Rapid change in the settling behavior of the arrowhead scale Unaspis yanonensis as an avoidance mechanism against introduced parasitoids, Aphytis yanonensis and Coccobius fulvus

Abstract

Parasitoids are thought to exert immense selection pressures that shape the traits of herbivores. We examined whether two species of parasitoid wasps, Aphytis yanonensis DeBach et Rosen and Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae), affect the settling behavior of the arrowhead scale Unaspis yanonensis Kuwana (Hemiptera: Diaspididae), in order to demonstrate the evolution of antiparasitism behavior of herbivores using parasitoids in the field. We used the following five methods: a comparison of parasitism risk in different settling behaviors-parasitoid introduction into a parasitoid-free population; a comparison of the settling behavior between parasitoid-present and parasitoid-free populations; a common garden experiment, in which scales were transferred from parasitoid-present and parasitoid-free populations into the same garden; and a laboratory observation of the settling behavior of the first instars derived from the two population categories. Both parasitoids were introduced into a parasitoid-free population in Wakayama in 1987, and the settling mode of the scales was examined in 1987, 1994, and 1995. The introduction of parasitoids modified the scale-settling mode so that more crawlers settled under another scale (called burrowing), which was consistent with the results observed in parasitoid-present (including South China) and parasitoid-free populations. Moreover, only the burrowing scale exhibited a lower parasitism rate compared to scales settling singly and being burrowed. The common garden experiment demonstrated that scales introduced from the parasitoid-present population had a greater proportion of burrowers than the parasitoid-free population, even in the same field cage under parasitoid-free conditions. Laboratory observations demonstrated that the population difference in parasitism rate was principally due to intrinsic differences in the settling behavior of nymphs; some first-instar nymphs derived from the parasitoid-present population burrowed under another scale settled. These results strongly suggest that the natural selection pressure imposed by the parasitoids modified the settling behavior of the arrowhead scale.