Posts Tagged ‘thrips’

Tomato Spotted Wilt Problems in High Tunnel/Greenhouse Tomatoes

Friday, May 6th, 2011

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

I recently visited a grower that grows both tomatoes and bedding plants. The plants are grown in a high tunnel-like setting, i.e., with heat. The grower was having problems in his tomato plants, but not in his bedding plants. The tomato plants looked like they had tomato spotted wilt virus (TSWV). The symptoms were found on most of his tomato plants, which would be unusual, as most of the time only an occassional plant here and there would be infected with the virus. The grower thought that is what he had as did an alert County Educator—and they were right. The grower unfortunaetly had a perfect storm in his operations that produced high percentages of TSWV infection in his tomato crop, but not his bedding plants.

Tomato spotted wilt virus is an obligate parasite; it must have a living host and must be moved from one plant to another by thrips or through cuttings or possibly seed. TSWV is transmitted most efficiently by Western flower thrips (WFT) (Frankliniella occidentalis), and less so by Onion thrips (Thrips tabaci), Tobacco thrips (Frankliniella fusca) and a few other thrips species. It is not transmitted by Eastern flower thrips (Frankliniella tritici).

WFT completes its life cycle in about 10-18 days. Eggs are laid in the leaf or tomato fruit. When WFT oviposit into tomato fruit they often cause a deeper dimple (black arrows Fig. 1) than other thrips species and very often the dimple is surrounded by a halo of white tomato tissue (white arrow Fig 1). This is how I could tell the grower had WFT present at one time (when I visited I could find no live thrips) at fairly high levels by the ovipositioning marks on his tomatoes. Larvae hatch in about three days and immediately begin to feed and in so doing pick up the virus. After four days, they pupate in the soil, and in a little over three days, the pupae become adults. Only immature thrips can acquire the virus, which they can acquire within 15 minutes of feeding, but adults are just about the only stage able to transmit the virus. Adults can transmit the virus for weeks.

TSWV infected leaves may show small, dark-brown spots (Fig. 2) or streaks on stems and leaf petioles (we found one prickly lettuce weed with such a symptom). Growing tips are usually affected with systemic necrosis and potentially stunted growth. Tomato fruit will have mottled, light green or yellow rings usually with raised centers (Fig 3).

Weed hosts function as important virus reservoirs for TSWV and can survive in and around greenhouses or even high tunnels through the winter. Some of these weeds include prickly lettuce, chickweed, (Fig. 4) spiny amaranth, lambquarters, black nightshade, shepherd’s purse, galinsoga and burdock. This grower unfortunately had a good crop of prickly lettuce at one end of his high tunnel.

The grower had been able to control his thrips populations with spinosad, but western flower thrips are notorious for developing resistance and sure enough have developed resistance to this insecticide in many greenhouses. So populations of WFT increased and with the weeds that were around and in the high tunnel some of which tested positive for TSWV, but negative for INSV, it was a perfect scenario for an outbreak of TSWV. I should note here that we tested for both INSV and TSWV on the tomatoes, weeds and impatiens. Only TSWV was found in the tomato and the weeds. No INSV was found in any sample. Although both viruses are transmitted by the same thrips species these viruses tend to infect either bedding plants (INSV) or tomato/pepper plants (TSWV). The grower threw out all his infected tomato plants and is in the process of killing his weeds in and around his high tunnel. He was able to get control of his thrips in his bedding plants using combinations of pylon and pyrethroids. One variety of tomato the grower was growing that did not show any symptoms of TSWV, even though it was right next to the other infected varieties was Mountain Glory.

Fig 1 Tomato fruit with WFT ovipostion marks

Fig. 2 Tomato leaves with TSWV symptoms and positive immunostrip (two black arrows; Agdia, Inc)

Fig. 3 Tomato fruit with TSWV symptoms

 

Fig. 4 Two common weed hosts of TSWV; prickly lettuce and chickweed

 

Thrips on Winter Annuals

Thursday, April 14th, 2011

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

 

Western Flower Thrips on Our Doorstep

Friday, June 20th, 2008

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

I have been working with researchers from Virginia Tech on thrips identification in their crops. They had a severe problem with thrips, especially Western flower thrips (WFT) Frankliniella occidentalis, last summer in tomato and cotton. This year they wanted to see if they were getting early (April and May) populations of Western flower thrips in their fields. Sure enough, the samples I saw from tomatoes, cotton and some winter annual weeds all had 20-50% of their thrips population as Western flower thrips. Southern parts of Maryland and the southernmost part of Delaware will probably see these Western flower thrips populations move into the area in late June and early July. This is another good reason NOT to apply any pesticides to tomato unless absolutely needed, especially early in the season. My research has shown that Western flower thrips are consistently worse on farms in our area that use pesticides on a weekly basis, whether their use is warranted or not. WFT are worse because they are usually resistant to many of the pesticides we commonly use and the frequent sprays greatly reduce natural enemies of WFT. Save your chemical sprays for later in the season when worms, thrips, mites and stink bugs may become major problems.

Thrips in Strawberry Flowers Rarely a Problem

Friday, May 23rd, 2008

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

In several strawberry fields I have visited over the last two weeks I have found thrips in the flowers. Almost all of the thrips were eastern flower thrips. The number of thrips found in a flower varied from 0 to 10. Thrips were feeding on pollen and not on any part of the ovary. Though studies vary as to the number of thrips needed per flower before they are considered above threshold, the studies agree that 10 thrips/flower are needed before any treatment is justified. After the fruit begins to develop and there is little pollen left in the flower most thrips will leave the fruit. However, if the thrips do not leave they will begin to feed on the developing fruit and seeds, interfering with proper fruit development. Densities of 3-5 thrips/fruit can lead to fruit damage and thrips populations need to be closely watched during the first two to three weeks after fruit set.

Thrips Overwinter on Winter Annuals in Maryland

Friday, April 11th, 2008

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.

 overwintering thrips December January 

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.

 overwintering thrips March

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.

 

Proportion overwintering thrips species

Figure 3. The proportion of thrips species found to overwinter at the 12 sample sites.