ReviewSpinosad: A new natural product for stored grain protection
Highlights
► Like most grain protectants spinosad effectiveness against a variety of stored product insects at the maximum registered use rate of 1 ppm varies with the species, life stage, commodity treated, and spinosad formulation type. ► Field tests show spinosad to be stable on corn and wheat for a period of six months to two years. ► Spinosad’s unique and non-cross resistant mode of action will make it a valuable new tool in stored grain resistance management programs. ► The practical pest spectrum controlled under commercial grain storage conditions still requires further confirmation after global launch of spinosad.
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.
Section snippets
Laboratory bioassays
The pest spectrum of spinosad includes nearly all of the major coleopteran and lepidopteran pests of stored grain, and at least two important psocid pests. However, numerous studies have shown that susceptibility varies by spinosad rate, pest species, and pest life stage (adults versus immatures), among other factors. Since spinosad is expected to be used as a grain protectant at a maximum use rate of 1 ppm, this upper rate limit will define the extent of its practical pest spectrum to some
Stability on stored grain
More field evaluations of spinosad are necessary to establish its future performance in commercial stored product environments. Field studies are particularly valuable as predictive tools, because they tend to reflect the combined impact of spinosad on adults, immatures, and progeny production under conditions of continuous exposure—aspects of which are difficult to simulate under laboratory conditions (Toews and Subramanyam, 2004). In most field studies reported here, spinosad was applied at
Impact of grain condition
In the single study of its kind Toews and Subramanyam (2003) investigated the impact of wheat condition (whole wheat, cracked wheat, wheat flour) on the toxicity of spinosad to a representative primary pest (R. dominica) and two secondary pests (T. castaneum and O. surinamensis). Adults of all three insect species were confined on wheat treated with 1 ppm of spinosad and mortality evaluated after 14 days. Mortality of R. dominica adults was 97–100% on whole wheat and wheat fractions—not
Impact of wheat class and variety
Fang et al. (2002a) were the first to note variation in the performance of spinosad applied to different classes of wheat. Adult mortality of R. dominica, S. oryzae, O. surinamensis, C. ferrugineus, and P. interpunctella was evaluated on four wheat classes: durum wheat, hard red spring wheat, hard red winter wheat, and soft red winter wheat. At a spinosad application rate of 1 ppm R. dominica and P. interpunctella were completely controlled on all wheat classes. Mortality of S. oryzae adults
Impact of grain type
Huang and Subramanyam (2003) were the first researchers to recognize the impact of grain type on performance of spinosad, noting that spinosad at equivalent rates controlled C. cephalonica more effectively on corn than on sunflower seeds. Since then, numerous studies have elaborated on this initial observation by comparing spinosad toxicity to the same pest species on different crops including wheat, corn, barley, rice, sorghum and oilseeds. Generalizations are difficult, as this “grain effect”
Impact of formulation type
Thus far, two basic spinosad formulation types have been evaluated–liquid suspension concentrate (SC) and dry formulations. Dry formulations are convenient for producers, because they are easy to measure and apply, and obviate the need for dilution calculations and complex calibration of spray equipment. Furthermore, in cold or isolated areas dry formulations are preferred to liquids because they do not require dilution in water. Also, in developing countries, a dry formulation may be adopted
Spinosad biological properties in relation to stored grain use patterns
In a commercial setting, spinosad’s predominant use pattern will be as a grain protectant in admixture with grain. This assumes that the entire bulk of stored grain will be treated with spinosad prior to storage. However, spinosad’s biological properties could lend themselves to other stored grain use patterns as well.
Spinosad is a highly contact-active insecticide, although its ingestion activity is clearly superior (Bret et al., 1997). Spinosad is toxic to adults of numerous stored product
Potential for combination products containing spinosad
Mixture products are not uncommon in grain protectant markets, either as tank mixes (Daglish, 2008) or as in-can mixtures such as Storicide II (3.0 ppm chlorpyrifos-methyl + 0.5 ppm deltamethrin). The components of mixture products are designed to be complementary, with one active ingredient making up for any weakness in its partner and thus providing full spectrum control of stored product insects. At present, the practical insect pest spectrum of spinosad at its maximum registered use rate of
Potential for cross resistance to other chemical classes
Spinosad acts on the post-synaptic nicotinic acetylcholine and GABA receptors of insects and has been demonstrated to possess a unique mode of action not shared by any other known insecticidal class of chemistry (Salgado, 1998, Watson, 2001, Salgado and Sparks, 2005). Lack of spinosad cross resistance to other chemical classes has been verified in a number of laboratory and field studies involving stored product insects. Laboratory bioassays comparing spinosad’s toxicity to susceptible and
Compatibility with beneficial insects in storage environments
In field crop markets, where it has been sold since 1997, spinosad is minimally disruptive to beneficial insects and compatible with Integrated Pest Management (IPM) programs in many crops (Miles, 2006, Arthurs et al., 2007). A portion of spinosad’s selectivity in crop markets derives from its intrinsic toxicity profile relative to beneficial insects, which can vary by taxa, but with spinosad generally being less toxic to predators than to parasitoids (Michaud, 2003, Williams et al., 2003). A
Commercialization timeline
The widespread global launch of spinosad as a grain protectant is anticipated during 2011. Spinosad as a 0.125% dust formulation was first registered as a grain protectant in Kenya in 2003. Since then it has received registration in a number of other African countries (∼15) but always for domestic consumption only, with no export of spinosad-treated grain. South Africa was the most recent African country to register spinosad for use in stored grain (March 2010). Spinosad has been registered as
Conclusions
The most critical issue with regard to spinosad as a grain protectant is the definition of its practical pest spectrum under commercial grain storage conditions, which still requires further confirmation (Fig. 3). Laboratory bioassays are predictive in this regard, yet by their very nature incompletely capture the full impact of spinosad’s toxicity when acting on an insect pest species’ life cycle over prolonged periods of exposure.
Laboratory studies reveal that spinosad’s overall protective
Acknowledgements
The body of literature reviewed in this paper would not have been possible without the hard work and contributions of many individuals in numerous countries around the world, and many are cited in this review. The contributing scientists acknowledged here are in no particular order, and include: Fang Liang, Rennie Roesli, Michael Toews, Fangneng Huang, Jaclyn Rowan, Boina Dhana Raj, Zeb Larson, Andy Allen, Anna Getchell, Michelle Hartzer, Thomas Phillips, Edmond Bonjour, Steve Brown, Roy
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