Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
Vegetable and bramble growers in Maryland have called me often over the last two 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 a two-year survey for their numbers and species. Over the last two winters I have taken weed samples from vegetable fields looking to see if any thrips were overwintering and, if so, what species they were. A sample site consisted of 5-12 fields from 2-5 farms that were in close proximity to one another. Pre-season weed samples consisted of 15 x 15 cm quadrats taken from a weedy area, 5 samples per field. Weed samples were placed in a 4-L Ziploc® bag with 20 ml of 70% isopropyl alcohol shaken in the bag ten times and the plants discarded. The bag was marked and placed in a cooler until transported back to lab where it was stored in a refrigerator until examined for content. Below is a 9-point summary of the overwintering sample program.
1. For most samples no thrips were found.
2. In 5 of the 12 sample sites thrips were found in December through January on winter annuals (Fig. 1).
3. At four sample sites thrips were found in March (Fig. 2).
4. The worst sample site had 25% of the sampled winter annual weeds with at least one thrips.
5. 81% of the thrips found were female adults, 11% were males and 8% were immatures or pupae.
6. Western flower thrips were found to overwinter in Maryland, Delaware and Virginia, although only in low numbers.
7. Chickweed was found to harbor 70% 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. They may have been there all along and we are just now discovering them. However, growers do need to watch for any early infestations in their brambles and vegetables 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.
Figures 1 and 2 show the 12 sample sites, nine in Maryland, and one each in Pennsylvania, Delaware and Virginia. The 4 to 5 sample sites where thrips were found to overwinter are indicated with red dots and the yellow dots indicate sites where no thrips were detected.
Figure 1. December and January sampling results, red dots indicate sites where thrips were found to overwinter and yellow dots indicate sites where no thrips were detected.
Figure 2. March sampling results, red dots indicate sites where thrips were found to overwinter and yellow dots indicate sites where no thrips were detected.
Figure 3. The proportion of thrips species found to overwinter at the 12 sample sites.