Spinosad and nucleopolyhedrovirus mixtures for control of Spodoptera frugiperda (Lepidoptera: Noctuidae) in maize
Introduction
The formulation of entomopathogens can greatly affect their efficiency as biological insecticides (Burges and Jones, 1998). Specifically, formulation can influence the stability of the pathogen in storage and the efficiency of the application to the crop. Moreover, certain formulation adjuvants can enhance the activity of the pathogen and improve environmental persistence (Jones et al., 1997). One way to increase the activity of the pathogen is to mix it with small quantities of synergistic substances such as optical brighteners (Shapiro and Dougherty, 1994), inorganic acids (Cisneros et al., 2002b; Shapiro and Bell, 1982) or sublethal concentrations of chemical insecticides (Peters and Coaker, 1993). However, the interaction between a pathogen and other compounds may also be antagonistic due to decreased feeding or a change of gut pH (Chancey et al., 1973; Fuxa, 1979; Pingel and Lewis, 1999) or each entity may act independently, leading to additive mortality (Koppenhöfer and Kaya, 2000; McVay et al., 1977).
Spinosad (Dow AgroSciences) is a mixture of spinosyns A and D produced during fermentation of the soil actinomycete Saccharopolyspora spinosa Mertz and Yao (Sparks et al., 1998). Spinosad is a neurotoxin with a novel mode of action involving the nicotinic acetylcholine receptor and probably GABA receptors as well (Salgado, 1997, Salgado, 1998). Exposure causes a cessation of feeding followed, some 24 h later, by paralysis and death. Spinosad is primarily a stomach poison with some contact activity and is particularly toxic to Lepidoptera and Diptera. However, toxicity tests indicate that Spinosad has virtually no toxicity to birds and mammals and relatively low toxicity to certain insect natural enemies (Bret et al., 1997), although a number of insect predators and parasitoids appear to be susceptible to Spinosad intoxication (Cisneros et al., 2002a; Elzen et al., 2000; Tillman and Mulrooney, 2000). Spinosad is classified by the US Environmental Protection Agency as an environmentally and toxicologically reduced risk material (Saunders and Bret, 1997).
Larvae of the fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) are the principal pests of maize production in Mesoamerica. Infestation levels over 55% can cause a 15–73% reduction in crop yield (Hruska and Gould, 1997). The crop damage caused by S. frugiperda larvae is highly apparent and growers often apply synthetic insecticides in spray and granular formulations to control the pest. However, the incorrect use of chemical insecticides by resource-poor rural growers results in a high prevalence of chronic pesticide poisoning in farm workers from southern Mexico and Nicaragua (Hunt et al., 1999; McConnell and Hruska, 1993).
Given the need for safe, sustainable, and economical pest control for Mesoamerican maize farmers, we have been evaluating the multinucleocapsid nucleopolyhedrovirus of S. frugiperda (SfMNPV) as a biological insecticide. Spray application between 1.2×1012 and 6×1012 viral occlusion bodies (OBs)/ha in water results in approximately 40% infection of S. frugiperda larvae collected at 2 days post-application and reared in the laboratory until death or pupation (Martı́nez et al., 2000). Natural parasitism typically contributes an additional 20% mortality giving an overall prevalence of around 60%.
Formulation may improve this degree of control in two different ways. First, the use of feeding stimulants may increase consumption of virus inoculum by the target pest. Granular phagostimulant formulations based on nixtamalized maize flour have recently been shown to significantly increase control of lepidopteran pests using Bacillus thuringiensis Berliner (Tamez-Guerra et al., 1998, Tamez-Guerra et al., 2000) and SfMNPV (Castillejos et al., 2002). Second, the efficacy of virus treatments may be increased by the incorporation of substances, such as optical brighteners, that enhance the activity of the virus (Hamm, 1999) or insecticidal substances that cause complimentary mortality, resulting in improved pest control (Morris et al., 1974).
The objectives of the present study were to characterize the interaction between SfMNPV and very low concentrations of Spinosad and to determine the feasibility of using SfMNPV–Spinosad mixtures for control of S. frugiperda in maize. For this, we performed laboratory bioassays of virus and Spinosad alone and in mixtures. We then performed a field trial to assess the degree of pest control achieved by SfMNPV–Spinosad mixtures. Finally, we evaluated the possible impact of low concentration Spinosad applications on non-target arthropods present in the maize crop.
Section snippets
Bioassays
To determine the activity of an SfMNPV isolate previously characterized by Escribano et al. (1999), bioassays were performed based on the technique described by Del Rincón-Castro and Ibarra (1997). All laboratory procedures were performed at , 75–85% RH, and 12 h:12 h L:D photoperiod. Occlusion bodies (OBs) were produced in fourth-instar S. frugiperda larvae individually maintained in 25 ml plastic cups containing a semi-synthetic diet based on soya and maize without formaldehyde (modified
Bioassay
The LC50 value for SfMNPV was calculated at 70.3 OBs/mm2 of diet surface (range of 95% C.L.: 53.0–91.6 OBs/mm2). The probit regression equation was y=0.81log(x)+3.50 . Larvae died 5–12 days post-inoculation as reported previously for this isolate (Cisneros et al., 2002b; Martı́nez et al., 2000). There were no viral deaths in the controls. The LC50 calculated for Spinosad was 2.98 ppm (range of 95% C.L.: 2.25–4.06 ppm). The logit regression equation was y=0.49loge(x)−1.47 (in
Discussion
A recent survey of biopesticide researchers working in developing countries indicated that formulation was the most important issue in the development of biological insecticides (Harris and Dent, 2000). As spinosyns are produced by fermentation of an actinomycete, Spinosad has been classified as a biopesticide (Copping and Menn, 2000), although it has clearly insecticidal characteristics that differ from the majority of entomopathogen-based biopesticides (Cisneros et al., 2002a; Salgado, 1998).
Acknowledgements
We are grateful to Vasty Castillejos, Jaime Jiménez, Anaximandro Gómez, Lulu Gálvez, and Maurilio López for assistance in the field trials, Haiganoush Preisler for very helpful comments on the threshold tolerance analysis, Juan Antonio Garcı́a (Dow AgroSciences, Mexico) for a gift of Spinosad, and Gerardo Hernández who provided ingredients for the Spodoptera colony. The work received financial support from SIBEJ 0501047 and through a British Council Link scheme.
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