Corn earworm (Helicoverpa zea)
Bertha armyworm (Mamestra configurata)
Spotted cutworm (Xestra c-nigrum)
Alfalfa looper (Autographa californica)
Cabbage looper (Trichoplusia ni)
Pest description and crop damage Several species of moth caterpillars (Order Lepidoptera: Family Noctuidae) can be found in hemp in the PNW. All these caterpillars or larvae have three pair of true legs behind their head. The corn earworm, armyworms, and cutworms are varied in color but all have five pair of pro-legs towards the rear end while larvae of both looper species appear as green caterpillars with white longitudinal stripes and just have three pair of pro-legs at the rear end. They move in a looping fashion, like an inchworm.
The corn earworm is the most damaging of these caterpillars as it usually feeds in the buds and if the main stem is chewed on then dieback of the inflorescence beyond the point of injury will occur. Loopers chew holes and ragged edges in hemp leaves. Damage to mature hemp plants from caterpillars other than corn earworm is usually minor and does not require control.
Biology and life history Corn earworm feeds on a number of plant species and is a well known pest of corn and tomatoes. The corn earworm caterpillars concentrate their feeding on fruiting structures. See Common Pests of Vegetable Crops for more information. Cutworms and armyworms feed on foliage. They overwinter as medium-sized larvae and can extensively damage small plants early in the season. Some cutworms are active mostly at night and therefore are difficult to sample and monitor. Moths of loopers are found from May through September.
Scouting and thresholds Brown, dead, and fed upon areas in the inflorescence are most often a sign of corn earworm activity. First sign of infestation by other caterpillars is holes in leaves, with infestations starting in early summer. Control of small larvae is easier than for big larvae. When plants are upright, caterpillars can easily be found during beating sheet/tray. There are no established treatment thresholds for corn earworm or defoliating caterpillars in hemp. Unlike cutworms and armyworms, loopers remain on the foliage all day long and are found easily during normal scouting operations using a beating sheet/tray. Nonetheless, the most obvious evidence of a looper infestation will be the feeding damage on the leaves and frass left behind.
Pheromone traps can be used to monitor corn earworm in sweet corn fields but there are no current thresholds or guidelines regarding the use of pheromone traps in hemp.
All these caterpillars are prey of many generalist predators in hemp fields, including Geocoris big-eyed bugs, Nabis damsel bugs, and probably various species of ground beetles (Carabidae) and rove beetles (Staphylinidae) that inhabit hemp fields. They are also commonly attacked by various pathogens, parasitoids, and birds. Cabbage looper populations can be severely impacted by the disease caused by the Trichoplusia ni, nuclear polyhedrosis virus, can spread rapidly in a population under certain conditions. Loopers dying from this disease often become limp, dark and blotchy, hanging in the foliage by their prolegs, and then burst, dripping virus-laden fluids onto the foliage which infects other loopers.