Thrips are generally bad news for the gardener, whether you have one type of thrips (the name is the same – singular and plural) or more. Not visible to the naked eye as they often feed within buds and furled leaves or in other enclosed parts of the plant, these extremely small insects cause damage that, while unsightly, is generally more cosmetic than harmful. And not all of them are bad – a number of thrips species are considered beneficial predators that go after mites and other insects, some even prefer fungal spores and pollen.
Adult thrips range in color from translucent white or yellowish to dark brown or blackish depending on the species, and are tiny with long, narrow wings. These tiny insects feed on prey with rasping-sucking mouthparts that resemble a soda straw, scarring leaf, flower or fruit surfaces and distorting plant parts, especially flower buds and rapidly growing parts. Their damage can stunt growth and cause leaves to become stippled, papery and distorted, or tightly rolled dropping prematurely. Dead spots or blotches may appear on flowers like the silver spots or brown edges on petals of light-colored flowers, punctuated with small varnish-like fecal droppings.
One of the most common pest types is the Western flower thrips which favors roses and many other ornamentals, vegetables, and some stone fruit trees.
Although thrips have wings, they tend to rely on wind currents to carry them as much as several miles, or to be transported on infested plants. As poor fliers, they tend to spread slowly through a plant or garden. Their lifecycle can be extremely short, development from egg to adult may be as little as two weeks during warm weather, with multiple generations each year.
Thrips are difficult to spot on a rose blossom, as they are generally located at the base of the petals. Monitor for these insects:
- Remove a physically healthy blossom from your plant
- Peel back the first layer of petals, checking the base of the blossom for tiny brown to orange insects approximately 1/16 inch long
- Check for browning areas, which can be especially visible on lighter colored blossoms
- Blow gently into the blossom and pay close attention for any small movements within
Managing thrips can be a challenge. Prevent infestation through good cultural and sanitation practices. Limit excessive applications of nitrogen fertilizers as these actually promote higher populations of the pests (along with aphids). If cultural controls are not effective, then physically removing infested or damaged parts should be a first step in an overall integrated pest management approach that includes encouraging beneficial predators like green lacewings and minute pirate bugs. You can also try a strong spray of water to knock them off plants. If the plant is otherwise healthy, it should outgrow the damage.
Visit the UC IPM website for more details about managing thrips.
By Nanette Londeree, Master Rosarian