Prepared for the California Avocado Commission by
Mark S.Hoddle, Dept. of Entomology, University of California,Riverside, CA 92521

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Historical Review. The major thrips pest attacking avocados in California is the avocado thrips, an undescribed species in the genus Scirtothrips (which means jumping thrips) (Fig. 1). This insect was first noticed in California in July 1996 when it was discovered damaging fruit in a Saticoy avocado orchard in Ventura County. Since this initial discovery the thrips population increased rapidly causing significant damage to foliage and fruit in Ventura. In little under a year the thrips spread north and south of Ventura and was found in San Diego County in May 1997. By July 1997 significant damage attributable to avocado thrips feeding was noticed in orchards in San Diego County. In 1971, a quarantine interception at the Port of San Diego resulted in the collection of a single female specimen of an undescribed species of Scirtotothrips on avocado from Oaxaca in Southern Mexico. This single specimen is very similar to avocado thrips. Avocado thrips is not an avocado-adapted strain of citrus thrips, Scirtothrips citri. Avocado thrips is morphologically more similar to Scirtothrips aceri found on oaks in California and Arizona and Scirtothrips abditus which inhabits pines and oaks in Mexico and Costa Rica.

Thrips Overview. Thrips belong to the insect order Thysanoptera, which means "fringe wings" (the wings of adult thrips are fringed with long hairs) and 5,000 species of thrips are known of which 1% are pests. Thrips is a Latin word derived from the Greek for wood louse, and is used in both the singular and plural. Thrip is not a correct term. Thrips are typically small, slender bodied insects around 0.5-15 mm in length. Although winged, thrips are poor fliers and can be transported long distances by winds and storm fronts. The majority of thrips feed on plant juices. Some species are predators, others feed on pollen, fungi, decaying vegetation, or are omnivorous. Thrips have unusual mouthparts in that they only have one mandible which is used like a needle to puncture plant tissue. Food and liquids are siphoned into the mouth through a straw-like structure which is formed from moveable appendages around the mouth.

Thrips Lifecycle. Female Scirtothrips lay eggs singly in an incision made into soft plant tissue with the ovipositor. Eggs are kidney shaped and whitish-yellow in color. In contrast, some species of predatory thrips lay eggs on leaf surfaces (e.g., black hunter thrips). Following egg hatch, developing thrips pass through two actively feeding immature stages called larvae. All thrips species have more than one pupal stage. The first Scirtothrips pupal stage is the propupa and the second is the pupa. Thrips do not feed as pupae and many drop into the soil and leaf litter below host plants to pupate. Following pupation adult thrips move back onto the host plant to commence feeding and reproduction. A generalized Scirtoibrips lifecycle is shown in Diagram A.

An important step in any pest management program is the accurate identification of the pest. This is particularly true for biological control because natural enemies are often specific to just one pest or group of pests (e.g., thrips). In California three thrips species are commonly found on avocados, these being: (1) western flower thrips (Frankliniella occidentalis), (2) greenhouse thrips (Heliothrips haemorrhoidalis), and (3) avocado thrips (Scirlothrips n. sp.).

Western Flower Thrips. Western flower thrips is commonly found in flowers of a wide variety of plants in California. Adults feed on flowers, females lay eggs within flowers and immature thrips feed there until development is complete. Although extremely high numbers of western flower thrips can be found in avocado flowers, this insect is not a pest during bloom periods and does not cause significant damage to avocado foliage. Adult females of western flower thrips range in color from light yellow (Fig. 2) to dark brown (Fig. 3). Males are smaller and uniformly yellow in color (Fig. 4). Dark colored bands that run across the abdomen are similar to those on avocado thrips. Both sexes have stout bristle-like hairs that protrude beyond the tip of the abdomen and are visible a hand lens. Western flower thrips can supplement their diet with mites. Western flower thrips is native to the western United States.

