San Diego Floral Association
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Friend or Foe: Thrips
Reprinted from: July/August 2009, Volume 100, Number 4
Thrips (Thysanoptera) are tiny, slender insects with fringed wings. They are so tiny (.5–.14 millimeters long) that you may skip over them while searching for pests in your garden. Also known as thunderflies, thunderbugs, stormflies and cornlice, thrips can range in color from translucent white to yellowish, dark brown to black. With over 5,000 different identified species of thrips, it is difficult to determine if the one in your plant is a friend or a foe.
For instance, there are some beneficial thrips, which include black hunter thrips, banded wing thrips and sixspotted thrips. However, even though there are some friends out there, many thrips—often in the Thripidae family—are classified as pests. These garden foes are plant feeders that scar leaves and petals and even distort plant parts.
More than 20 plant-infecting viruses are transmitted by non-beneficial thrips, and these viruses are considered to be some of the most damaging plant pathogens around the world. The viruses are spread by how the thrips feed. They first puncture the plant and then suck out its cell contents. Plants damaged by thrips can show tiny scars on the leaves, have their growth stunted or have their leaves turn papery and distorted. Thrips can cause scabby scarring on avocado skins and create “color breaks” on flower petals. If thrips infect roses, flower buds may become deformed and even fail to open. It is safe to say that these particular thrips are garden foes.
It is very difficult to control thrips populations because they are so small and slender and tend to hide in tiny crevices when they feed. It is important to know that the presence of thrips does not necessarily mean that feeding and damage will result. If damage is noticed there are some biocontrol agents that can be used. Parasitic wasps, predatory mites (Anystis agilis), egg parasitic wasps (Megaphragma mymaripenne) and minute pirate bugs (Orius spp.) are all natural enemies of pest thrips. While these are their natural enemies, little or no research has been conducted on the effectiveness of releasing thrips predators in gardens. Because of this, many growers are forced to occasionally use pesticides in fields and greenhouses. Regular pruning can help control thrips populations without relying on pesticides. –Alyssa Holderbein
Mission: To promote the knowledge and appreciation of horticulture and floriculture in the San Diego region.