Citrus Cutworm

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Of the cutworms, generally only citrus cutworm is an economic pest. Other species, most notably the variegated cutworm, Peridroma saucia, are occasionally found on citrus but rarely cause economic damage.

Citrus cutworm has only one generation a year. The grayish citrus cutworm moths emerge from early January to the end of April, with peak emergence during March. After mating, female moths lay their round, milky-white eggs mainly on the upper side of new leaves in clusters of 40 to 225. In a few days eggs turn dark as larvae develop inside; they hatch in 5 to 10 days. Young larvae are usually light green in the first three instars and pinkish or brown in the fourth and fifth. All but the youngest larvae have a whitish stripe along each side of the body. The skin appears smooth to the naked eye; it does not have conspicuous hairs or tubercles. When disturbed, older larvae curl up into a tight C-shape and drop to the ground.

Larvae mature in 3 to 6 weeks; the greatest number of larvae is usually found from mid-March to the first of May, but sometimes even later. Mature larvae drop to the ground and pupate in soil. Pupae remain dormant until the following spring.

Damage by citrus cutworm can be substantial because they feed on young fruit. In addition, larvae of the citrus cutworm move around while feeding, usually taking a few bites from numerous leaves, blossoms, or fruit. Young larvae feed mostly on the edges of tender leaves; older larvae eat holes through leaves and blossoms and into fruit. A smaller number of citrus cutworms cause more damage than larger numbers of other caterpillars because they are larger and move throughout the tree during feeding. After petal fall, young fruit often have feeding scars; maturing fruit are rarely attacked.

Citrus cutworm is not a pest in all orchards or every year, and its numbers have declined in the San Joaquin Valley with the shift from organophosphate insecticides to insect growth regulators for scale control and spinosad or spinetoram (Entrust, Success, or Delegate) for citrus thrips control. Natural enemies play a significant role in reducing cutworm numbers and their presence should be preserved by selecting the most selective (those that are only toxic to a narrow range of insects and mites) insecticides. Treatments may be necessary during bloom and when fruit are young if monitoring indicates a need.

Two parasites attack citrus cutworm larvae and are highly effective in reducing the next year's population. Ophion sp., a parasitic wasp, attacks cutworms just before they are ready to mature. Parasitized larvae pupate in the soil where they are consumed by the parasite larvae.

Another parasitic wasp, Banchus sp., also attacks cutworm larvae. In some groves, a fungal pathogen has been found to infect and kill up to 25% of the pupae.

Use biological control and sprays of Bacillus thuringiensis on organically certified citrus.

The Bacillus thuringiensis (Bt) insecticides, both the aizawai and kurstaki strains, are toxic only to caterpillar pests. The stomach poison cryolite is specific to foliage-feeding pests. These insecticides are relatively nontoxic to parasites that attack the caterpillars and to beneficial insects and mites that feed on other citrus pests. Broad-spectrum carbamates (methomyl-Lannate) are disruptive to many of the beneficial insects and mites.

Male moths of citrus cutworm can be monitored using a synthetic female pheromone-treated lure in a bucket trap with a no-pest strip inside to kill the moths. One trap per 10-acre orchard is sufficient. Place the trap in the orchard on January 15 and check the bucket for moths each week. This trap can be used to predict when the larvae will emerge so that larval sampling begins at the right time and selective insecticide applications can be applied when larvae are young. Using a lower developmental threshold of 46°F and a biofix of the second week of consecutive moth flight in January, begin sampling for larvae at 250 degree-days. You will see larvae emerging at about 350 to 400 degree-days after the biofix.

Monitor larvae using a time search method (number of larvae per hour search) or by shaking the foliage into a sweep net. Before petal fall, the thresholds are 40 larvae per hour search and 10 to 15 larvae per 25 net shakes. During or after petal fall, the cutworm can cause more damage and the thresholds drop to 10 to 15 larvae per hour search and 3 to 5 larvae per 25 net shakes.

Monitor orchards weekly from early to mid-April through post petal fall. The most critical period is late bloom until the end of petal fall when cutworm larvae are attracted to the small developing fruit. If cutworm numbers begin to approach the threshold during this period, monitor twice weekly until the population pupates.

Selective insecticides such as Bacillus thuringiensis or cryolite are slow acting because they are stomach poisons and require warm weather so that the larvae are actively feeding. These pesticides are most effective when applied about 400 to 500 degree-days after moths begin flying, when the population consists primarily of 1st and 2nd instar larvae.