Description of the Pest
Thrips are very small, slender insects that are best seen with a hand lens. Mature western flower thrips are 0.06 inch (1.5 mm) long, while onion thrips are slightly smaller at 0.05 inch (1.3 mm) long. The most distinctive characteristic of thrips is two pairs of wings that are fringed with long hairs. Adults are pale yellow to light brown in color. Immature stages have the same body shape as adults but are lighter in color and are wingless.
Thrips have a very extensive host range, including cereals, onions, garlic, and broadleaved crops, but it is only the species of plants that are infected by Tomato spotted wilt virus and on which the thrips can complete their entire life cycle that play an important role in the disease cycle. In California, the key crop hosts include tomato, pepper, lettuce, radicchio and fava bean. Important weed hosts include cheeseweed (Malva parviflora), sowthistle (Sonchus oleraceus), and prickly lettuce (Lactuca serriola).
The adults are the only life stage that can fly, but they are not strong fliers. Adult thrips can be carried on wind currents, on clothing, and in association with plants. The length of the thrips life cycle (from egg to adult) varies depending on environmental conditions but is generally 30 to 45 days, though it can be as little as 14 days.
The primary damage caused by thrips to tomatoes is the vectoring of Tomato spotted wilt virus. The virus can only be acquired by the immature stage of thrips, whereas plant-to-plant transmission primarily occurs by adults. The adult thrips can transmit the virus for the remainder of their lives, which can last 30 to 45 days. However, the adults do not pass the virus to their progeny (through the egg).
High numbers of thrips can cause damage with their feeding, which distorts plant growth, deforms flowers, and causes white-to-silvery patches on emerging leaves that often have tiny black fecal specks in them.
If possible, avoid planting tomatoes next to onions, garlic, or cereals, because high thrips numbers often build up on these crops. Also, avoid fields near greenhouses where ornamentals (cut flowers) are grown because these plants serve as hosts for the virus and thrips.
Insecticide treatments for thrips are usually not warranted in the Imperial Valley but may be needed for suppression of Tomato spotted wilt virus in the Central Valley and coastal growing areas.
Treatment with foliar insecticide sprays early in the season and continuing through the season as needed may limit in-field spread of Tomato spotted wilt virus to some extent. Soil applied imidacloprid has not been shown to lower virus incidence. Rotate classes of insecticides to minimize insecticide resistance in thrips.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(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.|
|(Radiant SC)||6–10 fl oz||4||1|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: Do not use on tomato varieties that are less than 2 inches (cherry or grape tomatoes). Apply to foliage not through drip irrigation to reduce thrips numbers.|
|(Lannate SP)||0.25–0.5 lb||48||1|
|(Lannate LV)||0.75–1.5 pt||48||1|
|MODE-OF-ACTION GROUP NUMBER1: 1A|
|COMMENTS: Do not use if psyllids are present. Efficacy increases when combined with a pyrethroid such as zeta-cypermethrin (Mustang).|
|(Dimethoate 400)||0.5–0.66 pt||48||0|
|(Dimethoate 2.67EC)||0.75–1 pt||48||0|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Do not use if psyllids are present. Efficacy increases when combined with a pyrethroid such as zeta-cypermethrin (Mustang). Do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|(Beleaf 50SG)||2.8 oz||12||0|
|MODE-OF-ACTION GROUP NUMBER1: 29|
|**||See label for dilution rates.|
|‡||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.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|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 are assigned by IRAC (Insecticide Resistance Action Committee).|
|#||Acceptable for use on organically grown produce.|