How to Manage Pests
UC Pest Management Guidelines
Amorbia (Western Avocado Leafroller)
Scientific name: Amorbia cuneana
(Reviewed 9/16, updated 9/16)
In this Guideline:
DESCRIPTION OF THE PEST (View photos to identify caterpillars)
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 lay eggs 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 often occur after spraying malathion, which kills 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.
Biological Control (View photos to identify natural enemies of caterpillars)
Birds, predaceous insects, and spiders commonly prey on caterpillars. Predators include assassin bugs, damsel bugs, lacewings, and pirate bugs. A naturally occurring nuclear polyhedrosis virus often kills many amorbia when caterpillar numbers become high. The caterpillar pathogen Bacillus thuringiensis is commercially available as a selective insecticide.
Parasites, especially flies (family Tachinidae) are the most important natural enemies that usually keep amorbia numbers below economically damaging levels. Tachinids attacking amorbia include Eumea (=Aplomya) caesar, Nilea (=Pseudoperichaeta) erecta, and at least five 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 toreddish, parchmentlike tachinid pupal cases are often found near the larger pupal cases of dead caterpillars.
At least eight 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.
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 and orange tortrix will move from citrus to nearby avocado. Control weeds near avocado that host these caterpillars. Reduce dust in groves by driving slowly and watering dirt roads. Dusty conditions reduce the effectiveness of parasites and predators that attack caterpillars and other pests including mites and scales.
Organically Acceptable Methods
Use biological and cultural controls as well as Bt-based pesticides or spinosad in an organically certified crop.
Monitoring and Treatment Decisions
Where problems may occur, monitor during spring and summer, especially after peaks in moth flights. Continue monitoring until the flight ceases. Good places to monitor include where bright lights such as security lights are used outdoors because the nocturnal moths are attracted by lights to lay eggs nearby. 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. See MONITORING CATERPILLARS AND THEIR NATURAL ENEMIES for details on identification and monitoring methods including inspecting foliage for caterpillars and their damage (timed counts), trapping adults, shaking foliage to dislodge larvae (primarily for avocado looper), or a combination of these methods.
There are no established amorbia thresholds for pesticide application. As a guideline, a 5% fruit infestation level can be used as a treatment threshold. Monitor parasites and other natural enemies several times to determine if their numbers are increasing. If they are, the amorbia presence will decrease. Spraying with malathion often leads to outbreaks of other pests and is not recommended. Bt sprays are the least disruptive to natural enemies.
UC IPM Pest Management Guidelines: Avocado
J. G. Morse, Entomology, UC Riverside
Acknowledgment for contributions to Invertebrates:P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County
M. Blua, Entomology, UC Riverside
P. Oevering, UC Cooperative Extension, Ventura County
D. Machlitt, Consulting Entomology Services, Moorpark, CA
T. Roberts, Integrated Consulting Entomology, Ventura, CA
B. B. Westerdahl, Nematology, UC Davis