Omnivorous Looper

(View photos to identify caterpillars)

The omnivorous looper (family Geometridae), also called looper or avocado looper, feeds on several dozen plant species. Omnivorous looper occurs in most avocado groves, generally in low numbers, unless natural enemies are disrupted by application of broad-spectrum insecticides.

Adults are mostly tan to orangish on top, with a narrow black band across the middle of the wings. They are white on the underside and have a wingspan of about 1.75 to 2 inches. Females live 2 to 3 weeks, laying eggs in clusters of 3 to 80 on the underside of leaves. Each barrel-shaped egg has a ring of tiny projections around one end. Eggs initially are pale green, then turn shiny reddish to brown. Eggs hatch about 8 or 9 days after oviposition, leaving transparent shells.

Young larvae are pale yellow and about 0.06 inch (1.5 mm) long. Mature larvae are 2 to 2.5 inches long and mostly yellow to pale green or pink, with a gold-colored head. Older larvae have variable dark brown, black, green, or orangish lines along their sides. In addition to three pairs of true legs behind the head, avocado looper has two pairs of appendages (prolegs) near its rear on abdominal segments 6 and 10. Larvae travel in a characteristic looping manner, where they extend their body forward, then draw their rear forward to meet their forelegs. This arches their body up into a loop. When disturbed, omnivorous loopers often drop and hang from leaves on a silken thread.

Larvae feed about 6 weeks, then pupate within rolled or webbed leaves. Pupae are 1 to 1.25 inches long and white when first formed. The case darkens as a moth with brownish wings develops and can be seen through the pupal case. Pupation lasts 1 to 4 weeks.

Numbers increase with increasing temperatures in spring. Omnivorous looper typically has four (and perhaps five) generations per year at warmer growing areas. From inland Ventura to San Diego Counties, most adults fly and lay eggs during January through March, May through June, August through September, and October through November. Three generations a year are typical in coastal Santa Barbara County, where moths typically emerge and lay eggs during March through April, June through July, and August through September. Depending on temperature, egg to adult development takes 2 to 5 months.

Damage is often associated with dense foliage, lots of new leaves, and overcast/cool conditions. Leaf damage is especially evident on terminal shoots. Very young larvae feed only on the leaf surface, leaving a characteristic brown membrane. Older larvae chew all the way through the leaf, often leaving only the midrib and large veins. Full-grown larvae can consume an entire leaf in 1 day. Healthy avocado trees tolerate considerable leaf damage without severe effects on growth or yield. Extensive feeding can result in sunburn and may reduce yield the following year.

Economic damage occurs primarily when caterpillars damage fruit. Both young and old larvae can chew fruit. If young fruit is fed on, it sometimes becomes misshapen. Chewing typically scars the fruit surface, which may cause fruit to be culled or downgraded.

Conserve natural enemies, which usually keep caterpillars below damaging levels. Modify cultural practices to reduce pest reproduction and survival–pruning to open up the tree canopy usually results in noneconomic looper populations. Avoid applying broad-spectrum or persistent insecticides for any pests. Caterpillar outbreaks commonly occur after spraying malathion, which poison 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.

(View photos to identify natural enemies of caterpillars)

Spiders are important looper predators, especially in orchards that have not been sprayed with pesticide or recently subjected to a freeze. Assassin bugs, birds, damsel bugs, lacewings, and pirate bugs and predatory insects also prey on caterpillars.

Granulosis virus frequently infects and kills larvae when they become common. A virus epidemic can cause the looper numbers to rapidly decline within 1 to 2 weeks. Virus-killed caterpillars are immobile and range in appearance from white and swollen to brownish and shriveled. Diseased larvae cease feeding, become lethargic, and eventually liquefy and then dry up.

Wasps, especially Trichogramma egg parasites and three larval parasites (family Braconidae), are the most important natural enemies. Apanteles caberatae and Meteorus tersus are solitary internal parasites of larvae. The Apanteles caberatae larva pupates in a 0.1 inch, whitish silken cocoon near its dead host. The Meteorus tersus larva pupates in a brown or yellowish parchmentlike cocoon, which often hangs suspended beneath leaves or twigs on a 1 to 2 inch longthread. One to several pale Habrobracon (=Bracon) xanthonotus larvae feed externally on each looper, then each pupates in a 0.12 inch (3 mm) long white silken cocoon near the shriveled dead caterpillar.

At least five fly species (family Tachinidae) attack omnivorous looper, including Eumea caesar, Hyphantrophaga (=Eusisyropa) virilis, and Nilea erecta. Their black to dark grayish adults are about 0.25 to 0.33 inch long and resemble a common house fly, but have more prominent stout hairs. White tachinid eggs may be observed on or near a caterpillars' head. Brown to reddish, parchmentlike tachinid pupal cases are often found near the larger pupal cases of dead caterpillars.

Trichogramma platneri naturally parasitizes looper eggs, which turn black when parasitized. Where natural biological control is inadequate, omnivorous looper has been controlled by releasing T. platneri in late spring or early summer during peak moth egg laying, as determined by monitoring using commercially available pheromone-baited or black light traps. Until all T. platneri have emerged, 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.

For more information about obtaining and effectively releasing natural enemies, see Natural Enemy Releases for Biological Control of Crop Pests.

Use biological and cultural controls and sprays of Bacillus thuringiensis on an organically certified crop.

Where caterpillar problems may occur, monitor during the spring and summer, especially after peaks in moth flights. Omnivorous looper is a nocturnal moth. Nocturnal moths are attracted to lights and lay eggs nearby, therefore, monitor areas where bright lights such as security lights are used. Be sure to correctly distinguish the cause of any damage since 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.

MONITORING CATERPILLARS AND THEIR NATURAL ENEMIES methods include shaking foliage to dislodge larvae, inspecting foliage for caterpillars and their damage (timed counts), trapping adults, or a combination of these methods.

When inspecting foliage, if 15 healthy omnivorous looper larvae are found per hour of search, treatment may be warranted. Modify this guideline based on orchard history and the extent of biological control. If caterpillar damage has previously been a problem or broad-spectrum pesticides have been applied, it is more likely that treatment will be needed. If natural enemies are increasing, this may indicate treatment can be delayed or avoided. If looper numbers are near the threshold, monitor parasites and other natural enemies several times. With higher levels of larvae, watch for evidence of viral disease. When a nuclear polyhedrosis virus is present, looper numbers will often crash within 2 weeks. Diseased larvae cease feeding, become lethargic, and eventually liquefy and then dry up in their nests. Spraying with malathion often leads to outbreaks of other pests and is not recommended. Bt sprays are the least disruptive to natural enemies.

• Photos of caterpillars and their damage
• Key to identifying caterpillars that damage avocado in California
• Photos of natural enemies of caterpillars