How to Manage Pests
UC Pest Management Guidelines
Western avocado leafroller (family Tortricidae), is primarily a pest of avocado. It occurs in most California groves and occasionally increases dramatically, causing severe fruit damage. This caterpillar also damages citrus, where it is called amorbia, its official common name. Amorbia is often called just "leafroller." However, amorbia, avocado looper, and orange tortrix all roll avocado leaves and web plant parts together with silk.
Amorbia (and orange tortrix) adults are bell-shaped when their wings are folded at rest. Their variably colored forewings are typically orangish to tan with dark markings. Adult amorbia are about 1 inch (2.5 cm) long, about twice the size of orange tortrix adults.
Each amorbia female lays about 150 to 200 eggs during her 2 to 3 week life. These light green, oval-shaped eggs occur mostly on the upper side of leaves close to the midrib. Amorbia (and orange tortrix) eggs are laid overlapping or shinglelike in a flat mass. Amorbia lays about 5 to 100 eggs per mass, with an average of 25 eggs per mass. Eggs darken and larvae emerge about 2 weeks after oviposition. Hatched egg masses resemble whitish patches on leaves.
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. Older larvae have one short dark horizontal line on their side on their thorax just behind the head and above the first pair of legs. Other avocado caterpillars lack these distinct black marks. Amorbia feed in nests of leaves and fruit tied together with silk. When disturbed, amorbia and orange tortrix larvae often wriggle violently and drop to the ground.
Amorbia pupate for 2 to 3 weeks in rolled leaves. The 0.5 to 0.75 inch long pupae initially are pale green, gradually turn tan, and become brown when mature.
Egg to adult development time is about 1.5 months at an average temperature of 75°F. Amorbia typically has three generations per year at warmer growing areas. From inland Ventura to San Diego Counties, most adults fly and females oviposit during January through April, May through June, and during September through October. Two generations a year occur on average in coastal groves. In Santa Barbara County most moths emerge and lay eggs during March through June, and August through November.
Young amorbia larvae chew the leaf surface, leaving a thin brown membrane or skeleton of leaf veins. Mature caterpillars consume the whole leaf, starting in the center or at the leaf edge. Young larvae often web terminal leaves together and feed within them. This damage becomes apparent when terminals grow and unfold. Mature avocado trees can tolerate considerable larval chewing without severe effects on tree growth or fruit yield.
Healthy trees tolerate some loss of chewed foliage and blossoms. Extensive defoliation can result in sunburned fruit and twigs. Economic damage occurs primarily when caterpillars damage fruit. When larvae web leaves to fruit or feed among touching fruit in a cluster, in these protected sites larvae feed on fruit skin. This scarring causes downgrading or culling of fruit.
Conserve natural enemies, which usually keep caterpillars below damaging levels. Modify cultural practices to reduce pest reproduction and survival. Avoid applying broad-spectrum or persistent insecticides for any pests. 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 from which natural enemies can recolonize after treatment.
Parasites, especially flies (family Tachinidae) are the most important natural enemies that usually keep amorbia populations below economically damaging levels. Tachinids attacking amorbia include Eumea (=Aplomya) caesar, Nilea (=Pseudoperichaeta) erecta, and at least 5 other species. Their black to dark grayish adults are about 0.25 to 0.33 inch (6–8 mm) long and resemble a common house fly, but have more prominent stout hairs. White tachinid eggs may be observed on or near a caterpillar's head. Brown to reddish, parchmentlike tachinid pupal cases are often found near the larger pupal cases of dead caterpillars.
At least 8 wasps species parasitize amorbia, including the external larval parasite Habrobracon (=Bracon) xanthonotus (Braconidae) and the internal pupal parasite Brachymeria ovata (Braconidae). Trichogramma spp., 0.04 inch (1 mm) long or smaller wasps, lay one to several eggs in each caterpillar egg. Black amorbia eggs are probably parasitized by Trichogramma.
Where naturally occurring parasitism is inadequate, amorbia has been controlled by releasing Trichogramma platneri during peak moth egg laying in late spring as determined by monitoring adults using commercially available pheromone-baited or black light traps. Commercial suppliers typically provide Trichogramma as parasitized moth eggs glued to cardboard. The adult wasps should emerge soon after the shipment arrives. Protect cards from Argentine ants and other predatory insects. Keep a small portion from any purchase in a shady location in a clear container covered with tightly woven cloth. Observe wasp emergence to assess product quality.
Organically Acceptable Methods
Monitoring and Treatment Decisions
There are no established amorbia thresholds for pesticide application. Monitor parasites and other natural enemies several times to determine if their populations are increasing. If they are, the amorbia population will decrease. Spraying with malathion often leads to outbreaks of other pests and is not recommended. Bt sprays are the least disruptive to beneficials.
UC IPM Pest Management Guidelines: Avocado