Ultraviolet-blocking Greenhouse Plastic Films for Management of Insect Pests

Abstract

Most greenhouse polyethylene plastic films contain ultraviolet (UV) light-blocking components added to prolong the life of the material and maintain high levels of transmission of photosynthetically active radiation (PAR). Standard greenhouse films generally block the majority of UV light transmission at wavelengths below 360 nm; however, some materials block a greater portion of the UV light spectrum by blocking transmission below 380 nm (Antignus et al., 1996; Costa and Robb, 1999). Previous studies conducted in completely enclosed greenhouses reported reductions in whitefly, aphid, and thrips infestations in vegetable crops grown under <380-nm blocking plastic compared to those grown under <360-nm blocking plastics (Antignus et al., 1996, 2001). The presumed method of insect population reduction is the alteration of normal UV wavelength patterns used by insects during orientation and flight (Antignus, 2000; Antignus et al., 1996, 2001). When silverleaf whitefly (Bemisia argentifolii Bellows and Perring), greenhouse whitefly (Trialeurodes vaporariorum Westwood), or western flower thrips [Frankliniella occidentalis (Pergande)] were released in small, completely enclosed tunnels, sticky trap catches were lower under <380-nm plastics than <360-nm plastics, indicating a preference to enter tunnels that transmitted more UV light (Costa and Robb 1999; Costa et al., 2002). Greenhouse structures in Southern California and other moderate climates often have sides or roof vents that open to allow natural ventilation and cooling. A previous study on flowering crops conducted in small (4 × 8 × 3 m) open-sided hoop-houses found significantly with six to eight yellow sticky traps per treatment (each 10 ×18 cm; Seabright, Emeryville, Calif.) adjusted to plant canopy height. Traps were replaced and the number of insects recorded weekly. In addition, three leaves from five plants per bay (30–40 plants per treatment) were randomly sampled weekly and the total number of insects present was recorded. Greenhouse whitefly and western flower thrips were the dominant insect species observed in these trials. Over the course of the three replicates, the mean number of whiteflies and thrips trapped on sticky cards under the <380-nm plastic-covered houses was not significantly different from the number trapped under the standard <360-nm plastic (whiteflies: F = 0.64, df = 1374, P = 0.426; thrips: F = 2.14, df = 1374, P < 0.145). Similarly, there was no significant difference in the mean numbers of whiteflies (F = 0.08 = df = 1374, P = 0.777) or thrips (F < 0.01, df = 1374, P < 0.963) counted on plants under the two types of plastics. However, the mean number of leafminers (Liriomyza sp.) trapped on sticky cards and counted on plants under the <380nm plastic-covered houses was significantly lower than the number trapped under <360-nm plastic (cards: F = 26.81, df = 1374, P < 0.001; plants: F = 14.35, df = 1374, P < 0.001). Aphid populations were too low to make meaningful statistical comparisons. In a previous study using smaller opensided greenhouses, we found that populations of thrips were lower under <380-nm plastics compared to <360-nm (Costa et al., 2002). However, no effects on thrips populations were observed in this study. Thus, it appears that increasing the overall covered area did not increase the effectiveness of the <380-nm plastic in reducing insect pests. In the open-sided greenhouses the presence of some unfiltered light and the passive movement of insects into the greenhouses by wind may have reduced effects of the UV-blocking materials. The benefits to integrating the use of high-UV-blocking plastics into an existing pest management program will vary with the type of greenhouse structures used and types of insect and disease pressures commonly encountered. The effect of the high UV-blocking material on insects has been greatest when used in completely closed structures with little or no unfiltered light entering the system.