Spotted Fireworm Control on Cranberries, 2021

This experiment tested the residual activity of Leprotec (Bacillus thuringiensis kurstaki) alone and in combination with Spear-Lep (GS-omega/kappa-Hxtx-Hv1a) or Basin Flex (U1-AGTX-Ta1b-QA), Verdepryn 100SL (cyclaniliprole), and Intrepid 2F (methoxyfenozide) for controlling early and late instars of spotted fireworm in cranberries. The insecticide treatments and rates were as follows: Leprotec at 1 pt/acre, Leprotec at 1 pt/acre + Spear-Lep at 2 pt/acre, Leprotec at 1pt/acre + Basin-Flex at 7.5 oz/acre, Verdepryn 100SL at 11 fl oz/acre, and Intrepid 2F at 16 fl oz/acre. In addition, an untreated (control) treatment that received no insecticide was included, for a total of six treatments. The experiment was conducted in a cranberry bed, var. Ben Lear, located at the Rutgers P.E. Marucci Center in Chatsworth, New Jersey. Treatments were applied to 3 ft × 3 ft (9 ft²) (= 0.91 × 0.91 m, 0.83 m²) plots, and each treatment was replicated seven times in a CRB design, blocked by replicate. Applications were made with an R&D CO₂ backpack sprayer, using (1-liter) plastic bottles. The sprayer was calibrated to deliver 50 gal of volume per acre at 25 psi, using a single T-jet VS 110015 nozzle, yielding 39.1 ml per plot. Plots were treated on 12 July 2021 at 5:00–6:00 h and were sampled 1, 3, and 7 DAT. Insecticide residual samples were taken on 13, 15, and 19 July 2021.

Treated uprights were randomly selected from the center of each plot, i.e., 15 cm from the plot’s edges, clipped, and taken to the laboratory. In the laboratory, three insecticide-treated uprights were inserted into a florists’ water pick that was fitted through a hole cut in the bottom of a 32-oz (946-ml) Deli cup container (PrimeSource; Chattanooga, TN). For both instars, seven containers were setup for each treatment, totaling 42 containers for all treatments for each of the three residual dates. Water picks with the containers were secured through holes in the lids of plastic shoeboxes filled with water. Three first instars or two third instars were placed inside each container. Larvae used in the experiment were obtained from a laboratory colony kept at the Rutgers P.E. Marucci Center. Each container was considered a replicate. Containers with plants and insects were placed in the laboratory at approximately 27°C, on a 15:9 L:D cycle. Mortality of both instars was assessed 6–7 d after setup, the 1-DAT assay was setup on 13 July and assessed on 19 July; the 3-DAT assay was setup on 15 July and assessed on 22 July; and the 7-DAT assay was setup on 19 July and assessed on 26 July. It rained on 12, 17, and 18 July, with a total daily precipitation of 0.07 inch (0.18 cm), 0.25 inch (0.64 cm), and 0.19 inch (0.48 cm), respectively. Number of larvae alive, moribund, dead, or missing was recorded for each container. The numbers of moribund and dead larvae were combined prior to analysis. Data were analyzed using ANOVA and means separation by Fisher’s LSD test at P = 0.05. Count data were Ln(X + 0.01) transformed prior to analysis.

At 1 DAT, Leprotec + Spear-Lep, Leprotec + Basin-Flex, Verdepryn, and Intrepid provided >90% control on early-instar spotted fireworm larvae (Table 1). For early-instar larvae, mortality remained high (>90%) for the Verdepryn and Intrepid treatments at 3 DAT and 7 DAT; however, it dropped to 0% at 7 DAT for the Leprotec + Spear-Lep and Leprotec + Basin-Flex treatments (Table 1). In comparison, Leprotec alone resulted in lower (68%) mortality of early instars at 1 DAT that declined to 0% at 7 DAT (Table 1).

At 1 DAT, Verdepryn and Intrepid provided >90% control on late-instar spotted fireworm larvae; the toxicity of these two insecticides remained relatively high after 1 wk of treatment (Table 2). In contrast, application of Leprotec alone or in combination with Spear-Lep or Basin-Flex resulted in ≤20% mortality of late-instar spotted fireworm larvae at all residual dates evaluated (Table 2).

No phytoxicity symptoms were observed following any of the insecticide treatments.¹

Footnotes