Using sex pheromone to control male Helicoverpa armigera Hubner (Lepidoptera: Noctuidae) in cotton

Abstract

The aim of this thesis was to develop and test slow-release sex pheromone-based formulations for Helicoverpa armigera (Hubner) (Noctuidae: Heliothinae) for use as part of an integrated pest management (IPM) program in cotton and associated crops. H. armigera is considered to be the most significant pest of cotton and is destructive in many other crops. It readily develops resistance to insecticides, hence alternative methods of control are highly desirable, especially those which are selective and environmentally safe. This thesis briefly reviews the pest status of H. armigera, then looks at the sex pheromones ofheliothines. The use of sex pheromones for attract and kill and mating disruption of H. armigera and other insect pests are discussed in detail. <br/> Two main formulations of H. armigera sex pheromones were considered in this thesis. The first one was for attract and kill. The basis for the formulation was a gel-like matrix called Sirene® which protects semiochemical components such as pheromones, whilst allowing their slow release into the environment. This matrix can also be laced with insecticide to make a formulation which kills insects attracted to it. Synthetic blends of H. armigera sex pheromone used here were based . on existing blends reported in the literature, and consisted of a 10: 1 ratio of (Z)-11- hexadecenal:(Z)-9-hexadecenal loaded at 1 % in the Sirene® matrix. The second one was the commercially available formulation for mating disruption (Selibate HA, AgriSense BCS Pty. Ltd., Pontypridd, South Wales, UK) which was used for a mating disruption trial. This contained the same blend of components at 5% in an extruded polymer dispenser. Both methods work by preventing mating by either killing male moths ( attract and kill), or by preventing male location (mating disruption). <br/> Sex pheromones attract only male moths. Male behaviour at sex pheromone sources was studied in detail. Male activity at the lures was influenced by diel periodicity, type of and stage of crop present, season, % relative humidity, wind run and overall climatic conditions. Of these factors the type of crop and seasonal factors were associated with the largest changes in number of moths flying to lures, and the percentage of moths contacting the lures. The effect of lure formulation, appearance and presentation method was evaluated by varying these factors and observing male behaviour in the field. The synthetic lure was compared with captive female H. armigera to determine if the synthetic blend was adequately attractive. Observations indicated that the synthetic blend was adequate, and was as least as attractive as calling females. Contact rates with standardised 200 mg droplets with 1 % pheromone were relatively low, with only 10% of approaching males contacting the lure. The percentages of contacting males could be significantly increased by placing a dead decoy female next to the lure, by placing the lure on a natural substrate and by increasing the active surface of the lure by smearing it or placing it as many small droplets. The addition of a synthetic pyrethroid (bifenthrin) did not significantly repel males from lures. <br/> The laboratory toxicology of the pyrethroid contact insecticide bifenthrin was evaluated for formulations made in Sirene®. A concentration of 6% bifenthrin gave close to 100% mortality over 4 h. Concentrations less than 1.5% were also effective, but took over 14 h to achieve > 90% mortality. Sublethal effects on reproductive behaviour were also tested. Treating males with concentrations of>0.01 % bifenthrin in Sirene® reduced the chance of successfully mating by more than 50%. The toxicity of formulations was compared in field conditions using a passive field wind tunnel. The 6% bifenthrin and 1 % pheromone formulation gave the best results when associated with dead decoy female moth, but mortality in the field wind tunnels was much lower than expected. This appeared to be due to the experimental conditions of the field wind tunnel altering the male behaviour. <br/> Weathering of the Sirene®-based pheromone formulations was studied under field conditions. The estimated life-span of 200 mg droplets was between 4-6 weeks. The life-span of the formulation was more than halved by smearing the lure over substrates compared to leaving formulations as a single droplet. <br/> A mark-recapture study of male behavior in flowering sorghum found that populations of male H armigera were non-resident in the crop. Population estimates in the field range from 97 to 4,008 males per hectare per night. Extrapolation from population estimates obtained in this study indicate that with only 12 pheromone sources in a 21 ha field it was possible to remove 10% of males present when population densities were very low. However, the number of pheromone sources would have to be increased to successfully control the large numbers (>4,000) seen on some nights. The technique used in this study allowed for an estimate of moth turnover on a nightly basis, and would be useful in other studies of insect movement. <br/> A full-scale mating disruption trial of H armigera was carried out in an isolated cropping area. Mating was reduced to almost zero in treated areas, and there was a significant reduction in the number of eggs laid in capsicum crops, and in the number of spermatophores per female. However, there was not a sufficient decrease in egg lays in any of the treated areas that would allow any decrease in conventional sprays. Mating was occurring on non-host plants adjacent to the treated areas, and mated females were flying back into treated areas to lay eggs. This is in contrast to published results of noctuid moths mating only on host plants. <br/> Information from the mark-recapture study was used in conjunction with a model developed from observing contact rates of moths with pheromone lures in the field. From this a recommended rate of 7 5-100 200 mg droplets per ha was estimated for the attract and kill formulation. The final recommended formulation based on laboratory and field testing used an existing protective base (Sirene®), 6% bifenthrin with 1 % of the H. armigera pheromone blend. <br/> The local movements of male moths and mobility and labile nature of mating behaviour in H. armigera indicate that any sex pheromone-based pest management program such as attract and kill or mating disruption is unlikely to succeed as a stand-alone system. It is possible that attract and kill may be a very useful tool when combined with mating disruption and other semiochemical techniques in an IPM program. The implications and future uses of sex pheromone for control of H. armigera are discussed.