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 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 mild spring 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. 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 worsen 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 downgrading 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 reduce 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
Use biological control and early harvest in an organically certified citrus crop.
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 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 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. Note that 25 thrips-weeks could be five thrips feeding for 5 weeks, 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.
Spray thrips in spring with pyrethrins plus piperonyl butoxide. Use of pyrethrins 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, applied to control scale. Generally if thrips are present in lemons when a spring scale spray is applied, this will be sufficient for the season. Adding 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 spray may be necessary to protect inside canopy fruit.
|Common name||Amount to use||REI‡||PHI‡|
|(Example trade name)||(type of coverage)**||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Pyrenone Crop Spray)||6–12 oz/acre (OC)||12||When dry|
|RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most|
|PERSISTENCE: Pests: short; Natural enemies: short|
|MODE-OF-ACTION GROUP NUMBER1: 3A/—|
|COMMENTS: Because there is no residual, repeat application may be needed in 2 to 3 weeks and control may be only partial.|
|**||OC - Outside coverage uses 100 to 250 gal water/acre.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers (un = unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).|