Michael J. Costello

Abundance, growth rate and parasitism of Brevicoryne brassicae and Myzus persicae (Homoptera: Aphididae) on broccoli grown in living mulches.

Abstract In 1990 and 1991, populations of the cabbage aphid, Brevicoryne brassicae, and the green peach aphid, Myzus persicae, were monitored on broccoli interplanted with three leguminous cover crops (the living mulches) and compared with broccoli without cover crop (clean cultivation). The cover crops used were white clover (Trifolium repens L.), strawberry clover (Trifolium fragiferum L.) and a mixture of birdsfoot trefoil (Lotus corniculatus L.) and red clover (Trifolium praetense L.). Fertilizer was applied as compost or synthetic fertilizer crossed with cropping system regime. Brevicoryne brassicae can reach pest status in Brassica cropping systems, whereas M. persicae is generally innocuous. Both aphids are attacked by the parasitoid Diaeretiella rapae. Seasonal aphid abundance, parasitism and population growth rates were analyzed. No significant effect of fertilizer was found for any of the parameters analyzed. The living mulches had lower aphid populations overall, although differences with clean cultivation were greatest early in the season for M. persicae and diminished over time. No effect of cropping system on population growth rate was seen for Brevicoryne brassicae, but for M. persicae growth rates were higher in living mulches compared with clean cultivation. Parasitism was highest on clean cultivated broccoli for both aphid species in 1990 and for M. persicae in 1991, but no differences were found for Brevicoryne brassicae in 1991. This suggests that differential population growth rates and parasitism seen for M. persicae are a result of inhibition of D. rapae in the living mulches. Possible reasons for differences in parasitism between the two aphid species in living mulches and clean cultivation are discussed.

Influence of Ground Cover on Spider Populations in a Table Grape Vineyard

1. Cover crops and/or resident ground vegetation have been used in California vineyards to increase the number of predators and decrease the number of pestiferous herbivores. The most common resident predators in vineyards are spiders (Araneae). Several observational studies suggest that the addition of cover crops results in an increase in spider density and a decrease in insect pest densities.

Spider and Leafhopper (Erythroneura spp.) Response to Vineyard Ground Cover

Abstract Ground cover is used in some vineyards to improve soil structure and help manage insect pests; previous studies have shown lower leafhopper (Erythroneura spp.) densities on vines grown with ground cover. We undertook a 2-yr study to determine why ground cover is associated with reduced leafhopper densities. Ground cover consisted of a fall-planted cover crop of purple vetch (Vicia benghalensis) and barley (Hordeum vulgare), which senesced in May and was replaced by a complex of resident vegetation comprised primarily of the grasses Echinochloa spp., Digitaria sanguinalis, and Setaria spp., as well as common knotweed (Polygonum aviculare). We compared three treatments during the growing season: Cover, No Cover, and Cover/Exclusion. Cover/Exclusion was similar to Cover treatment but with barriers to impede arthropod movement between ground cover and vines. We measured leafhopper density and egg parasitism, spider density and diversity, and grapevine vigor, and found that mid- and late-season leafhopper densities were significantly lower in Cover versus No Cover. Neither leafhopper egg parasitism nor spider density on the vines or ground cover could explain these differences; however, grapevine vigor was significantly lower in Cover than No Cover, and provides the best correlation to leafhopper density. Late-season leafhopper density was highest in the Exclusion treatment but cannot be explained by changes in grapevine vigor. Individual spider species composition and density on the grapevine canopy varied significantly among treatments: Trachelas pacificus (Chamberlin and Ivie) was higher in the Cover treatment, Hololena nedra Chamberlin and Ivie, Cheiracanthium inclusum (Hentz), and Neoscona oaxacensis (Keyserling) were lower in the Exclusion treatment, and Oxyopes spp. was higher in the Exclusion treatment. We suggest the lower densities of leafhoppers in the Cover treatment resulted from poorer host plant quality because of the competition between ground cover and grapevines. The higher late-season leafhopper densities in the Exclusion treatment may be due to changes in spider species composition, and subsequently, differences in rates of predation on leafhopper nymphs.

Broccoli Growth, Yield and Level of Aphid Infestation in Leguminous Living Mulches

ABSTRACT A living mulch is an intercropping system which combines an annual crop with a cover crop in order to achieve better weed and insect control, enhance soil fertility and reduce soil erosion and compaction. In this study broccoli (Brassica oleracea var. botrytis L.) was grown under clean cultivation (no cover crop) and in living mulches using three leguminous cover crops: white clover (Trifolium repens L.), strawberry clover (Trifolium fragiferum L.) and a mixture of birdsfoot trefoil (Lotus corniculatus L.) and red clover (Trifolium praetense L.). Aphid infestation of harvested broccoli heads, leaf area, leaf water content, leaf nitrate-nitrogen and yields were assessed. Fertiliser was applied at commercial rates as synthetic fertiliser or compost crossed with cover crop regime. In plots fertilised with compost aphid infestation on harvested broccoli heads was consistently less in living mulches than under clean cultivation. Nitrogen availability was lower in all compost subplots, as indicated by ...

Spectral reflectance from a broccoli crop with vegetation or soil as background: influence on immigration by Brevicoryne brassicae and Myzus persicae

Light reflectance in five wavebands of the spectrum was measured from broccoli (Brassicae oleracea var. botrytis [L].) interplanted with leguminous cover crops (cover crop background) or broccoli grown as monoculture (bare soil background), and fertilized with compost or synthetic fertilizer. Alate Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) (Homoptera: Aphididae) were monitored in yellow pan water traps and on broccoli leaves. Reflectance intensity was higher with a background of bare soil at all wavebands except blue (400–455 nm) in the early‐season. Intensity decreased as broccoli canopy cover increased at all wavebands except blue and green (515–550 nm), declining‐most dramatically in the yellow (550–590 nm). Highest late‐season intensities were in plots with bare soil background and fertilized with compost (those stressed for nitrogen). Few differences in spectral composition, expressed for each waveband as a percentage of total intensity, were recorded. Numbers of alatae were lowest in cover crop background plots in the early season, reached equivalency with bare soil background by mid‐season, and showed highest positive correlations with intensity in the yellow (550–590 nm). Results correspond to laboratory findings that aphids are attracted to higher intensity light, especially in the yellow waveband, and support a phototactic explanation for aphid orientation in the field.