Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/5/2004
Publication Date: 6/1/2004
Citation: Leland, J.E., Hardee, D.D. 2004. Cotton planting date and epizootiology of the cotton aphid fungus, neozygites fresenii. National Cotton Council Beltwide Cotton Conference.
Interpretive Summary: A fungus causes massive die offs of an important insect pest of cotton, the cotton aphid. This fungus occurs every year throughout the eastern United States and is an important factor in reducing the need to control cotton aphids with chemical insecticides. We investigated how the time at which cotton was planted interacted with the timing and severity of cotton aphid die offs caused by this fungus. In addition the study provided information related to how common the fungus was within the vertical structure of the cotton plant canopy and cotton aphid numbers, temperature, and relative humidity interacted with the occurrence of this fungus. This information helps answer the basic question of why cotton aphid die off from this fungus occurs when it does; which has applications for making decisions on how we grow cotton for causing the die off to occur earlier and with greater intensity. It may also be used for developing programs where the fungus could be applied to the field to start the die off. Causing the die off to occur earlier in the season or increasing its intensity will reduce the need for insecticide applications.
Technical Abstract: Studies were conducted to investigate the interaction of cotton planting date and epizootiology of the cotton aphid fungus, Neozygites fresenii, in cotton aphid (Aphis gossypii, Glover) populations. Delta Pine 491* cotton was planted on April 22, May 6, May 20, and June 3, 2003. Four, 1-acre plots were planted in a randomized complete block design for each planting date. Five plants were sampled from four locations in the northeast, northwest, southeast, and southwest quadrant of each 1-acre replicate on a biweekly schedule from mid June through August. The terminal and first true leaf were taken from each of the 5 plants, and single leaves were taken from within the canopy at 14" and 28" above ground from each of 5 plants once plants had reached sufficient maturity. These samples were kept separate to determine aphid populations and infection levels within each 1 acre replicate and within the vertical structure of the plants from each sampling location. Aphid populations from individual samples were estimated by direct counts of rinsed leaves, and aphids were stored in alcohol until they could be mounted for determining infection levels and stage of infection by light microscopy using the following criteria 1) uninfected, 2) with capilloconidia, 3) with hyphal bodies, 4) with conidiophores and sometimes saprophytic fungus also present, or 5) with saprophytic fungus and no signs of N. fresenii. Ten apteratae and up to ten alatae were evaluated for degree of infection from each quadrant within each acre for a total of 40 apteratae and up to 40 alatae per plant height per replicate. Temperature and relative humidity were monitored within the cotton canopy at a single central location within each 1-acre replicate of each planting date. This study provides information that may be used to correlate planting date, aphid population densites, vertical plant structure, presence and timing of infected alatae, temperature, and relative humidity with timing and intensity of N. fresenii epizootics. Such information contributes to the basic question of what initiates N. fresenii epizootics in the field, which has applications toward cultural practices for augmenting natural epizootics and selecting conditions for jump-starting epizootics using artificially disseminated inocula. Cultural and augmentative practices that could be used to initiate early epizootics or increase their intensity may reduce the need for aphicide applications and help ensure the occurrence of epizootics.