How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Greenhouse thrips (order Thysanoptera) occurs primarily on broadleaved evergreen plants including avocado, citrus, and many ornamentals. Adult greenhouse thrips are black with white legs and white wings. Adults seldom fly, and all stages of this tiny insect are sluggish. Males are not found in California, where each parthenogenic female can lay up to 60 eggs during her life. Eggs are inserted singly into fruit or the upper or lower leaf surface. Eggs hatch in about 4 to 5 weeks during summer, longer during the winter. Unhatched eggs gradually increase in size, causing a swelling (egg blister) in the leaf cuticle that can be seen with a hand lens.

Greenhouse thrips larvae and pupae are pale yellow to whitish with red eyes. Larvae carry a greenish red to black globule of liquid feces on the tip of their abdomen. They periodically drop this excrement, leaving dark specks on fruit and foliage that help to locate infestations during monitoring. Most greenhouse thrips occur in fruit clusters and where leaves and fruit touch.

Greenhouse thrips has about five to six generations a year. All life stages are usually present throughout the year. In some colder areas, overwintering is primarily as eggs, with newly hatched larvae appearing about mid-February. Greenhouse thrips populations are lowest during winter and spring, but can become abundant enough to damage fruit during early summer or fall. On Hass, where most greenhouse thrips reside on fruit, much of the population is removed annually at harvest.

DAMAGE

Greenhouse thrips occasionally is a serious pest in coastal avocado groves. Feeding on fruit skin causes scarring and the downgrading and culling of fruit at the packing house. Damage to leaves, although unsightly, is of no significance to tree health. Thrips injury on foliage begins to show in June as small, white-gray patches on upper leaf surfaces where thrips are found in the greatest numbers. The pale discoloration of foliage and fruit caused by early infestations turns brownish later in the season. The epidermis of injured leaves and fruit become thick, hard, and cracked. Black specks of thrips excrement may be noticeable.

Most economic damage occurs when fruit are 2 to 7 months old. Economic damage occurs when thrips cause scars or blemishes larger than 0.75 inches in diameter on fruit. Damage usually is most severe on fruit in clusters or where fruit touch leaves, as thrips are protected where fruit touch. Mexican seedling avocados and Hass are extremely susceptible. Least susceptible varieties include Anaheim, Dickinson, Fuerte, and Nabal, which are not widely planted. On green fruit avocado varieties like Bacon and Zutano, greenhouse thrips are not a pest as they feed primarily on foliage.

MANAGEMENT

Biological control, cultural practices, grove microclimate, and weather influence whether greenhouse thrips will be a problem on susceptible (Hass and Mexican seedling) avocado. Conserve natural enemies of thrips and other pests. Consider modifying harvest and pruning practices to control greenhouse thrips. If pesticide application is warranted, spot treat infested areas and avoid spraying the entire grove. Use selective materials for thrips and other pests whenever possible. Application of broad-spectrum pesticides often leads to outbreaks of pests such as caterpillars and mites.

Biological Control
An important egg parasite, Megaphragma mymaripenne (family Trichogrammatidae), often kills about 25 to 50% of greenhouse thrips eggs in coastal avocado. Parasitized eggs develop a relatively large round hole, usually in the middle of the egg blister, where the adult parasite emerges. When a greenhouse thrips emerges, part of the egg shell is often visible at the side of the egg blister.

Thripobius semiluteus (family Eulophidae) attacks second-instar larvae. The normally yellow to whitish thrips larvae turn black and swell around the head when a larva of this parasitic wasp matures inside. Thripobius egg to adult development time is about 3 weeks when temperatures average 70°F. Thrips populations decline when about 60% of larvae are parasitized. Natural control due to Thripobius semiluteus is inconsistent. Release of several thousand Thripobius per acre per week has controlled greenhouse thrips in coastal avocado, but Thripobius may not currently be commercially available.

Predaceous thrips including black hunter thrips and vespiform thrips (Franklinothrips spp., family Aeolothripidae), prey on greenhouse thrips. However, many predators apparently avoid greenhouse thrips because of their fecal excrement. Beneficial thrips and thrips-feeding general predators are discussed in AVOCADO THRIPS.

Cultural Control
The earlier the harvest, the less thrips damage on harvested fruit. Early harvest (about June or July) of all mature fruit on infested trees also reduces damage to next season's crop. Especially on Hass, where a large proportion of the greenhouse thrips feed and breed on fruit, early harvest minimizes the crop-to-crop overlap period, reducing the number of thrips that can move from old to new fruit.

When fruit prices are low, making early harvest less economical, selectively size pick the larger fruit in clusters and where fruit and leaves touch. Size picking reduces greenhouse thrips populations by removing some thrips. Thinning clustered fruit and pruning dense canopies eliminates harborage, which reduces the density of greenhouse thrips, as well as caterpillars and mealybugs.

Organically Acceptable Methods
Biological and cultural controls and sprays of pyrethrin (PyGanic) are acceptable for use on an organically certified crop.

Monitoring and Treatment Decisions
Map or record the locations of infestations and check these areas each year. Greenhouse thrips problems tend to reoccur at the same sites within groves, typically where the microclimate is moderate. From late March through July, monitor for greenhouse thrips about every 10 to 14 days, at least in coastal groves. Concentrate in less exposed and interior grove areas where temperature and humidity are moderate and where your records document greenhouse thrips were most abundant during previous seasons. If greenhouse thrips are present, also monitor trees where mature fruit was held the longest before harvest.

Monitor on the inside and the north side of trees, away from direct sun exposure. Examine where older fruit touch in clusters and the upper surface of older leaves. Look for colonies of greenhouse thrips, bleached tissue, and black excrement specks. Be sure to correctly distinguish the species of any thrips you find.

Record on a monitoring form the number of greenhouse thrips (adults and larvae combined) per fruit on 10 fruit from each of at least 10 trees per grove. Calculate the average number of thrips per fruit: divide the total number of greenhouse thrips by the total number of fruit sampled (100).

One study indicates greenhouse thrips damage can be predicted based on "thrips-weeks" (the number of thrips present X number of weeks they feed). When a colony of thrips are feeding in a group on a fruit, about 25 thrips-weeks (e.g., one thrips feeding for 25 weeks, or five thrips feeding for 5 weeks) may produce a 0.75 inch (19 mm) diameter, economically important scar. There are no more specific guidelines for when treatment is warranted.

IMPORTANT LINKS

• Photos of thrips and their damage

PUBLICATION

UC IPM Pest Management Guidelines: Avocado
UC ANR Publication 3436
Invertebrates
P. A. Phillips, UC IPM Program, UC Cooperative Extension, Ventura County
B. A. Faber, UC Cooperative Extension, Santa Barbara/Ventura counties
J. G. Morse, Entomology, UC Riverside
M. S. Hoddle, Entomology, UC Riverside
Acknowledgment for contributions to Invertebrates:
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

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