In a previous study we demonstrated greater abundance of the parasitoid Anagrus epos (Girault) in grape vineyards located downwind of prune trees that function as overwintering habitats. This study examines whether these higher A. epos numbers translated into higher egg parasitism rates of the grape leafhopper, Erythroneura elegantula (Osborn). Paired commercial wine-grape vineyard plots, one with and one without adjacent prune trees, were studied within a complete block design in northern and central California. A. epos was the key mortality factor affecting E. elegantula eggs. Point estimates of A. epos parasitism rates were significantly greater in vineyards associated with prune trees during the first E. elegantula generation in both 1991 and 1992. No consistent differences in parasitism rates were observed during the second or third generations. The results indicated that prune trees enhance early season parasitism rates. Cumulative estimates of egg parasitism across E. elegantula generations demonstrated that enhanced early-season parasitism resulted in a net season-long increase in the degree of mortality imposed by A. epos on E. elegantula eggs. Two factors were found to influence parasitism rates: the abundance of early-season A. epos adults moving into vineyards and the density of E. elegantula eggs in vineyards. Our results indicate that diversification of vineyards using prune trees supports overwintering populations of a specialist parasitoid and thereby alters host-parasitoid interactions to favor enhanced parasitism in vineyards.
A frequently cited habitat diversificationtactic is the use of prune tree refuges that support overwintering populations of Anagrus epos (Girault), a mymarid egg parasite of the western grape leafhopper, Erythroneura elegantula Osborn, in vineyards. Here we test the effect of prune trees on early-season abundance of adult A. epos in vineyards. A. epos was found in vineyards downwind of prune trees at more than twice the densities of vineyards lacking pnllle trees, despite significantvariation in A. epos immigration from sources outside the pnme tree-vineyard system. Densities of A. epos overwintering within prune trees explained a significantamount of the variation in A. epos trap capture in vineyards.Furthermore, another factor associated with prune trees was found to influence A. epos abundance in vineyards: a windbreak effect created by the prune trees concentrated dispersing A. epos on the leeward side of the prune trees, tllereby further enhancing A. epos numbers.
French prune trees provide overwintering habitat for the egg parasitoid Anagrns epos Girault (Hymenoptera: Mymaridae), an important natural enemy of the the grape leafhopper, Erythroneura elegantula Osborn (Homoptera: Cicadellidae). French prune trees were treated with rubidium during the fall of 1991-1993 to assess the potential for obtaining an elemental label in overwintering A. epos. Multiple applications of 5,000-ppm solutions of RbCI to French prune foliage from late August to early October resulted in increases in foliar rubidium content of up to 200 times the naturally occuring concentration. Rubidium content of adult prune leafhoppers, Edwanuiana prnnicola Edwards (Homoptera: Cicadellidae), collected from treated trees was up to 130 times the naturally occurring level. Naturally occuring rubidium content of A. epos was 0.052 ng Rb per individual based on a sample of 498 wasps collected from 3 separate, untreated grape vineyards. Mean rubidium content was 0.205 ng Rb in A. epos collected in the spring directly from French prune trees that had been treated in the fall with RbCI. This is 3.9 times the background level and indicates successful labeling of overwinteringA. epos. However, there was substantial overlap in rubidium content between A. epos from treated trees and those from untreated sites. A. epos reared from rubidiumenriched host eggs and maintained in the lab on a mixture of honey and water e1iliibited minimal decay of the elemental label during the adult lifetime. Success of this labeling approach has made possible the direct study of the dispersal of A. epos from treated French prune tree refuges into adjacent grape vineyards.
Western flower thrips cause considerable losses in a wide range of agricultural crops by feeding on leaves and fruit, layhgaggs in fruit and transmitting diseases. Repeated pesticide application is currently the only method that reduces populations to acceptable levels. Biological control efforts have focused on using predators and have been largely unsuccessful. However, entomopathogenic fungi could also be used as biological controls for western flower thrips. Laboratory and field trials show that commercial formulations of Beauveria bassiana (GHA strain) can infect and reduce western flower thrips numbers in greenhouse floriculture crops, thus demonstrating its potential as an alternative to conventional pesticides.
Field experiments were conducted with rates and methods of application of B to cotton. Broadcast was as effective as drilled, foliar application was as good as soil application, and there was no advantage to a combination of soil and foliar application. A highly soluble source of B was superior to a more slowly soluble one. Young leaves, the third or fourth node from the growing point, were found to have a critical level of about 15 ppm B. Mature leaves occurring at the fifth and sixth nodes from the growing point had a B content of 5 to 10 ppm higher. The critical level of hot‐water soluble B in the top 15 cm of soil was in the range of 0.10 to 0.16 ppm.
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