Enhancing the predatory potential of hoverflies on aphids in Oregon broccoli fields with floral resources

Abstract

Four key aspects of the relationship between predatory hoverflies and the aphid pest Brevicoryne brassicae L. on broccoli were investigated in the Willamette Valley, Oregon USA: 1) the relationship between aphid density and hoverfly oviposition, 2) the larval voracity of key hoverfly species, 3) the preferences of hoverflies and broccoli pests for candidate insectary plant species, and 4) the role of insectary plantings in enhancing hoverfly oviposition in aphid colonies in broccoli fields. Hoverfly oviposition at two commercial broccoli field sites increased at aphid densities greater than 50 aphids per plant, but did not peak at the highest aphid densities. A logistic regression model, selected with Akaike's Information Criterion, of the odds of oviposition in relation to aphid density included effects for sampling date and aphid species (B. brassicae and Myzus persicae (Suizer)). The magnitude of the response was similar for B. brassicae and M persicae. The oviposition response was greater in the upper part of the plant canopy, and different on individual leaves from that seen on whole plants. The voracity and development rate of Scaeva pyrastri (L.) were greater than those of Eupeodes fumipennis (Thomson), and both exceeded Syrphus opinator Osten Saken and Sphaerophoria suiphuripes (Thomson). Both E. fumipennis and S. opinator reduced aphid populations to low levels on plants over two weeks, but E. fumipennis spent much of the time foraging away form the caged plants. Alyssum (Lobularia maritima (L.) Desv.), coriander (Coriandrum sativa L.), buckwheat (Fagropyrum esculentum Moench), and phacelia (Phacelia tanacetifolia Benth.) were screened for arthropod visitation frequency. The three pest species Pieris rapae (L.), Diabrotica undecimpunctata undecimpunctata Mannerheim, and Lygus hesperus Knight visited phacelia the most frequently, while the hoverfly species visited mostly coriander, which was possibly influenced by competition from other foragers. Hoverfly oviposition was increased near within-field blocks of alyssum in broccoli fields at the end of the season however, hoverflies arriving at the field had pollen in their guts and were oviparous four to five weeks before their eggs were detected in aphid colonies. This late oviposition may have occurred because threshold densities for oviposition were not reached until this stage.