The corn earworm (CEW), Helicoverpa zea (Lepidoptera: Noctuidae) is a key pest on hemp in the United States. Caterpillars actively feed on hemp flowers, fruit, and seeds; less commonly on leaves. Although this insect is considered to be of economic importance in agricultural hemp, the impact on yield remains unclear, especially considering that CEW infests hemp varieties produced for CBD, for seeds, and for dual purpose (seed and fiber).
Due to the recurrent infestations of this pest, it is important to know how growers can control CEW populations in the field, particularly when the plants start producing aromatic terpenes (oils) on the flower buds, or inflorescences.
Natural Enemies of Corn Earworm
Among the natural enemies controlling the CEW, there are both predatory and parasitoid wasps, tachinid flies, lacewings, minute pirate bugs, big-eyed bug, damsel bugs, and lady beetles. Certainly, there should be occasional encounters between CEW and many other parasitoids, predatory arthropods, and entomopathogens (such as Bacillus thuringiensis or nuclear polyhedrosis virus).
Our Scouting "Find"
During our weekly scouting on hemp during September 2022, we found a cocoon (pupa) of Hymenoptera that had parasitized a CEW larva. The interaction was observed in a commercial hemp field for CBD production in Caldwell County. We found the cocoon still attached to the dead body of a CEW caterpillar. We collected the specimen and the adult wasp emerged 8 days later. The parasitic wasp was identified as Campoletis sonorensis (Ichneumonidae).
Parasitic Wasp Range
This small wasp (4.0-6.0 mm) is widespread in the New World, occurring from the United States, Mexico, Bermuda, Brazil to Chile. It is known to parasitize at least 18 moth species including those of economic importance, such as the fall armyworm, corn earworm, and tobacco budworm.
An Uncommon Interaction
Based on our personal experience, this interaction is not common in hemp considering we have already collected more than 1,000 CEW caterpillars from hemp fields in Fayette, Caldwell, and Calloway during 2021 and 2022. Although parasitism of CEW was reported in other crops such as corns, or soybeans, and even in hemp somewhere else, our team at the University of Kentucky's Research and Education Center at Princeton found this parasitism by a hymenopteran parasitoid in field hemp for the first time.
The relationship between CEW and hymenopteran parasitoids is complex, poorly studied, and not well understood. Presently we don't know why these wasp are not parasitizing CEW while they are exposed to feeding on hemp. Maybe the strong odor or scents of hemp plants repels wasps, or it has to do with the seasonal occurrence of wasps. Learning the biology of natural enemies and the trophic interactions between plants, natural enemies, and prey may clarify this situation.
Categories: Kentucky, Crops, Corn