Attractiveness of Manitoba traps to the headfly, Hydrotaea irritans (Fallén) (Diptera: Muscidae): the effects of short-term weather fluctuations, carbon dioxide and target temperature and size

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Abstract
The numbers of adults of Hydrotaea irritans (Fall.) caught in Manitoba traps in north-eastern England varied greatly between successive 30-min periods. Weather variables accounted for 49·4% of this variation, but only wind speed and sunshine showed significant simple correlations with fly catch. Using principal component analysis of weather data, three principal components accounted for 67·3% of the variation in catch. The two components most highly correlated with catch were composed mainly of wind speed, sunshine, shade temperature and radiant temperature. These two principal components were used as covariates in an analysis of the effects of a heated target and carbon dioxide on trap effectiveness. Heating the target to blood temperature increased the catch of females 1·8-fold but had only a slight effect on the catch of males. The presence of CO2 increased the female catch nearly fivefold and the male catch more than fourfold. There was no interaction between the two stimuli of heat and CO2. A separate experiment showed no significant difference in catch between CO2-baited traps using spheres 0·6 or 0·3 m in diameter as the target.