The trouble with thrips

Thrips — in this case western flower thrips (Frankliniella occidentalis) — is still the most destructive insect pest for greenhouse-grown horticultural crops. The fact that western flower thrips can cause both direct (feeding) and indirect (via vectoring viruses) damage to plants has resulted in a very low tolerance for this insect pest.

Management or regulation of western flower thrips populations typically involves extensive use of insecticides. Below are a series of questions and answers related to insecticide use against these pests.

Q: Why are western flower thrips populations so difficult to regulate with insecticides?

A: The natural thigmotactic, or cryptic, behavior of western flower thrips in which individuals reside in enclosed, concealed locations on plants (such as unopened flower buds) reduces direct exposure to spray applications of contact insecticides. In addition, it is possible that continuous spray applications may cause increased cryptic behavior.

Q: How prevalent is resistance among western flower thrips populations?

A: There are approximately 153 documented cases of insecticide resistance associated with western flower thrips populations worldwide. These cases of resistance involve insecticides in at least seven chemical classes based on the Insecticide Resistance Action Committee (IRAC). Furthermore, certain western flower thrips populations may possess multiple mechanisms of resistance, or resistance may involve multiple genes (polygenic). In general, there are three primary factors that exacerbate the ability of western flower thrips populations to develop resistance. These include rapid development time; high female reproductive capacity; and haplo-diploid breeding system, which enhances resistance being “fixed” rapidly in populations.
 

Q: What types of insecticides are most effective in regulating western flower thrips populations?

A: Contact insecticides are effective as long as applications are timed when the most susceptible life stages are present, including nymphs and adults; thorough coverage of all plant parts (leaves and flowers) is obtained; and frequent applications are made to kill nymphs and/or adults that were previously eggs and/or pupae. Translaminar insecticides may be most effective because the material (active ingredient) enters plant tissues, which increases the probability that those western flower thrips concealed in flowers actually ingest the toxins. When applied as a drench or granule to the growing medium, systemic insecticides tend to be less effective because the active ingredient is not readily transported into floral tissues.

Q: What is one of the most important factors associated with regulating western flower thrips populations with insecticides?

A: It is important to rotate insecticides with different modes of action to mitigate or delay resistance. One mode of action should be used within a generation (two to three weeks) before switching to another insecticide with a different mode of action.

Q: Is there a publication I can refer to for additional information on the management of western flower thrips?

A: Yes, there is an extension publication available online from Kansas State University titled “Western flower thrips: Management on greenhouse-grown crops (MF-2922).”

Cloyd is a frequent contributor to Greenhouse Management and a professor and extension specialist in ornamental entomology/integrated pest management for the department of entomology at Kansas State University. You can write Raymond at rcloyd@ksu.edu.