Biological Control: Western Flower Thrips in Spring Bedding Plants: Which Formulation of Mites is Best?

Western flower thrips (WFT) (Frankliniella occidentalis) is one of the most important insect pests of bedding plant in Massachusetts, New York, and the northeastern United States in general. Pesticides can be used for its control, but there are problems because this pest has developed resistance to many materials, including dimethoate, cyfluthrin, permethrin, bifenthrin, chlorpyrifos, methomyl, and abamectin (Robb, 1989, Robb et al. 1995, Immaraju et al. 1992). Thrips have also developed resistance to spinosad (Conserve®) which at the time of this research, was a new pesticide that provided good control of WFT.  Of the various natural enemies of WFT, the most promising species are predatory mites in the family Phytoseiidae (Van Driesche et al. 1998) and among these, Neoseiulus (=Amblyseius) cucumeris is the only species that is easily available from commercial suppliers.

Past studies of the ability of predacious mites to suppress thrips in flower crops have produced mixed results in different parts of the United States. In California, releases of the predacious mites Neoseiulus cucumeris (Ouds.) or N. barkeri (Hughes) on chrysanthemum at 2.5 mites per leaf were unable to reduce western flower thrips densities below 2-7 per leaf (Hessein and Parrella 1990), an unacceptable level for mums. However, in Maryland, Gill (1994) found that releases of N. cucumeris in a slow release formulation reduced the number of pesticide applications needed for management of western flower thrips in bedding plants from 3.6 to only 0.4.

To gain information on how well this mite would suppress WFT in bedding plant crops under commercial conditions in the northeastern United States we ran a trial in spring of 2000 at two commercial growers in Massachusetts and one in New York. N. cucumeris is sold in two different formulations: bulk material and slow release sachets. The former is just live mites mixed in a carrier like bran or vermiculite, while the latter is a small bag that contains a colony of the mite together with live prey (a harmless grain mite) together with food for the grain mite. Mites continue to emerge for up to 6 weeks from these sachets. The purposes of our trial were two: (1) to see how well biological mite control worked compared to chemical control in commercial bedding plants in our area and (2) to see if one formulation was better than the other. This information will be used in the future together with information on the compatibility of Conserve with this mite to design a integrated control program for WFT that should be suitable for long term use, and be less vulnerable to the development of WFT resistance to Conserve.

How the trial was run

Design. We replicated our experiment three times, each replicate being a set of two or three greenhouses at a different grower. We worked with two growers in the Connecticut River Valley, near Amherst, MA and with one in Warwick, New York (Orange Co.). At both of the MA sites, we followed thrips populations in three greenhouses. Two greenhouses received mite applications, one as bulk releases and the other as sachets. In the third greenhouse, WFT was suppressed by insecticide use, based on the grower's decisions of what and when to apply. At both MA sites, greenhouses were filled with flats of a diverse array of bedding plants. In NY, there were only two greenhouses, one a bulk release and one a sachet greenhouse. There was no chemical control check greenhouse. Both NY greenhouses were filled with potted dahlias.

Release rates. In the greenhouses receiving bulk releases of N. cucumeris, we followed the manufacturer's recommended release rate of 10,000 mites per 1000 sq. ft. We chose to make releases in weeks 1, 2, 3, 5 and 7 of the 10 week crop in MA, yielding a total application in the 10 week crop of 50,000 mites per 1000 sq. ft (or expressed per week, 5000 mites per wk per 1000 sq. ft.). In NY, releases were made at the rate of 20,000 mites per 1000 sq. ft in weeks 1, 3, 5, 7, 9, 11, 13 of a 14 week crop, yielding a total application in the 14 week cop of 140,000 mites (or expressed per week, 10,000 mites per week per 1000 sq. ft.).