Greenhouse Thrips. Greenhouse thrips is a serious pest in some coastal avocado orchards. Severe infestations have occurred in Santa Barbara, Ventura, and San Diego Counties where the marine influence provides optimal temperature and humidity ranges for survivorship and population growth. On green fruit avocado varieties like Bacon and Zutano greenhouse thrips feed primarily on foliage. On Hass avocados young fruit is preferred. Greenhouse thrips feed gregariously and populations are generally higher on plants where fruit contact each other as this provides protection and ideal micro-climates for greenhouse thrips. This species produces female offspring without mating and males are rare in the population. Females are black in color with white wings covering the thorax and abdomen at rest (Fig. 5). Females insert eggs into young fruit and foliage. Larvae produce globules of fecal material at the tip of the abdomen which increase in size until they are shed and another begins %Æ_ _to form (Fig. 5). Fecal globules are a defense against natural enemies since larvae are sluggish and adults rarely fly. Greenhouse thrips has a worldwide distribution.

Avocado Thrips. Avocado thrips are unusual amongst thrips in that adult and immature stages are readily observed on upper leaf surfaces. When disturbed they run to leaf edges and move to the leaf's under surface where larvae are most commonly found. Males are the smaller of the two sexes. Adults are straw yellow in color, and have thin dark lines running across the upper surface of the abdomen. The wings are brown in color when folded on top of the abdomen (Fig. 1). Larvae are pale yellow in color. Feeding damage to foliage is observed on upper and lower leaf surfaces and bronze colored damage initially follows leaf veins. As the thrips population and feeding damage increases bronzing is observed in random patterns between leaf veins (Fig. 6). Avocado thrips larvae and adults feed on developing fruit while hidden under the calyx. Fruit is susceptible to damage until it exceeds the size of a half dollar. Feeding scars develop from the calyx and as feeding continues scars radiate towards the top of fruit (Fig. 7). Fruit scarring can be severe resulting in 'alligator skin" (Fig. 8). Currently, very little is known about the reproductive and developmental biology of avocado thrips. It is thought that its biology does not differ radically from that of other Scirtotothrips spp. Consequently, it is assumed that females lay eggs in young fruit and leaves and that pupation occurs predominantly in the soil or in cracks and crevices on trees. Second instar larvae and females are probably the most damaging stage as they feed more than first instars, pupae and males. The overwintering stage of avocado thrips is unknown. Avocado thrips may overwinter as eggs like citrus thrips, or as adults like the chillie thrips, Scirtothrips dorsalis. Avocado thrips has only been found feeding on avocado. In greenhouse experiments, avocado thrips has been observed feeding on young foliage of Hass, Lamb Hass, Zutano, Bacon, Lulu, and Topa-Topa. The area of origin for avocado thrips is unknown but is thought to be central America.

Distinguishing Western Flower Thrips From Avocado Thrips. The majority of western flower thrips larvae and adults are found in avocado flowers and disperse after flowering is completed. Avocado thrips larvae and adults are predominantly found feeding under the calyxes of small fruit and on the upper and lower surfaces of young leaves. Western flower thrips larvae are slender, cigar shaped, white to pale yellow in color, slow moving and have stout hairs towards the end of the abdomen that can be seen with a hand lens. Avocado thrips larvae, on the other hand, are oval in shape, yellow to amber in color, are quite active, and have barely visible abdominal hairs. Adult avocado thrips are straw yellow in color, have fine brown-black lines running across the abdomen, and are very active. Western flower thrips adults are yellow-brown in color, are more sluggish in comparison to avocado thrips, and have obvious hairs at the end of the abdomen which adult avocado thrips lack (Fig. 9). The best way to distinguish between these two thrips is to collect western flower thrips from flowers and with a hand lens compare them to avocado thrips.

A high diversity of thrips natural enemies exist in avocado orchards. Although the impact of this fauna against avocado thrips is unknown, it is advisable whenever possible to conserve predators and parasitoids. To assist with avocado thrips management decisions a guide to identifying commonly encountered thrips natural enemies is provided.

Predators. Thrips are eaten by many generalist predators which are commonly found in avocado orchards and are listed below.

Pirate Bugs (Fig. 10). Pirate bugs in the genus Onus are sold commercially for thrips control and are commonly encountered in a variety of outdoor agricultural crops where they attack mites and thrips (plant eating thrips and predatory thrips are attacked by Onus). Host plants can have an important effect on the number of thrips killed by pirate bugs and whether pirate bugs can reproduce on the host plant. For example, pirate bugs are ineffective natural enemies for thrips on tomatoes because they can not reproduce on this plant, while hairy leaves on some cucumber varieties impede pirate bug searching.

