How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

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 in color 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 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

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.

MANAGEMENT

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

Biological Control
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.

Organically Acceptable Methods
Biological control and sprays of Bacillus thuringiensis are acceptable for use on organically certified citrus.

Selectivity
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 organophosphates (chlorpyrifos-Lorsban) and carbamates (methomyl-Lannate) are disruptive to many of the beneficial insects and mites. Some populations of the beneficial mite, Euseius tularensis, show resistance to chlorpyrifos, so this is the least toxic of the broad-spectrum pesticides.

Monitoring and Treatment Decisions
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 treatments 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 350-400 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 populations 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 400-500 degree-days after moths begin flying, when the population consists primarily of 1st and 2nd instar larvae.

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Insects, Mites, and Snails
E. E. Grafton-Cardwell, Kearney Agricultural Center, Parlier
J. G. Morse, Entomology, UC Riverside
N. V. O‘Connell, UC Cooperative Extension, Tulare County
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County
C. E. Kallsen, UC Cooperative Extension, Kern County
D. R. Haviland, UC Cooperative Extension, Kern County
Acknowledgments for contributions to Insect, Mites, and Snails:
J. Barcinas, E. S. I., Corona, CA
R. Dunn, Badger Farming County, Exeter, CA
J. Gorden, Pest Management Associates, Exeter, CA
H. Griffiths, E. S. I., Corona, CA
D. Machlitt, Consulting Entomology Services, Moorpark, CA
C. Musgrove, retired entomologist, Riverside, CA
K. Olsen, S & J Ranch, Pinedale, CA
T. Roberts, E. S. I., Corona, CA
J. Stewart, Pest Management Associates, Exeter, CA
P. Washburn, Washburn & Sons Citrus Pest Control, Riverside, CA
K. Godfrey, USDA Biological Control, Sacramento
D. Headrick, California Polytechnic State University, San Luis Obispo
B. Faber, UC Cooperative Extension, Ventura County
J. Kabashima, UC Cooperative Extension, South Coast Research and Extension Center

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