Undersowing rutabaga with white clover: impact on Delia radicum (Diptera: Anthomyiidae) and its natural enemies

Abstract

The cabbage maggot, Delia radicum (L.), is a serious pest of cruciferous crops in temperate regions of North America and Europe. The effects of undersowing rutabaga, Brassica napus L. subsp. rapifera Metzg. (Brassicaceae), with white clover, Trifolium repens L. (Leguminosae), on second-generation cabbage maggot and its natural enemies were studied in Newfoundland in 1997 and 1998. In 1997, totals of 1311 and 724 eggs were recovered from bare and undersown plots, respectively. More eggs were present in bare plots than undersown plots on various specific dates. In 1997, rutabagas from bare plots weighed more than those from undersown plots, although damage ratings were similar, suggesting that competition, not cabbage maggot feeding, caused the yield differences. In 1998, there were few cabbage maggots present and little damage or yield reduction in either treatment. Similar numbers of cabbage maggot pupae were extracted and reared from each treatment in each year. In 1997, of the pupae reared from undersown plots, 48% produced cabbage maggot flies, 14% produced parasitic Hymenoptera, and 8% produced Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae); 19% of the pupae from bare plots produced cabbage maggot flies, 8% produced parasitic Hymenoptera, and 36% produced A. bilineata. More A. bilineata were captured in pitfall traps in bare plots than in undersown plots. The effect of clover on carabid beetles was species specific. There were more Bembidion lampros (Herbst) and Amara bifrons (Gyllenhal) in bare plots in 1997, and more Pterostichus melanarius (Illiger) in undersown plots in both years. Despite consistently lower egg numbers in undersown plots than in bare plots, the numbers of pupae in the two treatments were similar at the end of the season. We speculate that this may be due to differential, density-dependent mortality of immature stages of cabbage maggot caused by predators and parasitoids.

Effects of Canola-Wheat Intercrops on <I>Delia</I> spp. (Diptera: Anthomyiidae) Oviposition, Larval Feeding Damage, and Adult Abundance

“…adults initially locating canola monocultures or intercrops with wheat were similar, as Bjö rkman et al (2007) also concluded for D. floralis. In agreement with our results, Dixon et al (2004) usually found nonsigniÞcant differences in D. radicum trapped in rutabaga (Brassica napus L. subsp. rapifera Metzg.)…”

“…In studies intercropping brassicaceous vegetables with nonhost plants, where monocultures and intercrops had equal densities of host plants, researchers also found reduced Delia spp. egg densities in the intercrops (Tukahirwa and Coaker 1982, Finch et al 2003, Dixon et al 2004, Bjö rkman et al 2007. Similarly, reduced root maggot egg populations per plant were observed by Dosdall et al (2003) in weedy compared with weed-free canola with equivalent canola plant densities in the weedy and weed-free situations.…”

“…Intercrops or weedy crops often harbor a greater abundance than monocultures of predators, parasites, and pathogens, particularly carabid and staphylinid beetles (Horn 1981(Horn , 1987Tukahirwa and Coaker 1982;Purvis and Curry 1984;Kromp 1999). However, some parasitoids of Delia spp., such as A. bilineata and T. rapae, are not favored in intercrops of cruciferous vegetables with nonhost ground cover (Langer 1996, Dixon et al 2004). Langer (1996) suggested that host-speciÞc parasitoids are better able to locate their hosts in the more homogenous conditions of a monoculture than in an intercrop.…”

Neighbouring monocultures enhance the effect of intercropping on the turnip root fly (Delia floralis)

“…The focal species in this study, the turnip root fly ( D. floralis ) and its congener the cabbage root fly ( Delia radicum L.), are herbivores on cruciferous crops. The cabbage root fly has been frequently studied for its potential to be controlled by intercropping (Theunissen et al., 1995; Finch & Kienegger, 1997; Lehmus et al., 1999; Dixon et al., 2004). The two species have similar biology and behaviour and react upon the same or related stimuli (Varis, 1967; Baur et al., 1996a).…”

In the presence of specialist root and shoot herbivory, invasive‐range Brassica nigra populations have stronger competitive effects than native‐range populations

“…; Dixon et al . ), there have been no reports of D. radicum in natural invasive‐range populations of B. nigra . Invasive B. nigra has thus likely escaped D. radicum .…”