In 2006 we initiated studies to determine if some non-chemical control tactics might be able to reduce thrips` infestations in onions. Because thrips locate suitable host plants through visual cues in the UV spectrum, we evaluated materials that reflect UV radiation to determine whether they could disrupt host finding by thrips. We undertook field studies to investigate the effect of straw mulch, kaolin particle film and their combination against onion thrips. Kaolin clay did not perform well because it was easily washed off in the rain. However, straw mulches delayed thrips colonization in the field and reduced the overall numbers of thrips on the plants during the season. At the peak time of colonization, straw mulches offered a significant reduction of 55-73 percent in the numbers of larvae compared with the control. The same trend was observed with adult thrips population peaks. Such reduction in numbers of thrips could allow widening the gap between insecticide applications, thus reducing application costs and environmental damage, and delaying insecticide resistance development. The delay in colonization caused by straw mulches would also likely reduce the impact of Iris Yellow Spot Virus (IYSV), a new disease of onions in New York, which is particularly damaging to young plants.

Onions are a major vegetable crop in New York, grown on 13,800 acres and valued at approximately $50 million annually. Onion thrips are small insects that account for most of the insecticide use on onions. Damage caused by onion thrips feeding results in leaf tissue silvering and photosynthesis reduction that leads to bulb size reduction and yield loss. Onion thrips have developed resistance to commonly used insecticides, and chemical control is therefore often unsuccessful. Furthermore, they have been identified as the main vector of an emerging disease, Iris Yellow Spot Virus (IYSV), which was detected in most onion-growing areas of New York state during the summer of 2006. All these factors initiated our interest in investigating alternative methods to control onion thrips in New York onion fields.

In 2006 we undertook field studies to investigate the effect of straw mulch, kaolin particle film, and their combination against onion thrips. Kaolin clay did not perform well because it was easily washed off in the rain. However, straw mulches delayed thrips colonization in the field and reduced the overall numbers of thrips on the plants during the season. At the peak time of colonization, straw mulches offered a significant reduction of 55-73 percent in the numbers of larvae compared with the control. The same trend was observed with adult thrips population peaks. Such reduction in numbers of thrips could allow widening the gap between insecticide applications, thus reducing application costs and environmental damage, and delaying insecticide resistance development. The delay in colonization caused by straw mulches would also likely reduce the impact of Iris Yellow Spot Virus (IYSV), a new disease of onions in New York, which is particularly damaging to young plants.

Straw mulch appears to be a promising non-chemical method of helping to reduce onion thrips populations in onions. The mechanical application of straw mulch may involve financial investment of new equipment and labor but the low material price and the reduction of the early colonization and subsequent thrips presence on onion plants could compensate for the investment. It also appears that these practices are compatible with the general onion production methods. Thus, straw mulches in onions should lead to increased income for onion growers, reduced use of insecticides, less environmental damage and reduced risk of insecticide resistance.

Dr. Eleni Larentzaki (Entomology Department Cornell/NYSAES)