Integrated management of the cabbage seedpod weevil and overwintering biology of canola pests

Key Result

The order of susceptibility to infestation by cabbage seedpod weevil was B. rapa > B. napus > B. juncea > S. alba. As well, an ectoparasitoid of cabbage seedpod weevil larvae, the wasp species Trichomalus perfectus (Walker), was discovered.

Project Summary

Several experiments were undertaken in 2001-2002 to develop an integrated management strategy for cabbage seedpod weevil in canola, and to investigate aspects of overwintering biology of lygus bugs and cabbage seedpod weevil. Research on integrated management of cabbage seedpod weevil focused on host-plant resistance, cultural control and biological control. Substantial research progress was achieved during this first year of study.

Brassicaceae species having high levels of resistance to infestation by cabbage seedpod weevil were identified, and include Sinapis alba, Crambe abyssinica, and Brassica carinata.  Of 236 intergeneric  hybrids developed from crosses of a resistant parent (S.alba) with a susceptible parent (B.napus), 18 were found to have high levels of resistance (< 5 weevil exit holes per 100 pods).

Manipulating canola plant stands by seeding early and increasing seeding rate resulted in higher levels of infestation by cabbage seedpod weevil. However, improved seed yields occurred at higher seeding rates and earlier seeding dates, indicating that canola can compensate for attack by this pest.

Trap borders of canola that flowered earlier than the main crop were effective for concentrating cabbage seedpod weevil adults, enabling growers to spray just the borders and not the entire crop. Even when two border sprays were required for adequate weevil control, growers could obtain an economic benefit from trap crops, and damage to non-target and beneficial insect species was minimized with this approach.

A parasitic wasp, previously unknown to Alberta, has apparently dispersed here and now attacks larval weevils feeding within pods. Tree shelters, rather than other habitats, appear to hold the key role in sustaining overwintering populations of lygus bugs and cabbage seedpod weevil.

Biological control initiatives for cabbage seedpod weevil resulted in the discovery of an ectoparasitoid of weevil larvae; the identity of the parasitoid remains to be confirmed, but it appears to be the wasp species Trichomalus perfectus (Walker). Studies on Microctonus melanopus (Ruthe), a parasitoid that attacks adult weevils, determined that the species is now widespread in mixed weed patches near Lethbridge, AB. It appears to overwinter as a larva or embryo.

Microcosm cages used to enclose weevils and lygus bugs and retrieved at varying intervals during winter indicated high levels of survival (ca. 70%) in both fields and treed areas. Although ambient air temperatures can vary widely, soil temperatures showed much less variance. At depths occupied by overwintering lygus bugs and cabbage seedpod weevil adults, mean minimum soil temperatures did not exceed –5 °C.