Effects of insect opportunists on a four-level trophobiotic system involving nectar-producing galls of the cynipid wasp Disholcaspis quercusmamma (Hymenoptera: Cynipidae)

Abstract

The induction of plant galls is considered an adaptive life history trait found in many insect groups. The formation of galls provides several advantages to the gall maker, such as enhanced nutrition, favorable microclimate, and protection from natural enemies, including parasitoids, inquilines, and predators. Order Hymenoptera has many gall-making species, belonging to the gall wasp family Cynipidae. As an extended phenotype of the gall makers, some galls exhibit very sophisticated adaptive mechanisms involving multilevel species interactions. In particular, the oak galls of the Cynipid species Disholcaspis quercusmamma, found in much of Illinois, produce a palatable, sugary nectar-like secretion, attracting other insects. It is hypothesized that visiting insects discourage potential parasitoids and inquilines questing for oviposition sites within the galls, possibly producing an enemy-free space. Several studies on the species interactions involved in this four-level trophobiotic association have examined the role of visiting ant species in the system. However, other insect species have also been observed to be attracted to the extrafloral nectar of the galls, and their roles in the gall wasp-natural enemy interaction system have not yet been studied. This study aimed to examine the effects of all visiting insects, including ants, on the success of the gall-makers as a result of potentially reduced parasitism. Exclusion experiments were carried out on three oak trees in study sites in central Illinois. Statistical analyses of field data found no effect of treatment on gall wasp success, parasitism, or inquilinism. However, additional tests showed a positive correlation between gall wasp success and inquilinism rate compared to gall cluster size. Gall diameter correlated positively with gall maker success and negatively with inquilinism rate. This suggests an underlying complexity to the system. Additional research is necessary to better understand this uniquely adapted relationship.

Date of Award	2020
Original language	American English
Awarding Institution	Eastern Illinois University
Supervisor	Zhiwei Liu (Supervisor)