Lacewings (Fig. 11). Lacewing larvae (Fig. 12) are probably the most voracious thrips predators found in avocado orchards. Lacewings are commercially available and can be purchased as eggs or larvae and applied to plants by hand. In the laboratory the lacewing Chrysoperla rufilabris can consume 100 2nd instar citrus thrips larvae or 80 citrus thrips adults during its first larval instar and 324 2nd instar citrus thrips larvae or 277 adults during its second larval instar. On potted plants in cages, C. cameo and C. rufilabris caused 82.5% and 77.1% mortality of citrus thrips respectively when compared to plants that were not treated with lacewings. Lacewing releases into citrus orchards significantly reduced fruit damage caused by citrus thrips on plants that were treated with lacewings when compared to plants that did not receive lacewings. Similar research with lacewings for biological control of avocado thrips is planned.

Predatory Thrips. Several predatory thrips species are found in avocado orchards. Predatory species tend to feed on small soft-bodied arthropods including other thrips, aphids, mites and mite eggs, whiteflies, scales, and moth and butterfly eggs. In the absence of prey, predatory thrips can survive on plant material but development and reproduction is reduced. Franklinothrips vespiformis has been observed in high numbers in avocado orchards with heavy infestations of avocado thrips. The larval stage of this predator is easily identified by the red band on the abdomen (Fig. 13). The adult thrips is black in color, has a thin waist and legs which have white bands and is an ant mimic (Fig 14). The ant-like appearance of adult F. vespiformis may protect it against other predators. Adults of six spotted thrips (Scolothrips sexmaculatus) are recognized by the six spots on the forewings (Fig. 15). This thrips is a specialized mite predator and is known to feed on eggs, nymphs, and adults of two spotted spider mite and persea mite. Six spotted thrips uses its forelegs to hold and rotate prey before piercing the ventral surface and feeding. Black hunter thrips (Leptothrips ma/i), larvae are reddish brown in color (Fig. 16) and adults are black with white wings that cross each other to lie in the center of the body when folded (Fig 17). This thrips feeds on mites and other thrips. When prey is unavailable, L. ma/i adults can survive for five days by feeding on plant juice. Banded wing thrips (Aeolothrips spp) attack a number of plant eating thrips (e.g., oat, onion, gladiolus and bean thrips). Mites and aphids are also attacked. Aeolothrips spp. supplement their diet with pollen and plant juice and can complete their lifecycle on plant material. Adult Aeolothrips spp. are black and have white wings with two distinguishing black bands (Fig 18).

Evidence from field experiments suggests that most predatory thrips are unable to control pest thrips on their own because they breed too slowly and lay fewer eggs than their prey. However, predatory thrips are an important component of the generalist predator fauna attacking avocado thrips.

Predatory Mites. The most common predatory mite found in avocado orchards is the phytoseiid Euseius hibisci (Fig 19). This predator is common in orchards year-round, and attacks plant feeding mites and supplements its diet with pollen and plant juice. The impact of this Euseius hibisci on avocado thrips is unknown but the closely related Euseius tularensis is an effective predator of citrus thrips. Experimental results have shown that E. tularensis can substantially reduce fruit damage caused by citrus thrips.

Parasitic Wasps. Two parasitic wasps found in avocado orchards that parasitize thrips larvae are Thripobius semiluteus (Fig 20) and Ceranisus menes. Thrips parasitoids lay their eggs inside the body cavity of thrips larvae and developing parasitoids feed internally killing the host. Wasp larvae pupate inside dead thrips and emerge as adults. Thrips parasitized by Thripobius turn black and have a mummified appearance (Fig. 5). Thripobius significantly reduces fruit scarring by greenhouse thrips when parasitism reaches 60% or more. It is unknown if Thripobius will parasitize avocado thrips. A trichogrammatid parasitoid, Megaphragma mymaripenne, is common in avocado orchards and parasitizes thrips eggs but does not provide effective thrips control on its own, As with predators, thrips parasitoids are a valuable element of the natural enemy fauna attacking avocado thrips.

To reduce the likelihood of resurgence (recovery of pest populations, sometimes to levels higher than before treatments began) and secondary pest outbreaks (release of non-pest insects from biological control due to natural enemy mortality from pesticides) it is necessary to use insecticides that are compatible with natural enemies and to provide refuges for biological control agents. Compatible insecticides have short residual activity or are non-toxic to natural enemies. The botanical insecticide sabadilla is compatible with natural enemies because it has short residual activity and is not toxic to natural enemies. Biological control agents can be protected in refuges. Untreated trees provide refuges for natural enemies allowing them to recolonize sprayed areas. Natural enemies can be purchased to re-inoculate orchards after insecticide treatments have been made or to augment the orchard's indigenous natural enemy fauna (see Hunter 1994 for suppliers of beneficial insects).

