Biocontrol Actually Working With North Carolina Cotton Aphids

Jack Bacheler, Extension Entomologist

North Carolina State University

For the past six years running, most of North Carolina's cotton producers have witnessed moderate to high aphid populations in at least some of their fields. Having been conditioned to respond to low levels of bollworms, the sheer numbers of these unwelcome aphid hordes were initially shocking. Prior to 1989, low numbers of cotton aphids were the rule, and could be routinely eliminated or reduced to subeconomic levels by any of a number of organophosphate (OP) insecticides.

As had happened earlier in the mid-south, by 1990 some aphid populations in NC appeared to be expressing high levels of resistance to recommended OP insecticides (see Table 1). Additionally, studies by a graduate student here confirmed both high levels of resistance and extreme variability in resistance levels throughout the state. Based upon replicated tests conducted in North Carolina in 1997, we found that these variably-resistant aphid populations were still the rule. Because North Carolina cotton producers have used essentially no OP insecticides since the end of boll weevil diapause applications more than a decade ago, these high levels of resistance were puzzling. Reasoned speculation about the cause of this sudden shift in resistance levels points toward widespread migration of resistant cotton aphids from our generous southern neighbors. However, unlike most situations, where resistance of an insect pest population to a class (or classes) of insecticide poses a dilemma, our situation has actually been a blessing. An explanation is in order.

Because producers here have not needed to treat for the boll weevil in over 15 years (over 20 in some of our northeastern counties), and because of the low acreage typically requiring treatment for tobacco budworms and plant bugs, predators and parasites (particularly the small mummy-causing wasps) have, with few exceptions, held aphids to subeconomic levels. These wasp parasites are often able control cotton aphids in the midst of the two to three pyrethroid treatments our producers typically use to control the major mid-July to mid-August generation(s). Also occurring from about mid-July and throughout the remainder of the season, the fungal pathogen, Neogygites fresenii, is another significant factor in limiting aphid numbers. Like the wasp parasites, this fungus is particularly effective in eliminating or greatly reducing large populations of aphids.

The publicized presence of insecticide-resistant aphid populations also appears to have contributed toward a producer and consultant reluctance to spray for aphids, a decision almost always rewarded by effective natural control.

Because the above scenario has played out so consistently for the past six years, we would again urge that North Carolina producers exercise restraint in controlling cotton aphids and regard natural control as the primary, most effective and economical option for managing aphids. This is one case where a quick insecticide response to an insect 'problem' will in all likelihood create more of a headache than a cure.