Expanded Oviposition Range by a Seed Beetle (Coleoptera: Bruchidae) in Proximity to a Normal Host

Abstract

The bruchid beetle Stator vachelliae Bottimer will sometimes oviposit on seeds of the nonhost Parkinsonia aculeata (Leguminosae = Fabaceae), but most larvae do not survive. Of 1,118 eggs on Parkinsonia seeds, 86% occurred in a narrow zone where seeds of Parkinsonia and the normal leguminous host Acacia flexuosa were mixed on the ground beneath overlapping canopies. A field experiment indicated that females use nonseed cues associated with Acacia (pod valves and cow and horse dung) to locate Acacia seeds, and when seeds of both hosts are encountered, females oviposit equally on seeds of both plants. Other factors that were not controlled in the experiments (amount of daylight, temperature, humidity) may also affect discrimination between seeds of different hosts. Survivorship on Parkinsonia was <0.1% in the field, whereas on Acacia it was 15%. Most (53%) mortality on Parkinsonia was attributable to inability of larvae to penetrate seed coats. Enemy-free space was available on Parkinsonia: only 3% of bruchid eggs on its seeds were parasitized, but on the normal host Acacia, 24% were parasitized. We suggest three steps for S. vachelliae to include Parkinsonia in its host range or for a host shift: (1) oviposition on the potential host in proximity to cues associated with the normal host, which does not appear to require a genetic change in search or oviposition behavior; (2) larvae of S. vachelliae must evolve the ability to complete development in the new host; and (3) the adult beetles must be able to track the new host. Only step 1 has occurred in this system. The results support previous suggestions that the initial colonization of new hosts may be behavioral.