Amorbia (Western Avocado Leafroller)

Amorbia is primarily a pest of avocado, but can also occasionally cause damage in citrus groves, primarily in Coastal and Southern California and the San Joaquin Valley. Adults are bell-shaped when their wings are folded at rest. Amorbia larvae develop through five instars. At maturity they are 0.75 to 1 inch long. Caterpillars are yellowish green when young, and mostly darker green when mature. The larva has two dark horizontal lines on each side of its head and prothoracic shield that distinguish it from other caterpillars that occur in citrus. The female lays about 150 to 200 eggs during her 2 to 3 week life.

Amorbia completes two generations per year in Northern California, where adults are present in May through June and again in October. In Southern California, adults have been recorded every month of the year.

Amorbia larvae may feed on young fruit at petal fall. They also feed on new growth flushes, often rolling the leaves or tying leaves to fruit and feeding on the peel of young or maturing fruit and under the calyx. Healthy trees tolerate some loss of chewed foliage and blossoms, however extensive defoliation can also result in sunburned fruit and twigs. Economic damage occurs primarily when caterpillars damage fruit. Damaged fruit often decays at the feeding site and scarring causes downgrading or culling of fruit. Infestations generally occur in groves planted near avocado.

Monitor for amorbia from petal fall through fall. Use selective (toxic to only a narrow group of insects) insecticides to preserve natural enemies. Caterpillar outbreaks commonly occur after spraying malathion, which poisons parasites and predators. When pesticides are warranted, limit application to the most infested spots to provide refuges where natural enemies can recolonize after treatment.

A variety of natural enemies such as birds, spiders, lacewings and predaceous bugs attack egg, larval, and pupal stages of amorbia. A naturally occurring virus often kills many amorbia when caterpillar numbers become high. A tachinid fly and several parasitic wasps attack the larvae. The tachinid fly attaches its eggs near the head of the larva and the emerging maggots bore into the amorbia larva to develop inside.

One of the most effective egg parasites is the tiny wasp, Trichogramma platneri. Parasitized eggs are black. Mass releases of T. platneri are used for control in avocado. Make at least two releases a week apart during the period of peak egg laying (as determined by pheromone traps and visual inspection). Place parasite egg cards on at least four trees/acre for a total minimum release of 100,000 parasites/acre per season.

Use biological control, sprays of Bacillus thuringiensis, and the Entrust formulation of spinosad in organically managed citrus orchards.

Cultural Control

• Prune to reduce foliage touching among adjacent trees and to minimize dead twig and plant debris accumulation in canopies.
• Thin or selectively harvest fruit in clusters. Pruning and thinning reduce protected sites and canopy bridges that facilitate insect movement between trees, thereby reducing the abundance of caterpillars, greenhouse thrips, and mealybugs.
• Remove abandoned citrus to reduce the likelihood that amorbia will move from citrus to nearby avocado, or vice versa.
• Control weeds that can host these caterpillars.
• Reduce dust in groves by driving slowly and oiling or watering dirt roads. Dusty conditions reduce the effectiveness of parasites and predators that attack caterpillars and other pests including mites and scales.

When checking the tree, inspect under the button on the base of fruit for small amorbia larvae when monitoring for citrus thrips at petal fall, especially in groves near avocados. Monitor larger larvae later in spring and summer especially after peaks in moth flights. During spring and summer, consider monitoring where bright lights, such as security lights, are used outdoors because the nocturnal moths are attracted by lights to lay eggs nearby. Search for webbing and leaf rolls in young foliage and feeding damage on young and mature fruit located on the outside canopy. Be sure to correctly distinguish the cause of any damage, as other insects and certain abiotic disorders cause leaf holes resembling caterpillar chewing.

• Correctly identify the species of caterpillars. Alternate host plants, damage potential, monitoring methods, and natural enemies vary depending on the species of caterpillar.
• Look for caterpillar predators and larval diseases and parasitism. Natural enemy prevalence affects treatment decision-making because there are no established amorbia thresholds for pesticide application.
• Monitor parasites and other natural enemies several times to determine if their numbers are increasing. If they are, the amorbia numbers will decrease.

Avoid malathion sprays, which often lead to outbreaks of other pests. Bacillus thuringiensis sprays are the least disruptive to natural enemies and pollinators.