Thrips: Plant Damage and Control

The thrips (order Thysanoptera) are tiny insects, generally no more than 1 mm long, distributed worldwide. The order consists of over 5,000 species. The order name means "fringe wing", a reference to the unusual, highly fringed wings most adult thrips possess. Thrips range in color from light golden yellow, brown (with and without stripes) to black (with and without stripes). Thrips might be confused with small flies and wasps were it not for the four distinctively fringed that distinguish them from all other insect groups. They have several common names including thunder bugs.

Thrips also have unusual mouthparts with a single mandible that functions like a spike to puncture the leaf surface and a pair of finer stylets that penetrate interior cells. Plant injuries usually appear as silver scars marked with small varnish like fecal droppings. [3,4] They feed on a variety of plant tissues, including plant pollen, fungal mycelia, spores, and they also are predatory. When thrips feed on pollen, they puncture the coat and drain the grains. They are so numerous on flowers that they can be effective pollinators of numerous plants. Generally, however, they are considered as minor or secondary pollinators. predatory thrips feed on minute organisms such as mites, and various small insects including other thrips.[6] Their more frequent role is plant pests.[3] While many are dry season pests of agricultural crops, relatively few are serious tree pests, and those affect mostly ornamental and fruit trees.

The biology of thrips is unusual too. Eggs of most species are inserted into plants. Two active feeding stages (larvae) are followed by two inactive and non-feeding stages which take place in soil. Eggs are kidney-shaped and are thrust into young leaves, stems, twigs, or fruits. Nymphs are yellowish-orange with red eyes. Thrips produce many generations a years, a new one emerging every 2 to three weeks.




Fruit Damage

Feeding by thrips may cause strawberry blossom to fall off or cause fruit remain small and hard. Feeding by thrips may also cause a bronzing of the fruit surface. This pale brown or "bronze" discoloration, called Type I bronzing, is localized to small areas of the fruit surface where the thrips fed and seldom is a serious damage. treatment for thrips is not recommended unless population exceeds 10 thrips per blossom.[5]

The feeding wounds are ideal sites for bacteria and fungi to gain entry. They can also transmit certain viral diseases, such as tomato spotted wilt virus (TSWV) and peanut yellow spot (YSV). The viruses are both acquired during the larval stages and are introduced into a plant with the saliva of a feeding adult.

Control

These pests can be hard to control because they are so small and can hide easily in flowers and bark cracks. Apply a dust of sulfur or diatomaceous earth if the thrips are truly injuring fruit crops. Thrips are known to readily develop resistance to several pesticides. For this reason it is important to rotate pesticides with different mode of action.[5] Neem oil will smother thrips eggs, nymphs, and adults. Repeated application is needed to ensure effectiveness; polish leaves weekly for four weeks. Spinosad, a biological pesticide, is effective against thrips. Apply for several weeks according to label directions. Keep in mind that spinosad does have some toxicity to other beneficial insects, so apply only on thrip-infested plants.[8]

Thrips Picture

References

  1. Fern Marshall Bradley. Rodale's Vegetable Garden Problem Solver.
  2. Miranda Smith and Anna Carr. Rodale's Garden Insect, Disease & Weed Identification Guide.
  3. Status of pollinators in North America. National Research Council (U.S.). Committee on the Status of Pollinators in North America.
  4. Whitney Cranshaw. Garden insects of North America: the ultimate guide to backyard bugs.
  5. Larry Strand. Integrated pest management for strawberries.
  6. Hans G. Schabel. Forest entomology in East Africa: forest insects of Tanzania.
  7. Ross H. Arnett. American insects: a handbook of the insects of America north of Mexico
  8. Laurelynn Martin, Laurelynn G. Martin, Byron Martin. Growing Tasty Tropical Plants: In Any Home, Anywhere.




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