Economic injury levels for Bemisia tabaci (Homoptera: Aleyrodidae) in cotton: impact of crop price, control costs, and efficacy of control

Field studies were conducted during 1993 and 1994 in the Imperial Valley, California and Maricopa, Arizona to examine relationships between densities of Bemisia tabaci (Gennadius) populations and cotton (Gossypium hirsutum L.) yields, and to estimate economic injury levels (EILs) for pest management application. Populations of B. tabaci were manipulated by applying different numbers of insecticide applications in replicated plots. Resulting insect populations and cotton lint yields were used to develop damage functions and to estimate EILs for all life stages in relation to variable cotton prices, insecticide costs, and pest control efficacy. Economic injury levels declined with increasing cotton prices and increased as the cost of control increased, however, these changes were relatively small, based on typical ranges in price and control costs. In contrast, the efficacy of control provided by insecticide applications had a large influence on EILs, with lower efficacies being associated with higher injury levels. We developed a multiple regression model that accounted for the dynamic changes in the EIL in relation to crop price, control costs, control efficacy, and potential yield. Based on average prices and reasonable control costs and efficacy, EILs ranged from 5.9–15.2 adults/leaf, 6.1–19.8 eggs cm⁻², and 1.7–4.7 nymphs cm⁻² of leaf area. Additional research is needed to more closely relate the costs of control to the suppression of insect populations, and to define economic thresholds that will enable pest managers to maintain pest populations below EILs.     

Evaluation of Allorhogas pyralophagus Marsh (Hymenoptera: Braconidae) for the biological control of Diatraea spp. (Lepidoptera: Pyralidae) in sugar-cane in Guyana

In 1989, a mass production and field release programme of Allorhogas pyralophagus Marsh (Hymenoptera: Braconidae) for the control of the Diatraea spp., particularly D. centrella (Möschler) (Lepidoptera: Pyralidae) in sugar-cane, was initiated by the Guyana Sugar Corporation. The programme was terminated in 1995 after the release of approximately two million adult parasites resulted in a mean field parasitism in Diatraea spp. larvae of only 0.22%. In contrast, 62% of Diatraea spp. larvae were parasitised in vitro as were 32% of larvae in young cane shoots exposed to A. pyralophagus in the laboratory. Neither Diatraea larvae in young cane plants caged in the field nor those recovered from Echinochloa polystachya (Kunth) Hitchc. and Paspalum virgatum L. around release fields showed any parasitism by A. pyralophagus. The reasons for the low rate of host acquisition outside the laboratory are not known, but it is clear that A. pyralophagus offers little prospect for significant control of Diatraea spp., particularly D. centrella in sugar-cane in Guyana.     

Responses of the alien leaf miners Liriomyza trifolii and Liriomyza huidobrensis (Diptera: Agromyzidae) to some pesticides scheduled for their control in the UK

Baseline responses to a range of pesticides scheduled for the statutory eradication of outbreaks of Liriomyza trifolii, a leaf miner alien to the UK, were established using a topical application bioassay against a susceptible laboratory population. Using the same technique the responses of outbreak populations of L. trifolii, and also the related species L. huidobrensis, have been investigated. Field populations of L. trifolii showed resistance to the pyrethroids tested but no evidence of resistance to organophosphates was found. There was some evidence of resistance to gamma-HCH. L. huidobrensis showed a response to pyrethroids similar to that of resistant L. trifolii, and levels of tolerance to organophosphates approximately eight times that of susceptible L. trifolii. The response to gamma-HCH was similar to that of susceptible L. trifolii.