Corn Earworm (Soybean Podworm)
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Biology. This caterpillar is the most serious insect pest of North Carolina soybeans. Soybean field infestation normally occurs in later July and into August, following several build-up generations in other crops. Corn earworm (Helicoverpa zea Boddie) overwinters as pupa in the soil. The pupa change to moths in the April/May period and fly to whorl stage corn, or wild hosts, and lay eggs for the first generation. Following several weeks of development, first generation moths emerge around the time when corn pollinates and are strongly attracted to fresh corn silks for laying the second generation eggs. Corn ears provide protection and a good food source and a large increase in population is realized during the second generation. After developing in the ears, larvae eat through the husk and drop to the ground and pupate in the soil. Moths of the second generation emerge from corn fields, from mid- July and into early/mid
August, and seek soybean, cotton, peanut, sorghum, and other crop and wild host plants. This is often concurrent with the time period when soybeans are attractive to corn earworm (immediately prior to flowering until early pod-set). There is also a September moth flight and very late planted soybeans are sometimes suitable (not past blooming and early podding) for egg laying and small caterpillar survival. Yearly fluctuations in population may be great, with high populations often occurring in seasons of early warm and dry weather. In some years, a fourth generation may emerge. This insect is usually called the cotton bollworm when it occurs in cotton. The July/August moth flight is monitored throughout NC each year and presented in extension reports as corn earworm or bollworm moth trap catches. This information can be very helpful for determining when to scout soybeans.
The adult corn earworm is a robust light brownish-tan, rust- tan or olive-tan to olive, fast flying moth with a 1” to 1 1/2” wingspan. This moth has greenish eyes when living and a prominent dark spot on each forewing. The moths are active during the evening and dark hours but can be commonly seen in fields during the day. Eggs are laid individually on soybean terminals, flowers and leaves. Eggs can be easily seen (round, white to yellowish egg of about the size of a period in this text). Caterpillars have five pair of fleshy prolegs and range in size from 1/16” (newly hatched) to 1 3/4” (last stage) long. When these larvae fall from the soybean plant they usually curl into a tight circle. Color varies considerably but small larvae are usually brown with a dark head and larger larvae may range from green, yellowish, to black with distinct cream-colored bands running the length of the body; the head color on large larvae is usually orange.
Injury and damage. On soybeans, the corn earworm may feed on foliage, flowers, and fruit. Eggs are laid over the entire plant, but small caterpillars must have tender vegetative terminals and/or flower clusters for high survival. Soybean fields that bloom during the moth flight are most likely to be infested above the economic threshold, although pre-bloom soybeans sometimes have high caterpillar abundance. Soybeans that bloom early, before the moth flight, usually escape infestation. The most serious yield losses occur when large corn earworm larvae coincide with soybean seed that have achieved almost full size. This is when caterpillars feed exclusively on seeds and have a large appetite. Feeding by small caterpillars on terminals, flowers, and small pods does not reduce yield. However, occasionally large caterpillars will cause severe defoliation that will reduce yield.
Corn earworm typically reaches the highest populations, and does the most severe damage, in flowering, open canopy, and stressed soybeans which have had the biological control organisms removed with insecticide. Therefore, management practices that avoid these causes will help keep caterpillar numbers below threshold levels. Tactics which help maintain low populations include: using early varieties (Grp. V and earlier) that are early planted, establishing favorable growth conditions (pH, fertility, moisture), using narrow rows (that close the canopy quickly), and avoiding soil systemic and foliar insecticides in early season. Creating an unattractive, healthy crop with high levels of biocontrol organisms will greatly reduce corn earworm infestation levels.
Insecticide management. Pyrethroid and carbamate insecticides are most commonly used to reduce corn earworm numbers below the economic threshold, although several newer chemistries are gaining in popularity (flubendiamide, indoxacarb and spinosad). Ideally, treatment should be directed to mid-size larvae since control of small larvae is sometimes difficult (they are within flower clusters, terminals, etc) and large larvae may have already caused economic loss.
Recently NC State University entomologists have encouraged growers to avoid using pyrethroid insecticides for corn earworm control in soybean. During recent seasons this insect has shown signs of becoming resistant to pyrethroids in the state and across the Southeast (Jacobson 2009). Furthermore, some populations of “corn earworm” in soybeans have recently been found to be composed of mixed populations of corn earworm and tobacco budworm (Heliothis virescens Fabricius). Tobacco budworm has long been resistant to pyrethroids (Sparks 1981) and corn earworm may soon follow. Soybeans can be used a resistance management refuge by using insecticides other than pyrethroids. Thus, growers may help increase the useful life of pyrethroids for use in cotton, where their value for bollworm (corn earworm) is greatest.
Barber, G. W. 1936. The cannibalistic habits of the corn earworm. U.S. Dep. Agric. Technical Bulletin No. 499: 1-19.
Flanders, K. L. 2010. Insect Pest Management, pp. 1-32. In IPM corn insect, disease, nematode, and weed control recommendations for 2010. Alabama Cooperative Extension System. 2010 IPM-0428.