The cotton aphid, Aphis gossypii Glover, was not considered a major cotton pest in the U.S. until population outbreaks, attributed to the elimination of natural enemies, followed insecticide applications for boll weevil control in the 1940s (Slosser et al., 1989). Outbreaks became less frequent and more easily controlled following the development and use of organophosphate insecticides (USDA, 1960). However, during the past decade the cotton aphid has reemerged as one of the most important pests of cotton in much of the cotton growing areas in the U.S. (Hardee and O'Brien,
1990) due to the development of insecticide-resistant populations (Grafton-Cardwell,
1991) and the elimination of natural enemies with broad-spectrum insecticides targeted toward other insect pests (Edelson, 1989; Kerns and Gaylor, 1991, 1993). From 1995 to 1997 an average of approximately 89 million pounds of cotton were lost to aphids nationwide, with approximately 29% of U.S. cotton being treated for aphids (Williams, 1998).
Since insecticide resistance was detected (Lee, 1992) and surveys begun eight years ago in North Carolina, less than 1% of cotton acreage in North Carolina has been treated annually for aphids (the exception was 1997 with 3.5%) (J. S. Bacheler, personal communication). Three factors are predominantly responsible for the difference in the North Carolina average and the national average during the past few years (J. S. Bacheler, personal communication).
First, the majority of aphid populations in North Carolina are resistant to insecticides, with the exception of imidacloprid (Bacheler, 1997). Second, the cotton aphid is attacked by a variety of natural enemies, the most important of which appears to be the parasitoid wasp, Lysiphlebus testaceipes (Cresson), and the ento-mopathogenic fungus, Neozygites fresenii (Nowakowski) (Frazer, 1988, Steinkraus et al., 1991, 1995; Kidd et al., 1994; Knutson and Ruberson, 1996). This natural enemy complex is very effective in keeping aphid populations below treatment thresholds, and scouting recommendations emphasize inclusion of natural enemy observations, and treatment recommendations call for pesticide applications only in the event of very high aphid populations without either L. testaceipes or N. fresenii present (Bacheler, 1997; Toth, 1998).
The third reason is that there are very few early to mid-season pest populations in cotton that require treatment with insecticides in North Carolina. The boll weevil has been eliminated as an economic pest for 15 years, over 90% of cotton acreage is treated with an at-planting systemic insecticide, and mid-season pest populations are not usually abundant enough to warrant treatment (Jack Bacheler, personal communication). This allows natural enemy populations to increase and maintain aphid populations below treatment thresholds. Generally, it is not until mid to late July, when foliar insecticides are first applied for heliothine control, that arthropod natural enemies are disrupted. However, by this time the entomopathogen N. fresenii is present in most fields, producing epizootics and suppressing late season aphid populations.
This example illustrates several points that may reflect the actual approach to implementing many conservation biological control efforts within IPM programs. Even though conservation of cotton aphid natural enemies saves growers approximately $5,000,000 annually on the approximately 670,000 acres of cotton grown in North Carolina (J. S. Bacheler, personal communication), recommendations which serve to conserve beneficial arthropods may exist more because of a set of fortuitous circumstances than purposeful planning. There are essentially no detailed ecological data to provide a foundation for this program. The recommendations evolved as the result of careful, though anecdotal, field observations of the high efficacy of natural control factors. Aphid population values provided for scouting and treatment decisions are qualitative rather than quantitative, ranging from "low" to "very high" (Bacheler, 1997). Also, as pointed out above, North Carolina is in a unique situation in terms of pest pressure when compared with the rest of the cotton belt. Finally, pesticides for cotton insect management in North Carolina are chosen primarily on the basis of cost and effectiveness, rather than selectivity for beneficials. It is fortuitous that these pesticides are spatially and temporally selective toward natural enemies (i.e., at-planting systemic insecticides early and bollworm treatments applied late in the season).
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