This is the time of year that two “nefarious” insect pests may hamper your ability to produce a quality crop. These are the western flower thrips (Frankliniella occidentalis) and fungus gnats (Bradysia spp.).
1. Why are western flower thrips and fungus gnats major insect pests in greenhouse production systems?
Both insect pests cause direct and indirect damage to plants. Western flower thrips directly injure plants by feeding on leaves and flowers and indirectly cause plant damage by vectoring impatiens necrotic spot virus.
Fungus gnat larvae, while feeding on plant roots, interfere with the plants’ ability to take up water and essential nutrients. In addition, both the larva and adult may transmit fungal diseases such as Botrytis and Pythium. Larval feeding may predispose plants to attack from fungal pathogens via creating wounds, which may allow entry of soil-borne plant pathogens such as Pythium and Phytophthora.
2. What are the visual symptoms of western flower thrips and fungus gnat larvae feeding damage on plants?
Western flower thrips may cause leaf scarring, distorted growth, sunken tissues on leaf undersides and deformation of flowers. Flowers and leaves have a characteristic “silvery” appearance due to the influx of air after the removal of plant fluids.
Fungus gnat larvae cause leaf yellowing, plant stunting and wilting. The larvae may actually tunnel into the crown of plants resulting in complete wilting of infected plants.
3. What is the best way to scout/monitor for both insect pests?
For western flower thrips, place either blue or yellow sticky cards above the crop canopy. Count the number of adult thrips on the cards weekly. In addition, visual inspection such as looking into open flowers and/or shaking open flowers over a white sheet of paper may be useful in scouting for thrips nymphs and adults.
For fungus gnat adults, place yellow sticky cards near the growing medium surface. For the larvae, insert potato wedges onto the surface of growing medium. Leave the wedges in place for 48 hours. Remove them and turn over to look for fungus gnat larvae feeding.
4. What are the preferred pest control materials for western flower thrips and fungus gnats?
There are a number of pest control materials that may be used against western flower thrips, including spinosad (Conserve), abamectin (Avid), chlorfenapyr (Pylon), methiocarb (Mesurol), novaluron (Pedestal) and acephate (Orthene). Be sure to regularly rotate pest control materials with different modes of action to avoid western flower thrips populations from developing resistance.
For fungus gnat larvae, control materials that may be effective as a drench are Bacillus thuringiensis spp. israelensis (Gnatrol), chlor-fenapyr (Pylon), cyromazine (Citation), dinotefuran (Safari), and pyriproxyfen (Distance).
5. Which biological control agents may be used?
There are a number of commercially available biological control agents for use on these two pests. Biological control of western flower thrips relies on using natural enemies including the predatory mites Neoseiulus (=Amblyseius) cucumeris and Amblyseius swirskii, and the minute pirate bug, Orius insidiosus. The predatory mites feed on the first and/or second instar nymphs. Minute pirate bugs may consume the nymphs and adult stages.
For fungus gnats, there are three commercially available natural enemies that are quite effective in suppressing larval populations including the soil-borne predatory mite Hypoaspis miles (=Stratiolaelaps scimitus), the rove beetle (Atheta coriaria) and the beneficial nematode species Steinernema feltiae.