Western flower thrips, Frankliniella occidentalis, is a major pest in greenhouse production systems. It directly damages plants by feeding on flowers and foliage. In addition, it causes indirect damage by vectoring tomato spotted wilt and impatiens necrotic spot viruses. The primary management strategy is the use of chemical insecticides. These are generally ineffective due to the cryptic habit of western flower thrips and resistance to many available insecticides. The use of biological control against western flower thrips has been limited due to the low tolerance for this pest. However, biological control has been used successfully in some greenhouse operations. One of the primary biological control agents is the phytoseiid mite, Amblyseius (= Neoseiulus) cucumeris. A. cucumeris was first associated as a predator of western flower thrips in Dutch greenhouses. It was later fully developed to control western flower thrips and used on a commercial basis in the early 1980's.
A. cucumeris is a tan-colored mite, approximately 0.5 mm long. Females lay small white eggs on plants that hatch into non-feeding larvae that molt into nymphs after approximately 2 days. The two nymphal stages last 7 to 10 days. Adult females can lay up to 35 eggs during their 30 day lifespan. At 77°F, development from egg to adult occurs between 9 to 12 days. A. cucumeris works best at 54 to 86°F and 70 to 80% relative humidity. The mite is generally located on the underside of leaves along veins or inside flowers. Both nymphal and adult stages feed on western flower thrips. Adults feed on 1 thrips per day. However, they only attack young (1st instar) western flower thrips. This is because large thrips strike the predatory mite with their abdomens. In addition, thrips excrete a wet substance that covers predatory mites, the mites then spend time cleaning themselves instead of attacking thrips. The mite will also feed on pollen and two spotted spider mite eggs. A. cucumeris will enter into a resting stage (diapause) in late fall in response to short daylengths and low temperatures. Diapause can be avoided by increasing daylengths with supplemental lighting or by maintaining nighttime temperatures above 70°F. Non-diapausing strains of A. cucumeris that do not respond to short-day photoperiods are commercially available.
One method of releasing A. cucumeris into greenhouses or interiorscapes is by lightly shaking them onto plants from containers, which have between 10,000 to 25,000 predatory mites mixed with bran and vermiculite. It is beneficial to lightly mist the plants before release, so the mites do not fall off. Another method involves placing slow-release sachet bags, containing approximately 300 mites, among the crop. Mites will emerge from these bags and migrate directly into the crop. These bags last approximately 6 to 8 weeks.
A. cucumeris is most effective when plant leaves are touching and thrips numbers are low. The success of this mite depends on the availability of alternative food sources such as pollen, and moderate to high relative humidities. Plants with hairy leaves can reduce the performance of A. cucumeris, because it is harder for the mite to move around on the plant. In addition, low relative humidities can severely affect egg hatch, as drier air will cause the small eggs to desiccate. Establishment of A. cucumeris can take about 1 month, so mites need to be released prior to thrips establishment.
- Raymond A. Cloyd, Purdue University
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