Insecticide Efficacy Against Tropical Sod Webworm in Concentration-Mortality Bioassay, 2021

The objective of this study was to determine the susceptibility of field-derived tropical sod webworm to insecticides commonly adopted to manage pests in the turfgrass system. This study was performed during 2021 at the West Florida Research and Education Center (WFREC), University of Florida at Jay, FL. A field-derived colony of tropical sod webworm was established from larvae collected in commercial and experimental turfgrass fields during the 2020 summer in the Florida Gulf Coast region. The insects captured were maintained at 25 ± 2°C, 70 ± 10% relative humidity, and 14:10 (L:D) h photoperiod. Emerged moths were placed in rearing cages (W30 × D30 × H30 cm) and fed with a solution of 10% honey. The eggs were collected and transferred to 473-ml polypropylene containers (Fabri-Kal Corp., Kalamazoo, MI) until hatched. In each generation, the tropical sod webworm neonates were placed in 2.5-ml plastic cups containing a multispecies lepidopteran diet (Southland Products, Lake Village, AR) until adult emergence.

Concentration‐mortality bioassays were conducted using third instar tropical sod webworm larvae. Dilutions of the commercial insecticides were prepared in distilled water without any adjuvants. Acelepryn (chlorantraniliprole), Advion (indoxacarb), Arena (clothianidin), Talstar (bifenthrin) insecticides were selected based on the high adoption in turfgrass system and seven concentrations of each insecticide were tested, including a control (water only). The concentrations varied according to each insecticide recommendation rate for tropical sod webworm management. In brief, the dilutions were prepared by taking the label rate to control the tropical sod webworm larvae in turfgrass as a starting point and then making serial dilutions in a logarithmic (multiplicative inverse) scale.

Since there is no recommended method from Insecticide Resistance Action Committee (IRAC) for this species, a method was adapted from the leaf-dip bioassay IRAC Method, n. 007. Method, n. 007 is recommended for insecticide bioassays with other lepidopteran pests. The bioassays were conducted using a 5-cm stolon of St. Augustine grass (Stenotaphrum secundatum). The stolon was cut, cleaned in a solution of 0.1% bleach, and dipped individually into the insecticide dilution for 5 s with gentle agitation. The stolon was placed on paper toweling, allowed to dry for ≈5 min, and carefully transferred to rearing cups covered in the bottom with a fine layer of agar. The agar was prepared previously and sterilized for 15 min in UV light. Four tropical sob webworm larvae were transferred to each stolon. The cups were closed and placed in a growth chamber at 25 ± 2°C, 70 ± 10% relative humidity and 14:10 (L:D) h photoperiod. The bioassays were replicated five times using 20 larvae per concentration. Larval mortality was recorded after 48 h of exposure. The larvae were considered dead if they did not move when prodded with a fine paintbrush. Larval mortality data were analyzed using probit regression in Polo Plus v1.0, adjusting for natural mortality when needed. The susceptibility parameters estimated were the LC₅₀ and their respective 95% confidence limits (95% CL), slope ± SE for the response curves, and % mortality at the field label rate of each insecticide.

The probit analysis and percent mortality at field label rate are underlined in Table 1. Tropical sod webworm showed similar and high susceptibility to Acelepryn (chlorantraniliprole), Advion (indoxacarb), and Arena (clothianidin) insecticides. The lethal concentration to kill 50% of the population varied from 0.40 to 0.57 g of active ingredient per liter of solution. The same insecticides caused over 95% mortality when tested at a field label recommended rate to manage the tropical sod webworm. Talstar insecticide caused low mortality of 5% at the field label rate. Our results showed evidence of bifenthrin field resistance on the tropical sod webworm population from the U.S. Gulf Coast. Our study provides information to support the design of insect resistance management in the turfgrass system.¹

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