How to Manage Pests
UC Pest Management Guidelines
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.
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.
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.
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.
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
Monitoring and Treatment Decisions
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 greehouse 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.
UC IPM Pest Management Guidelines: Avocado