Genetic improvement of a parasitoid: Response of Trioxys pallidus to laboratory selection with azinphosmethyl

Colonies of Trioxys pallidus Haliday (Hymenoptera: Aphidiidae) were collected from California walnut orchards during 1985–86 and screened for variability of responses to azinphosmethyl. Variability was found and laboratory selection was initiated with four colonies. All colonies responded to selection; after seven to 12 selections, three colonies were combined (Select colony) and selection was conducted an additional 27 times. The corresponding base colonies were also combined and maintained for comparisons (Base). Concentration/mortality lines obtained for the Select and Base colonies after five and 27 selections indicated seven‐ and 9.2‐fold differences in LC₅₀ values respectively. Survival of the resistant strain varied with bioassay method and decreased in the order: clip cages on treated foliage > treated plastic cups with untreated mesh tops > treated plastic cups with treated mesh tops> treated cups with treated solid lids. The initial bioassay results with treated plastic cups suggested that the selection response was inadequate to allow the resistant strain to survive field rats of azinphosmethyl. When the resistant strain's survival was evaluated with bioassay methods more closely mimicking field conditions (clip cages on field‐treated foliage), we concluded the selected strain could survive field rates of azinphosmethyl.