Sorghum Webworm
Egg - The egg, 0.5 by 0.3 mm, is round to oval and slightly flattened. It is white with a pale green-yellow tinge when laid, but changes to straw yellow over a 2-day period. As it matures the color darkens to deep yellow or brown.
Larva - The newly hatched, pale green caterpillar averages 0.7 mm in length. Later instars, still green to tan in color, are thickly covered with spines and hairs, bear four red-to-brown longitudinal stripes on their back and have four pairs of fleshy prolegs. The caterpillar is sluggish and has a maximum length of 9 to 14 mm. Color plate.
Pupa - The lower surface of the pupa is generally light yellow to brown in color, while the upper side is reddish-brown. Length ranges from 6.0 to 9.5 mm.
Host Plants - Although grain sorghums with compact seed heads seem to be their preferred food source, sorghum webworms will also feed on sweet sorghum, Sudan grass, Johnson grass, corn, rye and timothy.
Damage - On sorghum, the caterpillars feed only on the ripening grain, consuming the contents of individual kernels and leaving the outside hull intact. Losses in individual fields may be as high as 30 to 80 percent; however, extensive damage rarely occurs during seasons characterized by prolonged spells of hot, dry weather.
Life History - Sorghum webworms average four generations per year in North Carolina. Webworms spin cocoons and overwinter in the larval stage behind the oppressed leaves of the host plant stalk sheath. The overwintering caterpillars begin to pupate when mean daily temperatures reach 14 to 15 degrees C (57 to 59 degrees F). The pupal period may last 5 to 9 days. The adults are nocturnal and live 10 to 20 days during which time the females deposit an average of 88 eggs singly on protected areas of the flowering parts or seeds of the host plant. The population grows slowly up to the middle of the season after which these insects increase rapidly. The larvae usually develop through five instars in 13 days. Twenty-four days lapse from the egg stage to the emergence of adult moths.
An infestation of three larvae per sorghum head is currently recognized as a threshold in North Carolina. For up-to-date insecticide recommendations, consult the current North Carolina Agricultural Chemicals Manual.
