Determinants of host selection for species ofAphytis(Hymenoptera: Aphelinidae), parasites of diaspine scales

Host selection behavior in entomophagous insects is a series of reflex responses to physical and chemical stimuli. It was known that the host selection process for Aphytis, parasites of armored scales, utilized the stimulus of a water-soluble chemical contained in the scale cover and that physical features were unimportant. It was unknown whether the adult could exercise host discrimination and restrict oviposition to host species that were suitable for progeny development. When given no choice of host species, the female is under physiological pressure to oviposit and does not discriminate. However, when a choice is present the female prefers to oviposit on the suitable host, though discrimination is not absolute. Suitability is determined by stimuli perceived both by the antennae and by the ovipositor, neither having predominant importance. The scale molt stage is rejected as an oviposition site because of its hard condition, not because of a lack of an “air space” stimulus occurring between the cover and the body. With the cover removed, the second instar scale will continue development to the second molt stage. In that stage, it is free from the host fruit and can be transferred to another fruit where it will become a third instar, forming an entirely new cover. Quick-frozen and defrosted hosts are readily accepted, showing that a movement stimulus or indeed the living condition is not necessary for oviposition. Pre-imaginal conditioning may or may not occur, depending on the species of host. Such conditioning would not be a factor in mass rearing programs using A. melinus. While ovipositing, Aphytis injects a venom into the host, preventing sclerotizing or molting. This prevents loss of the parasite egg due to host ecdysis.