VARIEGATED CUTWORM LIFE CYCLE AND HABITS

Often the larvae of other species occur on and defoliate mint at about the same time as the variegated cutworm. Bertha armyworm, Memestra configurata, is often seen in mint fields, particularly west of the Cascade Mountains. Its life cycle is about the same as that of the variegated cutworm. However, at times Bertha armyworm larvae may be present before detectable populations of variegated cutworm. Because its leaf feeding damage usually occurs earlier in the growth of mint and its population density is generally less than that of variegated cutworm, it is important to distinguish between the two species. An early insecticide application against this species may not necessarily control variegated cutworm. One well-timed remedial application for cutworms and loopers reduces cost and pesticide load on the field, and minimizes other possible problems, such as the induction of spider mite problems, destruction of beneficial insects, and increased pressure for resistance development in pest insects.

MANAGEMENT AND CONTROL OF FOLIAGE FEEDING CUTWORMS

Foliage feeding cutworms and armyworms include the variegated cutworm and other foliage-feeding cutworms such as spotted cutworm, Amathes c- nigrum, western yellowstriped armyworm, Spodoptera praefica, and the Bertha armyworm. These different cutworm species may occur together in the same field during July and early August. The sampling program and treatment threshold described below for the variegated cutworm may also be used for the aggregate of these species.

For variegated cutworm, inspect fields closely from mid-June to just prior to harvest, remembering that if an insecticide application is considered, the preharvest interval must be observed. Growers may want to consider harvesting earlier to avoid further crop injury. Sweep net samples can be used to sample small larvae (first, second, and third instars). Usually 10 straight line sweeps at 5 different sites in fields up to 30 acres are sufficient to evaluate larval populations. Add an additional site for every additional 10 acres. Largest collections of these smaller larvae will occur on cool, overcast days, or when fields are sampled early in the morning or near dusk on still days. Avoid sweep net sampling when mint is water-stressed or foliage is wet. Very often, more than 50 per cent of the cutworms found in samples will be parasitized; this may alter the treatment thresholds.

The decision to apply an insecticide is usually based on the average number of larvae found per 1,000 sq cm (cm2)(an area slightly larger than 1 sq ft) on the soil surface. To estimate larval populations of fourth, fifth, and sixth instars, inspect the soil surface by first vigorously shaking mint foliage and closely observing and recording the number of larvae per 1000 cm2 randomly through the field. Take a ground search sample every 5 acres for fields up to 30 acres. Add an additional site for every 10 acres in fields that exceed 30 acres. Look very closely for small and curled-up larvae under and in folded leaves on the ground. Remember that larvae can fall into cracks on the soil surface. When leaf-chewing is quite evident and cutworm counts from ground searches are low, consider returning after dark and sampling with a sweep net when the larvae actively feed on the foliage.

Sequential sampling plans have been developed for variegated cutworm using sweep net samples to estimate larvae (instars 2 to 4) and for ground search samples (1000 cm2) to estimate larval instars 4 to 6 (Coop, 1987). Using these plans, treatment of larval instars 2 to 4, sampled with a sweep net, is recommended if 60 larvae are collected from a minimum of 11 different field sites (a minimum of 10 sweep net samples should be taken at each site). Treatment is not recommended if fewer than 44 larvae are collected in sweep net samples. For ground search sampling, treatment of larval instars 4 to 6 is recommended if 24 larvae are collected in 1000 cm2 samples taken from a minimum of 18 different sites. Treatment is not recommended if fewer than 17 larvae are collected in the 1000 cm2 samples.

A sex pheromone is commercially available and can be used to detect and monitor adult males of the variegated cutworm in the spring. Trapping males could provide valuable early season information to growers concerning the potential need to control cutworm larvae during June and July (Coop, 1987). Sticky traps baited with this lure can be set in fields in late April and monitored weekly or biweekly through June. Although action levels for an insecticide or Bt treatment have not been developed based on moth catches, it is likely that large and continual catches greater than 25 per week will result in similarly large populations of larvae being observed approximately 2 weeks following peak trap counts. The real value of pheromone traps lies in the fact that they signal when to begin inspecting fields for larvae, thereby greatly improving timing of an insecticide application, if one is necessary. Also be aware that, occasionally, large trap catches will not result in large larval populations. This is particularly true when the mint field does not have a resident population of variegated cutworm and the trap catches are a result of males being lured into traps from other crops. Conversely, a small trap catch does not necessarily mean that an action level will not be exceeded by the larvae.

# K+ Biological Control by parasitoids Naturally occurring predators and parasites play an important role in suppressing cutworm populations throughout all mint-growing areas in Oregon. The percentage of parasitism may reach 80 to 90 per cent in some fields. The principal parasites of cutworms and loopers on mint are Meteorus communis, Nepiera sp., Campoletis sp., and Copidosoma sp. (Coop, 1987; Coop and Berry, 1986). Growers and consultants are urged to consider that it is likely that a significant proportion of the larvae in samples may be parasitized. Leaf consumption by parasitized larvae is much reduced and there is very little reduction in oil yield caused by these larvae (Coop and Berry, 1986). Parasitized larvae can be distinguished from nonparasitized larvae only by dissection. Select some of the largest larvae from the sweep or ground search sample. With each, cut the head off, and pull the larva apart. If the larva is parasitized, another smaller larva of the pasrasite will be found inside the cutworm larva. If time permits, larvae can be reared in the laboratory to determine the number parasitized. Depending on the percentage of larvae parasitized, you can increase the suggested action thresholds.

If treatment is justified, refer to the insecticide table for a list of insecticides and rates that are registered for use on mint to control cutworms. Orthene generally has been more effective in controlling larger cutworm and looper larvae and grasshoppers. Lannate controls smaller larvae and has been shown to prevent egg hatch in laboratory studies and field observations in other crops. Lannate also satisfactorily controls populations of adult mint flea beetle when present at the time of application -- Orthene does not. Research has shown that variegated cutworm feeding on peppermint is less susceptible to insecticides because the terpenes found in mint leaves induce enzymes that detoxify the insecticides (Berry et al., 1980). On the other hand, certain peppermint terpenes have been shown to increase mortality of variegated cutworm larvae and pupae (Harwood, 1988; Harwood et al., 1990).

# K Bacillus thuringiensis (Bt) applied at 1 to 2 quarts/acre can reduce populations of small cutworms. It benefits from the addition of a spreader sticker or wetting agent and is more effective if applied at night, when larvae are actively feeding on foliage being sprayed, and in the absence of direct sunlight and extreme temperatures. It does not control pests other than loopers, cutworms, and armyworms. Activity on variegated cutworm and other cutworms and armyworms is variable, depending on the trade product and its formulation. The major disadvantage of the use of Bt is the fact that the dense mint foliage prevents penetration and coverage of the leaves with Bt.

Entomopathogenic nematodes have recently been tested to provide control of variegated cutworm in peppermint (Berry et al. 1993).