Morgan, D.J.W., & Hare, J.D. 1997 Uncoupling physical and chemical cues: the independent roles of scale cover size and kairomone concentration on host selection by Aphytis melinus DeBach (Hymenoptera: Aphelinidae).. Journal of Insect Behavior 10: 679-684.

Notes: The aphelinid ectoparasitoid Aphytis melinus initially selects its host, the California red scale, Aonidiella aurantii, using characteristics of the hosts cover before assessing the quality of the body beneath. Host suitability is known to increase with host size until A. aurantii reaches maturity, after which it is no longer available for parasitism. Aphytis melinus uses a combination of scale cover size and a kairomone, O-caffeoyltyrosine, in the cover for initial assessment. Under natural conditions these two factors are frequently coupled. The relative importance of scale cover size and kairomone concentration was quantified independently by removing and selectively reapplying controlled concentrations of synthetic O-caffeoyltyrosine to scale covers of known size. In the absence of the kairomone, aphelinids did not discriminate among scale covers differing in size. Aphelinids showed a curvilinear response to kairomone concentration for each scale cover age group. Aphytis melinus preferred low concentrations of O-caffeoyltyrosine on young, small scale covers, and high concentrations on old, large scale covers. It was concluded that the ability of A. melinus to respond quantitatively to the kairomone may be used in the field to differentiate small 2nd-instar larvae of Aonidiella aurantii from larger and more suitable 3rd-instar larvae.