The interaction between the moth, Cactoblastis cactorum, and the cactus, Opuntia stricta, is used as a model to examine the question of whether the CO sense of a herbivorous insect can detect the CO gradients associated with a plant's metabolic activity. Both the anatomical and the electrophysiological characteristics of CO-sensitive receptor neurons in C. cactorum indicate an adaptation to the detection of small fluctuations around the atmospheric background. Evidence is provided that further rises in background will impair the function of the sensory organ. In the habitat of the plant, during the diurnal window of the moth's activity, two types of CO gradients occur that are detectable by the moth's sensors. The first gradient, associated with soil respiration, is vertical and extends from the soil surface to an altitude of approximately 1 m. Its magnitude is well above the detectability limit of the sensors. The notion that this gradient provides, to a flying insect, a cue for the maintenance of a flight altitude favourable for host detection is supported by field observations of behaviour. The second gradient, associated with CO fixation by the plant, extends from the surfaces of photosynthetic organs (cladodes) over a boundary layer distance of approximately 5 mm. Again, its magnitude is well above the detectability limit. The notion that this gradient provides, to a walking insect, a cue to the physiological condition of the plant is supported by the observation that females of C. cactorum, prior to oviposition, actively probe the plant surface with their CO sensors. In a simulation of probing, pronounced responses of the sensors to the CO-fixing capacity of O. stricta are observed. We propose that by probing the boundary layer, females of C. cactorum can detect the healthiest, most active O. stricta cladodes, accounting for earlier observations that the most vigorous plants attract the greatest density of egg sticks.
Since the successful control of prickly pear cactus by Cactoblastis cactorum in Australia, populations of plants and moths have persisted at low densities in open woodland sites. A contagious egg distribution causes overcrowding of larvae on some plants but insures low levels or no attack of other plants. This prevents extinction of plants and insects. Cactoblastis moths choose plants with characteristics which may increase the success of their larvae. Field observations and cage experiments indicate that large, green cactuses near previously attacked cactuses receive more eggs. Plants which are actively photosynthesizing are also more attractive as oviposition sites. These oviposition preferences contribute to the observed contagious egg distribution.While open woodland Opuntia and Cactoblastis populations fluctuate around an equilibrium, pasture populations may better be described by the "hide and seek" model, with the woodland populations serving as refuges. Average plant quality and variation in quality are suggested as important components in the dynamics of this system.
This paper describes the methods devised and used in rearing, transporting, sterilizing, and releasing Dacus tryoni for field experiments aimed at controlling natural populations of this pest with sterile males. The general principles of economical mass-rearing, including the management of breeding stocks, and quality and economy in larval rearing, and previous work on mass-rearing Dacus tryoni are discussed. Adults required exposure either to natural or artificial dusk to stimulate mating, 120 cages each stocked with 4000-5000 flies provided sufficient eggs to achieve a production rate of up to l0*6 pupae per week; eggs were collected in apple-hollows; larvae were reared in a medium whose basic components were yeast and dried powdered carrot; pupae were stored at 25�C and 70-80% R.H. until they were ready for irradiation; they were irradiated on the eighth day of pupal life using a 60Co source to produce doses between 6000 and 10,000 rads. After they had been irradiated the pupae were consigned by commercial aircraft to country airports near the points of release. The sites of release were trees; a release platform protected the pupae from rain, sun, and predators.
The attractiveness of eight prospective lures for male Queensland fruit fly was tested in a simple olfactometer. Six of these compounds were found to be attractive and these were ranked in field trials in which traps were baited with a mixture of each lure and malathion.Trap catches varied with lure, trapping site, weather (including wind speed), and season. Cue lure, 4-(p-acetoxyphenyl)butan-2-one, was the most attractive compound. Baits made by mixing cue lure and malathion were of unchanged attractiveness after more than 6 months but mixtures of cue lure and DDVP declined in attractiveness within 6–12 days. Funnel traps baited with cue lure and malathion and spaced 0.4 km apart in a square grid pattern caught 4.1% of newly emergent flies and 9% of mature flies (2–3 weeks old) released in the centre of the grid. The use of such a grid in early warning systems is discussed.
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