Frequent use of a limited number of insecticides with similar modes of action (e.g., stomach poisons) can result in insecticide resistance. Insecticide resistance is the developed ability of an insect population to withstand insecticides that were formerly effective. The rate at which resistance develops in a population is related to intensity of insecticide use. To prolong insecticide efficacy for avocado thrips, it is advisable to limit frequency of applications by spraying only when necessary, to alternate between insecticides with different modes of action, and to leave areas of the orchard untreated (this will conserve natural enemies and allow survival of susceptible thrips that can breed with thrips with resistance genes thereby reducing the rate at which insecticide resistance develops).

Recommendations for insecticidal control of avocado thrips will improve as knowledge on biology and phenology of avocado thrips and its natural enemies increases, when key natural enemies are screened for susceptibility to registered insecticides, and as results of insecticide field trials are analyzed.

For further information on biological control and research on avocado thrips contact: (1) Mark Hoddle, Dept. of Entomology, University of California, Riverside, CA 92521, phone no. (909) 7874714, fax no. (909) 787 3086, e-mail mark.hoddle~cr.edu (2

Joseph Morse, Dept. of Entomology, University of California, Riverside, CA 92521, phone no. (909) 787 5814, fax no. (909) 783086, e-mail morse@ucracl.ucr.edu (3) Phil Phillips, Cooperative Extension, Ventura County, 669 County Square Drive, Ste. 100 Ventura, CA 93001, phone no. (805) 645-1457, fax no. (805) 645-1462, e-mail paph~'ips@ucdavis.edu (4) Ben Faber, Cooperative Extension, Ventura County, 669 County Square Drive, Ste. 100, Ventura, CA 93001, phone no. (805) 645-1462, fax no. (805) 645-1462 e-mail bafaber@ucdais.edu (5) Gary Bender, UCCE Office, 5555 Overland Ave, Bldg. 4, San Diego, CA 92123, phone no (619) 694-2848, fax no. (619) 694-2856, e-mail gsbender@ucdavis.edu (6) Peggy Mauk, UCCE Office, 21150 Box Springs Road, Moreno Valley, CA 92577, phone no. (909) 683-6491, fax no (909) 788-2615, e-mail pamauk@ucdavis.edu (7) Steve Peirce, California Avocado Commission Field Coordination Consultant, 31750 Machado St. No.56, Lake Elsinore, CA 92530, phone no. (909) 674-5046, fax no. (909) 674-2245, e-mail spai@iinet.com.

Photographs used in this factsheet were taken by: Mr. Jack Kelly Clark (UC Photographer), Dr. Michael Parrella (UC Davis), Dr. Margaret Skinner (Uni. of Vermont, Burlington), Dr. Joseph Morse (UC Riverside), Mr. Al Urena (UC Riverside), Dr. Phil Phillips (Coop. Extension, Ventura), Dr. Mark Hoddle (UC Riverside), and Karsten Drescher (Umversitat Bonn, Germany). Thrips lifecycle schematic was prepared by Dr. Vincent D'Amico III (USDA Forest Service, CT).

Hunter C.D. (1994). Suppliers of beneficial organisms in North America. Copies are available from California Environmental Protection Agency, Department of Pesticide Regulation, Environmental Monitoring and Pest Management Branch, 1020 N Street, Room 161, Sacramento, California 95814-5604, phone no. (916) 324-4100.

Lewis T. (1973). Thrips: their biology, ecology, and economic importance. Academic Press, London. 349 pp.

Parker, B.L., Skinner, M. & Lewis, T. (Eds) (1995). Thrips biology and management. Plenum Press, New York. 636 pp.

Philips P. (1 997). Managing greenhouse thrips in coastal avocados. Subtropical Fruit News 5:1-3.

UC IPM Thrips Web Page. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pnO3O.html (you can load the following keywords into a search engine: UC 1PM Thrips Pestnotes).

Van Driesche R.G. & Bellows T.S. Jr. (1996). Biological Control. Chapman and Hall, New York. 539 pp.