypothesized counterstrategies. Surveying certain key species and disentangling the micro-evolution of oviposition strategies within a species would allow us to further investigate potential counter-adaptations that evolved against HR-like cell death. This study provides the basis for further investigation of potential counter-adaptations to egg-killing defences.