Abstract
Recently in Poland, cases of host expansion have frequently been observed in the typically monophagous bird-cherry ermine moth (Yponomeuta evonymella), which has moved from its native host plant, bird cherry (Prunus padus), to a new, widely distributed plant that is invasive in Europe, black cherry (P. serotina). We attempted to verify the reasons behind this host change in the context of the enemy-free space hypothesis by focusing on parasitoids attacking larval Y. evonymella on one of three host plant variants: The primary host, P. padus; initially P. padus and later P. serotina (P. padus/P. serotina); or the new host, P. serotina. This experiment investigated if changing the host plant could be beneficial to Y. evonymella in terms of escaping from harmful parasitoids and improving survival rate. We identified nine species of parasitoids that attack larval Y. evonymella, and we found that the number of parasitoid species showed a downward trend from the primary host plant to the P. padus/P. serotina combination to the new host plant alone. We observed a significant difference among variants in relation to the percentage of cocoons killed by specific parasitoids, but no effects of non-specific parasitoids or other factors. Total mortality did not significantly differ (ca. 37%) among larval rearing variants. Changing the host plant caused differences in the structure of the parasitoid complex of Y. evonymella but did not improve its survival rate. This study does not indicate that the host expansion of Y. evonymella is associated with the enemy-free space hypothesis; we therefore discuss alternative scenarios that may be more likely.
Highlights
Plants and the herbivores that feed on their tissues and sap have coevolved, creating a relationship wherein plants produce new defences and herbivores attempt to mitigate those defences to further feed on the plants
We focused on the bird-cherry ermine moth Yponomeuta evonymella (L.) (Lepidoptera; Yponomeutidae), which feeds on the European bird cherry (Prunus padus L.) and, recently, on the non-native, invasive black cherry (P. serotina Ehrh.)
We were searching for an explanation of a phenomenon that has been observed in nature over the past ten years: Y. evonymella more and more frequently utilising a new plant, P. serotina [22], a non-native and invasive species [24,31]
Summary
Plants and the herbivores that feed on their tissues and sap have coevolved, creating a relationship wherein plants produce new defences and herbivores attempt to mitigate those defences to further feed on the plants. Generalists, which include polyphagous and oligophagous insects, feed on a wide range of hosts because they are not specialised to feed on a particular range of plants, whereas specialists are insects that are monophagous. When choosing their host, monophagous insects search for the best food quality and the best environment in terms of the survival of their offspring The enemy-free space hypothesis states that insects will use new host plants to escape from natural enemies [6]. Choosing a new host plant can result in a decrease in adult insect mass [9]; this may extend larval development time, thereby increasing larval mortality due to prolonged exposure to predators and parasitoids [10,11,12]
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