The value of reduced-rate insecticide applications in achieving differential mortality of the lettuce aphid Nasonovia ribisnigri (Mosley) (Hemiptera: Aphididae) and one of its predators, the Tasmanian lacewing Micromus tasmaniae (Walker) (Neuroptera: Hemerobiidae : A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Science with Honours

Abstract

The application of reduced-rate concentrations of insecticides has potential as a component of integrated pest management (IPM). This is because there is theoretical and practical evidence that the survival of predatory insects is increased at a greater rate than is that of herbivorous insects as insecticide concentrations are reduced. In the current work, the insecticide Karate Zeon® (250 g/L¹ lambda-cyhalothrin) was applied to Petri dishes in the laboratory and alatae of the lettuce aphid, Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae) (a serious new pest of lettuce, Lactuca sativa L. in New Zealand) were exposed to range-test concentrations via contact exposure. Second and third instar larvae of the Tasmanian lacewing, Micromus tasmaniae (Walker) (N europtera: Hemerobiidae) (an assumed predator of N. ribisnigri) were exposed in the same way. The range-test results were used to establish, for the aphid, eight concentrations of lambdacyhalothrin (0.05, 0.1, 0.125, 0.25, 0.35, 0.375, 0.4 and 0.5 mg a.i./L) and a water control, and, for the lacewing, 11 concentrations (0.5, 0.5, 1.1, 2.3, 2.5, 5.0, 7.5, 11.0, 12.5, 17.5 and 23.0 mg a.i./L) and a water control, for the final experiments.

The resultant mortality/pesticide-concentration relationships were compared statistically, using the probit model, between aphid and lacewing, for slope of the regression lines, and the intercepts on the x- and y-axes. Lethal concentration values (LC₁₀, LC₅₀ and LC₉₀) were also calculated for each species, as were the 95% confidence limits. The slopes of the two regression lines did not differ significantly (P>0.05), but there was a large difference in the threshold concentration above which mortality began (i.e., the intercepts on the x-axis). That for the aphid was approximately 0.0009 mg a.i./L, whereas that for the predator was 556 times higher at 0.5 mg a.i./L. The intercepts of the regression lines on the y-axis differed significantly (P<0.05). One regression line was not suitable for both species; the herbivore was more susceptible than the predator. The lethal concentration values were also very different for the aphid (very low compared with those for its predator). An insecticide concentration range of between 0.1 mg a.i./L and 1.0 mg a.i./L (several orders of magnitude smaller than the recommended field rate of 50 mg a.i./L) gave an estimated 60-90% mortality for the herbivore, but caused only 0-10% mortality of the predator. The potential value of these results in reducing pesticide inputs into lettuce crops, while satisfactorily managing the aphid pest and maintaining relatively high rates of predator survival, are discussed.