Corn earworm (Helicoverpa zea) and fall armyworm (Spodoptera frugiperda) threshold review and recommendation in Mississippi grain sorghum

Grain sorghum, Sorghum bicolor L. Moench, is a low input rotational crop that is often planted in Mississippi to reduce disease, insect, and weed pressure in other crops. The corn earworm, Helicoverpa zea, and fall armyworm, Spodoptera frugiperda J.E. Smith, are economically important insect pests of grain sorghum because they feed directly on the grain. Inconsistent control of both pests with the pyrethroid class of chemistry has caused crop production costs to increase due to use of more expensive insecticides. Data from efficacy trials conducted in 2012 by the MSU Extension Service suggested that the current threshold of one larva per head of both pests either in combination or alone may be too high. During the summers of 2013 and 2014, a series of experiments were conducted at the Delta Research and Extension Center in Stoneville, MS. Experiments were conducted by artificially infesting individual panicles of grain sorghum at eight infestation levels ranging from one larva to eight larvae per panicle to determine both an economic injury level and an economic threshold for these pests. Infested and uninfested grain sorghum panicles were harvested by hand and damage ratings were collected by visually examining each panicle looking for the presence of frass and partially devoured seed. Panicle weights were measured and seed were removed from the main stem and panicle branches. Grain weights were then determined, allowing the amount of grain consumed by one individual larva to be calculated. Results suggest that the current economic threshold may not change very much.