Sensory cue use by insects associated with Arizona walnut

Abstract

This work focused on how visual and olfactory cue use by insects affect tritrophic interactions among the Arizona walnut, Juglans major (Juglandaceae), the fly Rhagoletis juglandis (Diptera: Tephritidae), whose larvae feed on the husk surrounding walnut fruits, and the parasitic wasp Biosteres juglandis (Hymenoptera: Braconidae: Opiinae) which oviposits in the fly larvae. The foraging behavior of female Biosteres juglandis was studied in the field and greenhouse, with specific attention paid to their use of visual and olfactory cues produced by walnut fruits harboring R. juglandis larvae. Field work demonstrated that wasps are successful in locating fruits infested with host larvae interspersed among uninfested fruits, and that they use fruit phenological traits (i.e. stage of rot) to do so. The method by which females appear to choose fruits in the field is functional, provided there are high host infestation levels. In greenhouse assays, fruit damage (apart from larval presence) was specifically identified as an important cue affecting wasp fruit choice. Wasps are also able to orient to infested fruits using only one type of cue, either olfactory or visual. Preliminary data from a pilot analysis of volatile compounds associated with infested, uninfested, and artificially damaged walnuts indicated that visual cues were more important than olfactory cues to free-foraging wasps. Rhagoletis juglandis adult females visit walnut fruits for oviposition, while adult males visit them to obtain matings. The effect of fruit color pattern on the behavior of male and female flies both inexperienced and experienced with real ripe walnuts was studied in the greenhouse. Overall, both sexes of flies exhibited a landing preference for plastic fruit models that appear ripe and uninfested, over models that appear infested. The behavior of both sexes may be driven by females who are attempting to provide offspring with the most possible food resources. Finally, the growth and germination of Arizona walnut seeds was followed to determine whether either was affected by infestation of the husk by R. juglandis. Although infested fruits were more likely to fall off a tree sooner, this did not affect the size of a walnut or its ability to germinate.