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Thrips

 

Thrips can become a problem when weather turns hot and dry. You likely won’t see the beasts themselves because they’re tiny and reclusive. Instead, you will see the damage caused by their rasping feeding which resembles the damage caused by a file run across the leaf tissue.

Pest Description:

Thrips are very small, elongate, cylindrical, gregarious insects ranging from 1/25 to 1/8 inch in length. The nymph are frequently pale yellow and highly active. The antennae and legs are relatively short. Adults are usually black or yellow-brown, but may be translucent and have red, black or white markings They often jump when disturbed. They may have wings or may be wingless. Thrips can be distinguished with the head and central area of the body having a distinct network of lines. If wings are present, they are long, narrow and fringed with hairs. For this reason, thrips are commonly referred to as fringed-winged insects. Thrips are poor flyers. As a result, damage often occurs in one part of the plant then slowly spreads throughout it.

thrip  (38255 bytes)

Size of thrip compared to a dime (21969 bytes)

Size of thrip compared to a dime

 

Life cycle of thrips: 

Thrips insert their eggs into leaf or petal tissue, and are hence protected from insecticides.  Each female deposits 25 to 50 eggs.  Eggs hatch into larvae, which usually remain protected. The insects pass through two larval stages, both feed in flower buds or foliage terminals. Toward the end of the second larval stage, the insects stop feeding and move down into the soil or leaf litter to pupate. The thrips pass through two "pupal" stages (prepupal and pupal), no feeding and little movement occurs at this time. While in these pupal stages in the soil, they are protected from insecticides directed at the crop. Like aphids, female thrips can reproduce without mating. One generation will occur every 17-30 days – the warmer the weather, the shorter the generation time.  The adults can live 7 weeks on plants growing in the greenhouse. 

 

Damage: 

Thrips feed on the foliage and flowers, as well as young tissues in shoot apexes where the leaves are expanding. They puncture the plant cells with their rasping-sucking mouthparts and withdraw cell sap. Feeding activities produce bleached, silvered or deformed leaves and small scars or blotches on flower petals and foliage. Eventually the damaged foliage becomes papery, wilts and drops prematurely. Flowers may be deformed and fail to open properly.  Thrips produce large quantities of a varnish-like excrement which collects on leaves. This excrement is tiny, black and shiny, which may be a clue to their presence.

Control of Thrips:

Natural control:

  • Natural control is difficult because the Thrip is embedded in  tightly curled leaves or deep in the shoot apex.  Dislodge them by applying a strong stream of water to the affected plant.
  • This is one time you will want to use overhead watering as it kills many of the thrips.  Remove and discard affected blossoms and plant parts.
  • Thrips prefer tender new growth. Avoid excess pruning which may stimulate new growth.
  • Avoid planting near dry, weed or grassy areas. Thrips migrate from these areas into the garden
  • Spray with horticultural oils weekly until no sign of thrip is present.
  • Spray with insecticidal soaps.
  • Two biological controls have shown promise in Colorado. They are Beauvaria bassiana (Naruralis O, Botanigard) and Spinosad. 
  • Michigan State is evaluating the use of calcium nitrate at a rate of 20% to see if this organic method will provide thrips control.

Chemical control:

  • Add systemic to soil such as Marathon
  • Spray with Orthene or any pesticide labeled for Thrip. The location of the insects makes it difficult to reach them with insecticides.
  •  Insecticide efficacy studies at Michigan State have shown that Lannate LV (methomyl) was the best product for thrips control.
  • Synthetic pyrethroids (such as Warrior) are another choice but there have been reports out of New York that indicate thrip populations are becoming resistant to Warrior.

  As with any chemical be sure to follow label directions and wear the appropriate safety equipment.  

NOTE: Disclaimer - Pesticide recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. 

Excerpts or photos taken from the following references:

The Trouble with Thrips, Dan Rahn, www.growquest.com

Thrips, Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

Thrips, Plant Ranch Wholesale Foliage

Thrip, www.ifas.ufl.edu

Bilogy, Natural History, and Ecology of Thysanoptera 

 


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