by Sebe Brown, Dr. David Kerns, LSU AgCenter Entomologists,
Dr. John Kruse, Cotton and Feed Grain Specialist
Cotton aphids can be a persistent problem in Louisiana cotton throughout the growing season. The cotton aphid is considerably variable in size and color with adults and nymphs ranging from yellow to green to black with winged and wingless forms appearing in mixed populations throughout a field. Cotton aphids have piercing-sucking mouth parts that allow them to feed on the phloem of developing cotton plants. Phloem is primarily composed of carbohydrates and contains little amino acids requiring aphids to continuously feed to satisfy their amino acid requirements. As a result of constant feeding, aphids must excrete large amounts of waste termed “honey dew” that has a very high sugar content and facilitates the growth of sooty mold fungi. Large amounts of sooty mold can coat the surface of leaves blocking sunlight and interfering with photosynthesis. Honey dew, when deposited on open bolls, can also cause sticky cotton resulting in ginning problems.
Cotton aphids prefer cooler temperatures and outbreaks of this pest are often associated with cool snaps. They are most often considered a secondary pest because natural enemies such as green lacewings and lady beetles often keep populations below damaging levels. However, insecticide applications for other pests often remove many of these natural enemies allowing populations to rapidly increase. Pyrethroids are notorious for causing cotton aphid outbreaks due to natural enemy destruction. Cotton aphids are also controlled by a naturally occurring fungus Neozygites fresenii. Epizootics of this fungus often cause aphid populations to crash and remained suppressed for the duration of the season.
Cotton aphids can cause significant reductions in yield, especially if the cotton is stressed, i.e. water stressed or during boll filling. Under non-stressed conditions, treatments may be considered when populations approach 50 per leaf, but stressed cotton should be treated earlier. Although most outbreaks of cotton aphid are controlled by fungal epizootics, these may not occur soon enough to prevent yield loss. Adequate control of cotton aphids with insecticides can be difficult due to this pest’s biology.
Aphids reproduce parthenogenetically, this is where unmated females will give birth to live female offspring. These nymphs are born pregnant and will begin to reproduce in about 5 days. Thus, a population of 20 aphids per leaf can explode into hundreds per leaf in a matter of days. Insecticide resistance and rebounding populations can occur very quickly, with resistance to neonicotinoid insecticides becoming common. Cotton aphid infestations will typically begin in the terminal and quickly spread throughout the plant canopy with feeding primarily taking place on the underside of leaves. This behavior necessitates the use of a translaminar or systemic insecticide limiting the options producers have for control. Intruder tends to be one of the most commonly used insecticides for cotton aphid control, but do not use less than 1 oz per acre and the addition of MSO will often enhance control. If neonicotinoid insecticides fail to provide effective control, producers should switch to Carbine 50WP.