Gaining control of thrips

Dan Gilrein

Judging from the numerous grower calls, the main story this spring was thrips, specifically western flower thrips. Control problems have been widespread and insecticide resistance seems to be the common theme. Fortunately, tospoviruses are not being seen, which western flower thrips are known to vector. Disease symptoms of infection are often ambiguous and easily overlooked on some plants so the diseases can’t be counted out quite yet.

Thrips control
Growers seeking to control thrips should discard older and infested plants, eliminate weeds and other alternate hosts, keep areas around greenhouses mowed or mulched and use sticky cards to help detect low-level infestations. In the research greenhouses at Cornell University we regularly remove, bag and discard infested flowers and opening buds (with thrips inside). Outdoor sources in spring are not a significant problem, though we suspect thrips can overwinter here (USDA Hardiness Zone 7) to some extent.

Consider incorporating biological controls too, which New York growers are trying. Insecticides still are needed for rescue situations and even early intervention. Current experience with insecticide resistance should weigh heavily before use.

Overture is the most recent effective control among insecticides and it has tested well in university trials. Both Pylon and Mesurol are still useful controls. Note that Overture and Pylon are for greenhouse or indoor use only.

In one trial Orthene TTO provided partial control on blooming chrysanthemums, but only a few ornamental plants are actually labeled and repeat sprays may lead to phytotoxicity (edge burn and yellowing) on some plants. Orthene TTO is discontinued, but generic versions of the active ingredient acephate are still available for ornamental plants.

Chilli thrips
Three cases of chilli thrips on ornamental plants have been confirmed in New York. Unlike western flower thrips, which generally prefer flowers, chilli thrips appears to be mainly a foliar pest causing stunting and leaf scarring. The symptoms in some cases (i.e., recent schefflera and hedera samples) are nearly identical to those caused by broad mite, which itself has been on the rise in recent years.

Chilli thrips are much smaller than western flower thrips and very easily overlooked. They were found on the newest unexpanded leaves in both foliage plant cases and hardly seen at all on older growth.

University of Florida entomologist Lance Osborne maintains a website with chilli thrips photos and control options. The insect is not difficult to manage using products effective against western flower thrips, but substantial damage may occur before action is taken.