Sampling to predict fungal epizootics in cotton aphids (Homoptera: Aphididae)

We monitored prevalence of Neozygites fresenii (Nowakowski) Batko in populations of cotton aphids, Aphis gossypii Glover, during a 3-yr period and used these data to develop sampling strategies for predicting aphid population decline caused by disease. Aphid populations began to decline when average fungal prevalence (percentage of fungus-killed aphids) reached approximately 15%. Populations declined to low levels 5-16 d later. Declines were more rapid in fields with higher initial densities of aphids. Fungus-killed aphids were detectable up to 10 d before prevalences reached approximately 15%. Detection at this early stage required (theoretically) the examination of 70-150 leaves per field (95% probability of detection). However, a sample size of only 4-5 leaves was required for detection after prevalence reached 4%, which would be expected to occur approximately 4 d before initial decline of aphid populations. With microscopic examination of 100 aphids per leaf, detection of fungus-infected aphids (95% probability) required between 9 and 17 leaf samples 10 d before decline of aphid populations, but only 1 sample was required 4 d before decline of aphid populations. During early stages of epizootics (prevalences 15%), fungal prevalences were greater on leaves 6 nodes below terminals, compared with leaves at the 1st or 2nd positions below terminals. During this time, fungal prevalences were correlated in subsamples collected 2 m apart. In samples collected 100 m apart during the very early stages of epizootics (prevalences 1%), distributions of infected aphids were clumped in 1 of 3 sample fields. These results underscore the importance of representative sampling of cotton fields if the goal is early prediction of epizootics