Oviposition Behavior of the Wheat Stem Sawfly When Encountering Plants Infested with Cryptic Conspecifics

Insect herbivores typically oviposit on the most suitable hosts, but choices can be modulated by detection of potential competition among conspecifics, especially when eggs are deposited cryptically. Larvae of the wheat stem sawfly, Cephus cinctus Norton, developing within an already infested stem, experience elevated risk when only one will survive because of cannibalism. To increase our understanding of host selection when the choices made by females can lead to severe intraspecific competition, females were presented with either uninfested wheat plants or with plants previously exposed to other females in laboratory choice tests. The oviposition behavior of this insect was described by recording the behavioral sequences that lead to and follow the insertion of the ovipositor in both previously infested and uninfested stems. No significant differences were found in frequencies of specific behaviors or behavioral transitions associated with oviposition. In choice tests, there was no difference in the numbers of eggs laid in infested and uninfested plants. Taller plants received more eggs, irrespective of infestation. Females neither preferred nor avoided previously infested hosts. Other characteristics of the host, such as stem height, may be more important in determining suitability for oviposition. These findings support the use of management tactics relying on the manipulation of oviposition behavior, such as trap cropping. Given that there is no evidence for response to previously infested hosts, the infested plants in a trap crop would remain as suitable as they were when uninfested, which could also lead to an increase in mortality caused by intraspecific competition.