Fungus Helps Control Louisiana Cotton Aphids

Billy Leonard, Bagwell, Ralph D.  |  11/18/2004 2:35:13 AM

Cotton aphids on leaves from a cotton field with high incidence of N. fresenii infection. (Photo by Rogers Leonard)

Cotton aphids on leaves from a cotton field with high incidence of N. fresenii infection. (Photo by Rogers Leonard)

“Honey-dew” stain on a cotton leaf (left) caused by cotton aphids. (Photo by Rogers Leonard)

Figure 1. Cotton aphid density per cotton plant terminal and percent Neozygites fresenii infected aphids at the Macon Ridge Research Station, Winnsboro, La.

Robert H. Jones, B. Rogers Leonard and Ralph D. Bagwell

The cotton aphid is a common secondary pest of cotton in Louisiana. Cotton aphids can infest cotton plants from seedling emergence until harvest and injure plants by continuously feeding on them. Injury symptoms may include a downward cupping of infested leaves, inter-veinal discoloration, compressed main stem nodes and reduced plant height. Severe cotton aphid infestations can cause sufficient plant stress to result in square and boll shed and delay crop maturity. In addition, cotton aphids secrete a liquid known as “honeydew.” The accumulation of “honeydew” on exposed cotton lint causes the lint to become sticky, reduces harvest efficiency and creates problems at textile mills.

Numerous agronomic and pest management practices used in cotton production can affect cotton aphid populations. Cotton aphid densities in Louisiana fields have been higher in no-till production systems. High numbers also commonly occur as resurgent populations following applications of selected insecticides for other insect pests. Natural enemies of cotton aphids are often reduced after these applications, causing cotton aphids to increase to damaging levels. Cotton aphid predators include lady beetles, big-eyed bugs, minute pirate bugs, lacewings and parasitic wasps.

During the past 10 years, the most valuable and effective non-pesticidal control of cotton aphids in Louisiana has been provided by a naturally occurring fungus, Neozygites fresenii Batko. This fungus, which spreads by airborne spores, kills cotton aphids within three days after infection. Entire field populations are reduced from peak densities to nearly non-existent levels within five to 10 days after the initial infection by N. fresenii (Figure 1). These epizootics (insect epidemics) normally coincide with peak cotton aphid populations during mid-June to early July. In many instances, producers can delay insecticide applications for a few days during this time, allow the disease to develop and not be required to provide any sup-plemental control. Usually, cotton aphid populations will not resurge during the remainder of the season after an epizootic is established, thus making N. fresenii more effective than insecticides.

N. fresenii has been a cost-effective and environmentally friendly pest management tool for Louisiana cotton producers. LSU AgCenter entomologists have been cooperating with scientists from the University of Arkansas to monitor the temporal and spatial occur-rence of infected cotton aphids. Information on the status of the disease is distributed throughout the season, giving agricultural consultants, county agents and producers additional information to select the appropriate integrated pest management (IPM) strategy.
 
Robert H. Jones is a former graduate student in the Department of Entomology. B. Rogers Leonard is a professor, and Ralph D. Bagwell is an associate professor at the Macon Ridge Research Station.

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