SAMPLING TO PREDICT FUNGAL EPIZOOTICS IN COTTON APHIDS (HOMOPTERA, APHIDIDAE)

We monitored prevalence of Neozygites fresenii (Nowakowski) Batko in p opulations of cotton aphids, Aphis gossypii Glover, during a 3-yr peri od and used these data to develop sampling strategies for predicting a phid population decline caused by disease. Aphid populations began to decline when average fungal prevalence (percentage of fungus-killed ap hids) reached approximate to 15%. Populations declined to low levels 5 -16 d later. Declines were more rapid in fields with higher initial de nsities of aphids. Fungus-killed aphids were detectable up to 10 d bef ore prevalences reached approximate to 15%. Detection at this early st age required (theoretically) the examination of 70-150 leaves per held (95% probability of detection). However, a sample size of only 4-5 le aves was required for detection after prevalence reached 4%, which wou ld be expected to occur approximate to 4 d before initial decline of a phid populations. With microscopic examination of 100 aphids per leaf, detection of fungus-infected aphids (95% probability) required betwee n 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. Du ring 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 samp les collected 100 m apart during the very early stages of epizootics ( prevalences <1%), distributions of infected aphids were clumped in 1 o f 3 sample fields. These results underscore the importance of represen tative sampling of cotton fields if the goal is early prediction of ep izootics.