Evaluation of deltamethrin-impregnated nets as a protective barrier against Western flower thrips, Frankliniella occidentalis (Thysanoptera: Thripidae) under laboratory and greenhouse conditions

Thrips are major pests of vegetables and ornamental plants grown under protective structures, and can penetrate all but the finest insect screens; i.e. thrips exclusion screening has recommended hole-diameter < 0.2 mm. We investigated a modern long-lasting insecticidal net (LLIN), with relatively larger hole size compared with thrips exclusion screening, as a barrier treatment for Western flower thrips, Frankliniella occidentalis Pergande under laboratory and greenhouse conditions. Theoretically, treated nets permit larger mesh sizes compared with untreated insect screening, improving light penetration and ventilation inside protected environments. In residual exposure bioassays, the LT50 and LT90 values for adult female F. occidentalis exposed to a commercial 0.4% w/w deltamethrin net (D-Terrence) with hole size 1.8 mm diameter, were 3.96 and 8.99 min, compared with 1.86 and 5.30 min, respectively, for males. In greenhouse cage tests, 43% fewer thrips penetrated black D-Terrence netting when compared with equivalent untreated nets, confirming the benefit of the insecticidal treatments. In another test, fewer thrips penetrated yellow D-Terrence netting when compared with black net. However, treated netting failed to prevent all thrips from penetrating through the net and, in subsequent tests, establishing on bean plants covered by the D-Terrence nets. We conclude that LLIN show promise in greenhouse applications, but require screening with smaller hole sizes (i.e. < 1.8 mm) for protection protect against thrips or similarly sized pests.