How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Heliothrips haemorrhoidalis
(Reviewed 9/08, updated 9/08)
In this Guideline:
Description of the Pest
Adult greenhouse thrips are tiny, black insects with whitish to translucent wings folded back over their thorax and abdomen. Legs are also a whitish color.
This species is parthenogenic (i.e. reproduces without mating), and males are rarely found. Adult females insert eggs into the leaf or fruit surface. Just before hatching, egg blisters, which denote where the eggs are, develop and can be seen with a hand lens.
Nymphs are whitish to slightly yellowish in color and produce a globule of fecal fluid at the tip of their abdomen. These globules of fluid increase in size until they fall off and another one begins to form, resulting in a characteristic spotting of the infestation area with black specks of fecal material. The globules serve as deterrents to predators.
The propupa and pupa are also whitish to slightly yellow. Neither pupal stage feeds, but remains among the feeding congregation of thrips. All stages of this thrips are sluggish and the adults rarely fly. There are generally from five to six generations per year along coastal southern California.
Greenhouse thrips prefer moderate coastal temperatures and humidity. In general, periods of stressful temperatures such as very cold winters or hot dry Santa Ana wind conditions will result in high mortality of all active stages. Temperatures below freezing or above 100°F cause significant mortality, particularly if they occur over several days.
Greenhouse thrips has historically been of greatest economic importance on coastal avocados. However, it is not unusual for coastal Valencia oranges to sustain significant damage from this pest when a mild winter is followed by mildspring and summer conditions. Lemons are also occasionally attacked. Navel oranges are generally not attacked because they are grown too far inland from the moderating effects of the ocean.
Greenhouse thrips suck out the contents of epidermal cells on leaves and fruit, including the chlorophyll or pigment, causing cells to turn pale in color. The injury is most likely to be found where two or more fruits are in contact, for this is where the thrips tend to congregate. Consequently, heavier than normal crops may exacerbate greenhouse thrips damage by providing the thrips with more places to congregate.
Affected areas on fruit and leaves take on a dirty, spotted appearance as thrips continue to deposit darkened droplets of liquid excrement while feeding. Congregations of greenhouse thrips expand their feeding area out from the initial feeding point, leaving an ever increasing area of tissue devoid of pigment. Although no actual scars or leaf deformities develop, as is the case with citrus thrips feeding, greenhouse thrips feeding injury can result in a degrading of fruit.
Greenhouse thrips is best managed by occasional inoculative releases of parasitic wasps, early harvest, and the selection of insecticides that do not destroy natural enemies.
Only one effective natural enemy is known to attack greenhouse thrips. The minute larval parasite Thripobius semiluteus, which was introduced from Brazil and Australia in the mid-1980s, has been successfully established on greenhouse thrips in many coastal avocado orchards. Parasitized thrips larvae appear swollen and the sides of their body are more parallel than tapered as in the case of healthy thrips larvae. The immobile parasite pupae appear black among the colonies of translucent, unparasitized thrips. The intermittent nature of thrips populations in coastal citrus makes it difficult to have sustained biological control in citrus without occasional inoculative releases of this parasite, but Thripobius is no longer produced and sold by commercial insectaries. Parasites either move into the orchard on their own or must be located in nearby avocado orchards and relocated to the citrus.
Other less effective natural enemies are known, including an egg parasite, Megaphragma mymaripenne, and three predatory thrips species, Franklinothrips orizabensis, F. vespiformis, and Leptothrips mali, also known as the black hunter.
Greenhouse thrips feeding injury is cumulative over the season, so planning for an early harvest in severely affected areas of citrus production can minimize the amount of damage. In addition, because much of the greenhouse thrips population resides on the fruit, it is removed from the orchard at harvest. An early harvest strategy can thus reduce the crop-to-crop overlap time and minimize the greenhouse thrips movement to (and resultant damage of) the following year's crop.
Organically Acceptable Methods
Biological control and early harvest are organically acceptable methods.
Monitoring and Treatment Decisions
If the locations of previous years' infestations have been marked, check these areas in late March or during April to determine the potential for damage in the current year. Greenhouse thrips populations tend to occur within the most moderate microclimatic areas of the orchard, which are consistent from one year to the next. Carefully inspect fruit where it contacts other fruit or foliage for signs of initial feeding injury.
If thrips populations are present, they are generally more easily found by early to mid-May. at this time light economic damage has already occurred. There is no established threshold except to know that without the intervention of extreme weather conditions or successful biological control, feeding populations of greenhouse thrips will enlarge along with the damaged areas on the fruit rinds. Generally, only 25 thrips-weeks (1 thrips-week = one thrips feeding for 1 week) are necessary to produce a damaged area about 1 inch in diameter. This could be five thrips feeding for 5 weeks or 25 thrips feeding for 1 week or some other similar product of feeding time and numbers of thrips. Areas of feeding injury that are larger than 1 inch in diameter are readily seen on the fruit packing line and rejected from top grade.
Thrips populations may be treated in spring with pyrethrin. Use of a pyrethrin is recommended to avoid severe mortality of natural enemies. Sabadilla, commonly used for citrus thrips, is not as effective. Greenhouse thrips are easily killed by organophosphates, such as malathion and chlorpyriphos (Lorsban), applied to control scale. Generally if thrips are present in lemons when a spring scale treatment is applied, this will be sufficient for the season. The addition of an oil helps the insecticide reach the eggs, which are located just below the plant cuticle. Where parasites have been introduced for thrips or other orchard pests, only use organophosphates for spot treatments, or avoid them completely. On Valencia oranges outside coverage should be sufficient to protect most of the fruit, while on lemons a full coverage treatment may be necessary to protect inside canopy fruit.
UC IPM Pest Management Guidelines:
Insects, Mites, and Snails
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