For mites applied in the sachet formulation, the recommended release rate is 1 sachet per 2.5 m2 (=37 sachets per 1000 sq. ft, or 1 per 27 sq. ft), as a preventative measure. Sachets are reported to last 8 weeks, but the manufacturer recommends replacing after 6 weeks for bedding plants. In MA, two deployments of sachets (37 per 1000 sq. ft) were made, one in week 1 and one in week 6, the middle of the cropping period. In NY, sachets were deployed in week 1 at a rate of 40 sachets per 1000 sq. ft. Subsequently one third of the original sachets in NY were replaced with new sachets in weeks 3, 5 and 7. The second set of sachets was then replaced, in thirds, in weeks 9, 11, and 13.

In both MA and NY, one release was made of the ground dwelling mite Hypoasis miles at a rate of 10,000 per 1000 sq. ft onto the media (or in the pots) at the beginning of the crop. In both the sachet and bulk release houses, some of the bulk release mixture of A. cucumeris was placed into each hanging basket because mites cannot reach baskets easily by natural dispersal. This will be done by allocating about 5% of the total release in the house to this purpose. In the sachet greenhouse, sachets were torn open and material sprinkled into pots.

Thrips sampling. To measure the efficacy of each method of WFT control, we counted the number of thrips caught on yellow sticky cards made by cutting standard sized cards (3x5") in half. Each half card was a sample unit, and was counted on each side (summed), once per week, and old cards replaced by new ones. There were 20 such half card sample units per greenhouse. Cards were held by clips on sticks stuck into pots or flats. Cards were distributed throughout the greenhouse more or less evenly, placing some cards in most of the various kinds of bedding plants present. Counts were made in the greenhouse with an Optivisor, supplemented as needed with a 10X hand lens.

Results of trial

At one of the two MA sites, where there were grower-run pesticide check greenhouses, the grower did not actually apply any insecticides, even though that was the grower's intention. Consequently, we cannot compare the effect of the mites to chemical control, except in one case, for which biological control gave a greater degree of thrips suppression than did chemical control. At all three sites, WFT thrips catches were lower for the bulk release formulation than for the use of sachets, although at one MA site, this difference was small. Also, all three growers were satisfied with the quality of their plants at the end of the crop, indicating that use of biological control led to production of high quality plants. Finally, the cost of the use of the better method (bulk release), given the rate and release frequency used in the MA sites in this trial, was calculated as 2.9 cents per sq. ft. of production, not including shipping cost, given a purchase price of $57.70 per 100,000 mites and a total seasonal application in the crop of 50,000 mites per 1000 sq. ft. The effect of shipping cost on total cost per sq. ft of using biological control varies with grower size, being smallest in larger greenhouses because shipping cost (about $11 per shipment) is spread over more square feet of production at larger greenhouses.

Acknowledgments

We thank the participating greenhouse growers and Massachusetts IPM Program for financial support.

References

• Gill, S. 1994. Thrips management and biological control. GrowerTalks 58(6):36-40.
• Hessien, N.A. and M.P. Parrella. 1990. Predatory mites help control thrips on floriculture crops. California Agriculture. 44:19-21.
• Immaraju, J.A., T.D. Paine, J.A. Bethke, K.L. Robb and J.P. Newman. 1992. Western flower thrips (Thysanoptera: Thripidae) resistance to insecticides in coastal California greenhouses. Journal of Economic Entomology 85: 9-14.
• Robb, K.L. 1989. Analysis of Frankliniella occidentalis (Pergande) as a pest of floricultural crops in California greenhouses. Ph.D. Dissertation, University of California, Riverside. 135 pp.
• Robb, K.L., J. Newman, J.K. Virzi, and M.P. Parrella. 1995. Insecticide resistance in western flower thrips, pp. 341-346. In B.L. Parker, M. Skinner and T. Lewis (eds.). Thrips Biology and Management. NATO ASI Series. Series A.: Life Sciences Vol. 276.
• Van Driesche, R. G., K. M. Heinz, J. C. van Lenteren, A. Loomans, R. Wick, T. Smith, P. Lopes, J. P. Sanderson, M. Daughtrey, and M. Brownbridge. 1998. Western flower thrips in greenhouses: A review of its biological control and other methods. UMass Extension Floral Facts, University of Massachusetts, Amherst, MA