Sorghum Webworm, Nola sorghiella, Lepidoptera: Nolidae

Description

Adult sorghum webworms are white moths with a wingspan of about ½ inch. Larvae are smaller than corn earworm and fall armyworm larvae and at maturity are only about ½ inch long. Many setae occur on the sides and back of the caterpillar giving it a hairy appearance (Photo 10). Larval color is greenish to yellowish and four red to brown longitudinal lines occur on the back.

Distribution, damage and impact

The sorghum webworm occurs throughout the state and the larvae damage grain sorghum by feeding directly on the seed. Impact varies considerably but in recent years sorghum webworm has produced considerable damage in late planted sorghum in eastern Arkansas.

Life History

Little is known about the biology of the sorghum webworm in Arkansas. Based on information from adjacent states, the webworm overwinters in plant debris as larvae and moths emerge in late spring. Early generations may develop on grass hosts. Adults emerging from these alternate host plants migrate to late planted grain sorghum where they mate and lay eggs. The eggs hatch in about 5 days and larvae feed on sorghum seed for about two weeks. Pupation occurs in plant debris and lasts for seven to ten days. The entire generation is about one month.

Management

Lepidopterous caterpillars often feed within the heads of grain sorghum and visual examination of the head my not detect all larvae. By shaking the seed head over a white bucket, the larvae will be dislodged and can be more easily counted. At least 30 plants per 80 acre field should be sampled. Thresholds for caterpillars on grain sorghum heads vary greatly due to the profit potential of the crop. In general when lepidopterous caterpillars (corn earworm, fall armyworm and sorghum webworm) number two or more per head in grain sorghum nearing maturity foliar insecticides may be warranted. If webworms are the only caterpillar found on grain sorghum, the threshold may be adjusted to five per head. Use of foliar insecticides to reduce damage may not produce economic benefits and should only be used against high insect populations. Numerous beneficial organisms affect webworm including naturally occurring insect pathogens, parasites and predators and insecticide use may reduce their effectiveness. Fall plowing to destroy and bury crop residue will reduce overwintering populations of webworm. Finally, “open headed”grain sorghum hybrids better expose caterpillars to foliar insecticide sprays and to beneficial organisms.

Insecticide recommendations for sorghum webworm on grain sorghum

See Table 2 for insecticide recommendations current at time of publishing. Current updated recommendations can be found in the Cooperative Extension Service publication MP-144 and on the world wide web at WWW.cdms.net/manuf/default.asp. Always follow instructions on pesticide labels.