Fundamental host range of Trissolcus japonicus in Europe

The brown marmorated stink bug, Halyomorpha halys, native to East Asia, is an invasive alien pest that arrived in Europe in the early 2000s and poses an imminent threat to a wide variety of crops. Adventive populations of the Asian egg parasitoid Trissolcus japonicus, the most promising agent for classical biological control of H. halys, have recently been detected in Italy and Switzerland. Its prospective fundamental host range in Europe was evaluated in behavioural no-choice tests, followed by large-arena choice tests presenting host plants with naturally laid egg masses of target and non-target hosts. Developmental suitability of European non-target host species for T. japonicus was demonstrated, via no-choice tests, by offspring emergence (successful parasitism) from eleven out of thirteen non-target species tested (85%). Whereas successful parasitism of most non-target species was significantly lower, acceptance of Arma custos, Palomena prasina, Pentatoma rufipes, and Rhaphigaster nebulosa was not significantly different from H. halys controls. When eggs of H. halys and non-target species were exposed in a semi-natural situation in large-arena choice tests, the degree of non-target parasitism was substantially reduced for three out of four tested species, whereas parasitism of Pa. prasina eggs was not. It remains unclear if there are behavioural barriers to parasitism that may exist and preclude excessive parasitism of potentially threatened species in the field, but field data from the invaded areas in Switzerland and Italy could contribute to a risk–benefit evaluation of releasing or re-locating adventive T. japonicus populations into other parts of Europe.