Spinosad: A new natural product for stored grain protection

Abstract

Spinosad is a reduced-risk insecticide derived by fermentation from the soil actinomycete, Saccharopolyspora spinosa Mertz & Yao. Spinosad is currently registered in several countries as a grain protectant at a maximum labeled use rate of 1 ppm (1 mg a.i./kg of grain) and with the Maximum Residue Level (MRL) or tolerance on grains set at 1 or 1.5 ppm. Global launch of spinosad as a grain protectant is expected in the near future, pending final acceptance of international residue tolerances for spinosad by major grain importing and exporting countries. Spinosad effectively controls economically important beetle and moth pests associated with stored grain and is also effective against certain psocid species. Spinosad provides grain protection through control of adult and/or immature life stages of pest insects. The pest spectrum of spinosad under commercial grain storage conditions is still being defined, but it is clear from available laboratory and field evaluations on various grains that the lesser grain borer, Rhyzopertha dominica (F.); larger grain borer, Prostephanus truncatus (Horn); rusty grain beetle, Cryptolestes ferrugineus (Stephens); flat grain beetle, Cryptolestes pusillus (Schönherr); red flour beetle, Tribolium castaneum (Herbst); confused flour beetle, Tribolium confusum Jacquelin du Val; Indian meal moth, Plodia interpunctella (Hübner); rice moth, Corcyra cephalonica (Stainton); Angoumois grain moth, Sitotroga cerealella (Olivier); almond moth, Cadra cautella Walker; and the psocid species Lepinotus reticulatus Enderlein and Liposcelis entomophila (Enderlein) are susceptible to spinosad and complete control is to be expected. Other pest species such as the maize weevil, Sitophilus zeamais Motchulsky; rice weevil, Sitophilus oryzae (L.); and sawtoothed grain beetle, Oryzaephilus surinamensis (L.) are susceptible to spinosad to varying degrees, but their overall level of control remains to be verified under commercial grain storage conditions. Spinosad residues are highly stable on grains stored in bins, with a length of protection ranging from 6 months to 2 years. Numerous factors have been shown to impact the overall performance of spinosad, including insect pest species, pest life stage, grain type, grain variety, and formulation type. Spinosad possesses a unique mode of action in insects and controls insect strains resistant to other grain protectants. When launched globally, spinosad will represent a valuable new addition to the limited arsenal of grain protectants and can positively impact global food security. Its combination of high efficacy, broad insect pest spectrum, low mammalian toxicity, and sound environmental profile is unique among existing products currently used for stored-grain protection.

Introduction

Based on current estimates, world population is projected to double from ∼6 billion to more than 12 billion over the next 50 years (Pimentel et al., 1994). This inevitable population growth will place increasing demands on the production of cereal and other food grains, which currently comprise 67–80% of human food supply and diet (Kendall and Pimentel, 1994, Dyson, 1999). Losses of cereal grains in storage can range from 10 to 20% of overall production, and a primary factor in these losses is the depredations of stored product insect pests (Phillips and Throne, 2010).

Control of stored product insects is best achieved through an integration of physical, chemical, and biological methods (Arthur, 1996, Hagstrum et al., 1999, Phillips and Throne, 2010). However, in practice there is still a strong reliance on the use of chemicals applied to grains at the time of storage. These chemicals are known as grain protectants and they provide protection to stored grains for 4–12 months of storage. To control an existing infestation, especially in grain that is not treated with a protectant, fumigants such as phosphine are used. Existing chemical control products are few, and of these many are under intense scrutiny due to concerns about human safety, insect resistance, environmental impacts, and presence of chemical residues in raw and processed foods (Daglish and Wallbank, 2002, Nayak et al., 2005, Daglish, 2006, Subramanyam, 2006a). Alternative chemical control options to protect grain that do not suffer from the concerns outlined above are urgently needed, and spinosad is one such product that fills this void.

Spinosad is an insecticide product from Dow AgroSciences (Indianapolis, Indiana, U.S.A.), derived via fermentation from a naturally-occurring soil actinomycete, Saccharopolyspora spinosa Mertz and Yao (Bacteria: Actinobacteridae). Spinosad contains two insecticidal factors, spinosyns A and D, present in an approximately 85:15% ratio in the final product (Mertz and Yao, 1990, Kirst et al., 1992, Sparks et al., 1999). Spinosad is highly active by both contact and ingestion to numerous pests in the orders Lepidoptera, Diptera, Thysanoptera, Coleoptera, Orthoptera, Hymenoptera, and others (Sparks et al., 1995, Bret et al., 1997). It affects nicotinic acetylcholine and gamma amino butyric acid (GABA) receptors sites of the insect nervous system, and so far has proved non-cross-resistant to any other known insecticide (Salgado and Sparks, 2005). In addition, spinosad exhibits low mammalian toxicity and a highly favorable environmental profile (Cleveland et al., 2001). Spinosad is considered a natural product and thus approved for use in organic agriculture by numerous national and international certification bodies (Cleveland, 2007, Racke, 2007).

Spinosad’s suitability as a stored grain protectant has been progressively highlighted in a series of scientific publications dating from 1999 (Subramanyam et al., 1999, Subramanyam et al., 2002, Fang et al., 2002a, Mutambuki et al., 2002). Since then, spinosad has been shown to provide highly effective and long-lasting control of numerous key stored product pests on various grains (Toews and Subramanyam, 2003, Nayak et al., 2005, Maier et al., 2006, Subramanyam, 2006a, Subramanyam, 2006b, Huang and Subramanyam, 2007, Huang et al., 2007, Subramanyam et al., 2007, Daglish et al., 2008, Athanassiou et al., 2008a, Athanassiou et al., 2008b, Chintzoglou et al., 2008a, Chintzoglou et al., 2008b, Vayias et al., 2010a, Vayias et al., 2010b). Spinosad was first registered for use as a grain protectant in Kenya in 2003. Registration in the United States was achieved in early 2005, with spinosad’s labeled use rate set at 1 ppm (1 mg a.i./kg of grain) and its Maximum Residue Limit (MRL) or tolerance established at 1.5 ppm. Spinosad is also currently registered for grain protection in a number of other countries, but widespread commercial launch has been deferred while awaiting final MRL or tolerance approvals in a few remaining key grain-importing countries. With global launch now imminent, this paper attempts to review and summarize available published information on spinosad as a grain protectant, with specific reference to spectrum of activity against stored product insect pest species, rate, residual efficacy, use patterns, potential for cross resistance, compatibility with beneficial insects, potential for combination products, and the impact of numerous factors on spinosad performance including pest species, pest life stage, grain condition, grain variety, grain type, formulation, exposure periods, temperature, and humidity.