Mistaken Identity Impacts PA Greenhouse Tomato Grower
Posted: March 4, 2011
In 2010 I received a frantic call from a grower when a pest management issue in his tomato greenhouse suddenly arose. The grower had managed his fertility expertly, kept good records, and was heading towards a very profitable year. The grower used biocontrols in his greenhouse to control thrips and spider mites and was at the top of his game when he began to notice gnat-like insects flying through his tomato crop. The grower was a bit perplexed by the appearance of this “new” pest on the scene and decided to consult with a local “expert” regarding the identity of this pest. The “expert” informed the grower that the pest in question was a fungus gnat and that the grower should drench his coir media with the appropriately labeled insecticide.
Two to three drenches later the problem began to worsen and the grower began to notice significant injury to the lower foliage of his now 6’ tall tomato plants. Realizing that he was up against either an insect or disease that he had not encountered before, the grower frantically called my office and requested that I make the 130 mile round-trip to his farm immediately.
When I arrived in his greenhouse I noticed that the top 1-2 feet of tomato growth looked fine, the bottom two feet of canopy was necrotic, and that each tomato leaf in the middle of the plant was marred by 3-7 mines (caused by leafminers). The loss of foliage in the plants increased the incidence of sun-scalded fruit and had encouraged an upward swing in his nitrogen-potassium ratio in the plant which resulted in the wide-spread development of green shouldered fruit.
Leafminers have a relatively short life cycle that can range from 21 to 28 days (egg to adult). Female leafminers tend to deposit their eggs in the middle of the plant on the lower surface of the leaves just below the epidermis. Adult female leafminers can live for about a week and can deposit up to 400 eggs during their brief life.
Leafminer feeding causes a significant reduction in the photosynthetic potential in the plant. Tomatoes are more forgiving than many of the 50-plus hosts of this pest, but 1-3 mines per leaf will still result in yield reductions.
Leafminers can be monitored through the use of yellow sticky cards in the greenhouse. When leafminer adults are detected, a grower should first consider employing biocontrols since insecticide resistance is a major concern with this pest. Parasitic wasps in the families Braconidae, Eulophidae, and Pteromalidae are listed in the literature biocontrols for this pest.
Area tomato greenhouse growers dealing with leafminer problems in their tomato crop should apply insecticides at 7 day intervals or as directed by the product label. Azadirachtin, pyrethrin, malathion, and spinosad formulations are labeled for leafminer use in the greenhouse. Please review all pesticide labels carefully since many of our commercial products labeled for leafminer control are not labeled for greenhouse usage. For additional information, please contact me at (814) 940-5989.