Bertha armyworm (Mamestra configurata Walker)
Spotted cutworm (Xestra c-nigrum Linnaeus)
Pest description and crop damage Several species of moth caterpillars (order Lepidoptera) of various colors can sometimes be found in potatoes, but recent research has shown that the most important pests in the PNW are bertha armyworm, spotted cutworm, and cabbage looper (see separate looper section, below). Armyworms and cutworms have three pair of true legs in the front, and five pair of pro-legs behind. Damage to mature potato plants is usually minor, and these caterpillars usually do not require control.
Biology and life history Cutworms and armyworms feed on foliage at various times of the season depending on which species is involved. Some species overwinter as medium-size larvae and can do a lot of damage to small plants early in the season. Some cutworms are active mostly at night and therefore are difficult to sample and monitor.
Scouting and thresholds The first thing usually noticed during an infestation of defoliating caterpillars is holes in leaves, with infestations starting in early summer. It is important that caterpillars are found before any control measures are implemented. When plants are upright, caterpillars can easily be found during beating sheet/tray sampling for aphids. There are no established treatment thresholds for defoliating caterpillars in potatoes. However, knowing that only a few species have potential to be important as potato pests offers the chance to deploy specific monitoring tools such as pheromone-baited traps for adults. There are effective pheromone lures for both spotted cutworm (z7-tetradecenyl acetate) and bertha armyworm (z9-tetradecen-1-ol acetate and z11-hexadecen-1-ol), both commercially available.
These caterpillars are prey of most generalist predators in potato fields, including Geocoris big-eyed bugs, Nabis damsel bugs, and probably various species of ground beetles (Carabidae) and rove beetles (Staphylinidae) that are known to inhabit potato fields. They are also commonly attacked by various pathogens and parasitoids.
Management-chemical control: HOME USE
- azadirachtin (neem oil)-Some formulations are OMRI-listed.
- Bacillus thuringiensis var. kurstaki (Btk)-Some formulations are OMRI-listed for organic use.
- Beauvaria bassiana-Some formulations are OMRI-listed.
- kaolin-Applied as a spray to foliage it acts to repel certain insect pests. Some formulations are OMRI-listed for organic use.
- plant essential oils (peppermint, rosemary, etc.)-Some have demonstrated efficacy against Lepidopteran larvae. Some formulations are OMRI-listed for organic use.
- pyrethrins-Some formulations are OMRI-listed for organic use.
- spinosad-Some formulations are OMRI-listed for organic use.
Management-chemical control: COMMERCIAL USE
Pyrethroid insecticide (Group 3 in Tables 1-2) applications make aphid management more difficult and can lead to outbreaks of spider mites.