Spreading-the-risk hypothesis may explain Cameraria ohridella oviposition in relation to leaf blotch disease

Abstract

The horse chestnut, Aesculus hippocastanum L., is a common tree in urban and rural environments of Europe. Invasion by Cameraria ohridella (Deschka & Dimić) started three decades ago, and to date, the whole European range of A. hippocastanum has been infected. With the addition of the fungal agent of leaf blotch Phyllosticta paviae (Desmazières), a new tripartite interaction arose, and a possible complex disease has been established. In this study, we aimed to answer the following question: do females of C. ohridella deposit eggs more frequently on healthy leaflets than on infected ones? In two experiments, leaves were previously infected by leaf blotch, and leaf miners were absent. In the choice test, one fertilised female was put into a box with healthy and infected leaflets. The second experiment was conducted under greenhouse conditions; an unknown number of females were exposed to leaf-blotch-infected and healthy saplings. Eggs were deposited mainly on healthy areas of leaves and always in leaf depressions. A positive relationship between the number of chosen leaflets and the number of deposited eggs was found when a single female was allowed to choose. In both experiments, we found that healthy leaflet area rather than the presence or absence of leaf blotch disease was the factor explaining the number of deposited eggs. We conclude that C. ohridella females are spreading the risk by ovipositing on many leaflets rather than avoiding the potential decrease in offspring survival.