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| Figure 1. Flower Thrips: Water Traps, Ohio, 1995 | Figure 2. Thrips Data (Sticky Traps): Water Traps, Ohio, 1995 | |
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| Figure 13. Flower Thrips: Sticky Traps, Ohio, 1996 |
Roger N. Williams, M. Sean Ellis, Dan S. Fickle, and Carl M. Felland
A major outbreak of flower thrips, Frankliniella tritici (Fitch), in 1994 in northeastern North America caused extensive damage in commercial strawberries in several states including Indiana, Ohio, Pennsylvania, and Maryland (3, 7). Infestations were first observed in late May just prior to the beginning of harvest. Losses ranged from 0 to 75 percent, with significant losses in pick-your-own operations in which insecticides were discontinued early or not applied at all to allay concerns about pesticide residues as compared to conventionally treated fields.
Flower thrips invade annually from the South and are carried northward on frontal systems in early spring (5). This pest was formerly known as the strawberry thrips and was reported to have caused destructive outbreaks from Florida to Illinois (6). There was great concern by growers and crop consultants that the flower thrips might return again in high numbers in 1995 and 1996. This speculation was strengthened by the fact that in the Canadian Maritime Province of New Brunswick the flower thrips has been a major concern annually since the mid-1980s (4).
Three sites in two Ohio counties were monitored for the flower thrips, Franklinella tritici (Fitch), in 1995 and 1996.
1995
In order to monitor the occurrence of flower thrips in Ohio and determine the best trapping method to achieve this goal, two different trapping regimes were employed in 1995, water traps and yellow sticky traps. The water traps consisted of rectangular, clear plastic containers (32 x 25 x 10 cm) filled with a solution of surfactant, glycerin, salt, and water in a ratio of 1:2:3:220. These traps were placed within the strawberry canopy (matted rows) allowing the foliage to eventually envelop the sides of the traps.
On May 2, 1995, six water traps were employed at two Wayne County sites, Moreland Fruit Farm and Snyder Farm, and one Warren County site, Valley Vineyard. The Moreland Fruit Farm is a commercial fruit and vegetable farm approximately 12.9 km south of Wooster, Ohio. Snyder Farm lies approximately 3.2 km south of Wooster, Ohio, and is the primary research farm for The Ohio State University's Ohio Agricultural Research and Development Center. However, the latter site differed from the others in that strawberries were not being grown in the immediate vicinity of the traps. The Warren County site is approximately 1.6 km south of Morrow, Ohio, and produces many types of fruits and vegetables for commercial sales. Water traps were collected and refilled weekly for the duration of the trial which ended on July 11. Specimens were removed from the aqueous solution by straining with a fine, organdy material in the field, then separated and stored in hexane in the lab until they were mounted on slides for identification.
The second trap employed was an unbaited yellow sticky trap, Pherocon® AM, manufactured by Sandoz Ltd., Basel, Switzerland. These traps measure 23 x 28 cm when fully open, and when employed, are folded over backwards, exposing two opposite yellow, sticky surfaces each measuring 23 x 14 cm. Yellow has been shown to be significantly better than other colors in attracting Frankliniella tritici (2). Traps were fastened to the tops of 51 cm metal plot stakes with large paper clamps. The stakes were driven into the ground approximately 10 cm prior to attaching the trap. This arrangement placed the bottoms of the traps about 30 cm above the soil level.
On May 4, 1995, four yellow sticky traps were set out at Valley Vineyards in Warren County, Ohio. These traps were placed in the center of the four perimeter borders of a 15 m x 91 m section of a 0.4 ha 'Earliglow' strawberry planting. They were oriented parallel to the borders with two traps facing north-south and two facing east-west. Traps were collected and replaced weekly until they were removed on July 12. Immediately upon returning from each collection trip, all traps were refrigerated until the thrips could be extracted. Upon careful extraction with an insect pin the following day, thrips specimens were placed directly in hexane to remove the adhesive and stored until slide preparation.
All thrips specimens collected in the water and sticky traps were mounted on slides for identification and counting. Specimens were taken out of hexane storage with a fine paint brush and placed in 5% lactic acid (Janusz Piatkowski, personal communication) for approximately one hour for clearing. Thrips were then removed from the lactic acid and placed directly on the slide and then covered with two drops of Permount (Fisher Scientific) and a cover slip. No more than 200 thrips were placed on a single slide.
1996
Only the yellow sticky traps were employed in 1996. The 1995 data had indicated that these traps were superior in collection and much easier in maintenance than the water traps. On April 23, 1996, five Pherocon® AM (no-bait) traps were set out in a new 1 hectare (2.5 acre) strawberry field at the Warren County site. This new planting was established in 1995 and consists of several varieties including 'Earliglow,' 'Red Chief,' and 'Allstar.' A 30 m x 91 m portion of the field was utilized for our trials. The five traps were placed across this area on a northeast-southwest diagonal. The first trap in the series was set with an east-west orientation. Each successive trap was oriented perpendicular to the one that preceded it (three traps faced E-W, two traps faced N-S) to evaluate the effect of trap orientation on collections. Traps were collected on a weekly basis, at which time each trap was replaced with a new one. The exposed traps were returned to the lab for processing in the same manner as in 1995. Trap location and orientation were recorded along with numbers collected on each respective trap. Slides were also prepared in a similar manner as described previously.
