Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Vegetable and bramble growers in Maryland have called me often over the last couple of years about fruit problems in their fields possibly caused by thrips. As an overall study of the possible impact thrips may be having on vegetable and fruit quality I have been conducting surveys for their numbers and species. I have taken weed samples throughout the winter and early spring from vegetable fields and high tunnels looking to see if any thrips were overwintering and if so what species they were. Below is a 9-point summary of the sampling program.
1. For most samples very few thrips were found.
2. In 14 of the 20 sample sites thrips were found in December through January on winter annuals.
3. At 9 sample sites thrips were found in March.
4. The worse sample sites were high tunnels that had chickweed and/or henbit winter annuals growing along the outer or inner edge of the base of the high tunnel (Fig 1). 87% of the sampled winter annual weeds at these sites over the last two years had at least 3 female thrips (one sample had 23 female thrips).
5. Of the total thrips found 76% were female adults, 19% were males and 5% were immatures or pupae.
6. Western flower thrips were found to overwinter in Maryland, Delaware, SE Pennsylvania and NE Virginia, although only in low numbers (Fig 2).
7. Chickweed was found to harbor 66% of all thrips with wild mustards and henbit being the next best winter hosts.
8. Sampling-sites near high tunnels or woods had a greater probability of containing thrips than sites out in a field.
9. Farms where thrips were found to overwinter had greater probabilities of infestations during the season.
Even though several thrips species, including Western flower thrips, were found to overwinter in the mid-Atlantic area it does not mean we have a thrips problem. However, growers do need to watch for any early season infestations in their field and high tunnel brambles and not overreact by spraying an insecticide unless really needed. Most brambles can have at least 5 thrips or more per fruit/flower before there is any possibility of damage. The species of thrips you have should be determined only if you think thrips are causing fruit quality problems at low densities. I would be glad to look at your thrips if you send them to me: 2005 Largo Rd, Upper Marlboro, MD 20774 or you can call 301-627-8440 or email me: jbrust@umd.edu.
Figure 1. Winter annual weeds along outside (under snow) and inside border of high tunnel
Figure 2. The proportion of thrips species found to overwinter at the 20 sample sites