Effect of Enzone™ as a Soil Fumigant on Survival of Various Developmental Stages of Diaprepes abbreviatus (Coleoptera: Curculionidae) in Container-Grown Citrus

Diaprepes abbreviatus L., a root weevil native to the Caribbean region (Woodruff 1985), has become an important pest of citrus and ornamental plants in Florida since its introduction over 30 years ago (McCoy 1999). Both the adult and larval stages of D. abbreviatus are polyphagous, feeding on the leaves and roots of about 270 plant species (Simpson et al. 1996). Plants that support larval development to pupation, a period of 5-15 months, include citrus, sugarcane, various woody ornamentals, and several agronomic crops (Schroeder et al. 1979). Injury to citrus by the adult is characterized by a notching of the leaf margin while larval feeding destroys fibrous roots and the bark of the tap, lateral, and crown roots (Quintela et al. 1998). D. abbreviatus is a univoltine species with the adult, egg, and neonate stages appearing on the host plant aboveground, whereas all larval stages, pupae, and teneral adults reside belowground (Wolcott 1933). The potential economic impact of D. abbreviatus on commercial growers of citrus and ornamental plants in nurseries and the field is significant. An estimated loss of $75 million annually has been reported from tree decline and lost production in open forum among citrus growers (Diaprepes Task Force 1997). About 100 commercial plant nurseries are infested throughout Florida. Sale of infested trees from nurseries offer one of many ways by which the weevil can be disseminated throughout the state. It is imperative that these nurseries do not sell liners infected with weevil larvae. All weevil-infested nurseries operate under a compliance agreement with the Florida Department of Agriculture and Consumer Services that regulates the movement of nursery stock (McCoy 1999). Larval control in infested nurseries must be performed using an approved chemical or mechanical treatment of potting media in containers. Currently, bifenthrin, formulated as Talstar® 10 WP and Talstar T & O granular are the only approved regulatory chemical treatments (McCoy et al. 1995). Talstar® is a soil barrier treatment applied to prevent neonate invasion of the soil. It is, however, ineffective against later instars already infesting the plant roots. To find a soil treatment effective against all developmental stages of D. abbreviatus , two greenhouse studies were initiated to determine the effect of sodium tetrathiocarbonate (EnzoneTM 31.8%) as a soil fumigant on all developmental stages of D. abbreviatus . The active ingredient decomposes in the soil environment to release carbon disulphide, a broad spectrum biocide of plant parasitic nematodes, grape phylloxera and some soil fungi (Hinds 1902, Young 1990, Weber et al. 1996). The formulation exhibits very low phytotoxicity and is environmentally benign. In both tests, 3-year-old Marsh grapefruit ( Citrus × paradisi Macfad) trees grafted to Swingle Citrumelo rootstocks ( × Citron cirus ‘Swingle’) were bare rooted and pruned lightly for transplanting into 15 liter plastic containers. Each tree was planted in sieved Candler soil (Entisol type; 92% sand, 2.9% clay, 2.0% silt) at a maximum soil depth of 26.7 cm (soil volume/pot = 0.0179 m 3 , soil surface area = 670 cm 2 ). Trees were placed on a bench in an air-conditioned greenhouse maintained at 25.5-26.5 ° C, where they received regular watering and 60 ml of liquid fertilizer (8:4:8) per tree every 2 weeks. Any weeds growing on the soil surface were removed by hand. In test 1, 100 neonate D. abbreviatus (48 hours old) were scattered on the soil surface next to the trunk of each containerized tree each week for 6 consecutive weeks prior to treatment. In addition, five, 6 th instars, were placed in each pot on four different occasions beginning at the 6 th week after neonate inoculations were begun. In test 2, 100 neonates of the above age were added to the containerized trees twice at 5 and 6 weeks prior to treatment. Ten, 7 th