On April 26, 1996, another series of five sticky traps was set out on the southwest-northeast diagonal of a 15 m x 182 m field of 'Earliglow' strawberries at the Moreland Fruit Farm site. Trap direction and orientation were similar to those at Valley Vineyard (three traps faced E-W, two traps faced N-S). Traps were collected weekly in the same fashion as in 1995 until they were removed on July 19, 1996. The Snyder Farm site was not surveyed in 1996.
Both collection methods utilized in 1995 were successful in sampling for flower thrips. However, sticky traps were preferred because they required less maintenance and were equal to or better than the water traps at trapping thrips. Monitoring indicated the presence of flower thrips by mid to late May at all locations prior to full bloom (Figures 1 and 2). Numbers were extremely low and did not reach their peaks until early July in northern Ohio and late June in southern Ohio. This was late enough to avoid inflicting any serious economic injury to the strawberry crop. Monitoring of the flower thrips in 1996 again indicated their presence within the northern and southern Ohio plantings prior to and during bloom (Figure 3), but numbers were once again not sufficient to inflict significant crop loss. However, damage to fruit set and development was more prevalent in the northern Ohio planting, which had experienced higher thrips numbers prior to fruit set.
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| Figure 1. Flower Thrips: Water Traps, Ohio, 1995 | Figure 2. Thrips Data (Sticky Traps): Water Traps, Ohio, 1995 | |
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| Figure 13. Flower Thrips: Sticky Traps, Ohio, 1996 |
It is interesting to note that population trends were inverse in 1996 from 1995. The flower thrips population was greater in southern Ohio during 1995, while in 1996 greater numbers were collected at the northern Ohio site. Environmental conditions such as temperature, precipitation, and wind currents along with host plant development play an important role in the early season evolution of the flower thrips population. One such condition, rainfall, in Wayne County, was significant during the month of May in 1995 (avg. 115.4 mm) and 1996 (114.6 mm) as compared with 1994 (46.7 mm) in Wayne County, and a similar pattern was recorded for Warren County. The long-term average is 99 mm. Drought conditions are favorable to allowing a rapid build-up in thrips populations and may have played an important role in the elevated population in 1994 and not in subsequent years which yielded above average rainfall and were unfavorable to thrips reproduction conditions (6). This trend was also true for the three-year period at our Warren County site.
In conclusion, a total of 19,546 flower thrips were collected in 1996, 12,228 at Moreland and 7,318 at Valley. Other thrips were collected in our sampling, but only the F. tritici flower thrips was identified by species and counted. Populations peaked at both sites in mid-June with the Valley peak occurring one week after the Moreland peak on June 14. A relatively steady decline in the population occurs after the last week in June through renovation in mid-July.
Trap orientation (N, S, E, W) was analyzed for both sites and found not to play a significant role in the thrips collections. However, when weekly collections for each trap were analyzed, an interesting trend was indicated. Trap No. 2 collected the greater number of thrips at both sites (Table 1). This was the second trap deployed in the plantings from the northeast corner of the field. At the Valley site this trap was statistically separable from Trap Nos. 4 and 5, and at both sites there was a trend for the first three traps (1, 2, and 3) to capture more thrips than the last two traps (4 and 5), which were in the southwest corner of the plantings.
Table 1. Comparison of Flower Thrips Collections on Sticky Traps in Two Ohio Strawberry Fields, 1996. |
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| Mean No. Thrips | ||
| Trap No. | Wayne Co. | Warren Co. |
| 5 |
157.75 az
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53.09 b
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| 4 |
158.00 a
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55.36 b
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| 3 |
199.58 a
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122.36 ab
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| 2 |
240.92 a
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150.55 ab
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| 1 |
262.50 a
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283.91 a
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| z Means followed by the same letter do not differ significantly according to Duncan's multiple range test (P = 0.05). | ||
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| Figure 4. Map of sticky-trap deployment. |
Although numbers appeared high throughout June and early July in the traps, blossom and fruit samples never reached the level of 10 thrips per blossom which may cause significant crop loss (1). A threshold for the flower thrips has not been firmly established; however, in the New Brunswick area of Canada it has been estimated that as few as two thrips per berry in the early maturity stage may cause damage to 20% of the crop (4). It appears that in 1996, as in 1995, conditions were not conducive to a build-up in the thrips population prior to and during the vulnerable bloom and fruit-development growth stage. The population peaks in mid-June clearly occurred when most plantings were approaching harvest.
In the future, growers who are not using any chemicals for control of the tarnish plant bug or meadow spittle bug, prior to berry set, should be alert as to weather conditions during the pre-bloom period. Dry and warm conditions may be the precursor to a build up in the flower thrips population, which in turn may cause a consequential crop failure. Growers who utilized Thiodan (endosulfan) for insect control in their spray program experienced less crop loss than those who used no insecticide.