Effects of Cry1F and Cry34Ab1/35Ab1 on storage pests

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Abstract

Two crystalline protoxins from Bacillus thuringiensis (Bt), Cry1Fa1 and Cry34Ab1/Cry35Ab1 (Cry1F, Cry34/35Ab1), were evaluated for efficacy against lepidopteran and coleopteran storage pests. Cry1F was tested against the lepidopterans Sitotroga cerealella (Angoumois grain moth) and colonies of Plodia interpunctella (Indian mealmoth) that are susceptible or resistant to Bt Cry1Ab and Cry1Ac toxins, Bt subspecies entomocidus, and the commercial formulation Dipel®. Cry1F was also tested against the coleopterans Cryptolestes pusillus (flat grain beetle) and Tribolium castaneum (red flour beetle). Cry34/35Ab1 was tested against S. cerealella, C. pusillus, and T. castaneum, and against additional coleopteran storage pests, including Tenebrio molitor (yellow mealworm), Trogoderma variabile (warehouse beetle), Oryzaephilus surinamensis (sawtoothed grain beetle), Rhyzopertha dominica (lesser grain borer), and Sitophilus oryzae (rice weevil). Strains of Bt-susceptible or -resistant P. interpunctella generally were more sensitive to Cry1A protoxin or toxin than either Cry1F protoxin or Dipel. Despite difficulties with the bioassay of S. cerealella larvae, the data suggest that Cry1F and Cry34/35Ab1 caused increased larval mortality, and a developmental delay was observed and no pupae emerged with 0.9% Cry1F. Neither Cry1F nor the corn rootworm-active toxin Cry34/35Ab1 significantly affected the biological parameters of the coleopteran species evaluated.

Introduction

Storage pests cause economic losses to stored grain and grain products worldwide. For example, in 2008 22.6 million bushels of corn and 4.3 million bushels of wheat were harvested in the U.S., with receipts of $64 billion (http://www.nass.usda.gov). Postharvest losses due to insect pests, estimated at 5–10% (Cuperus, 1995), would have contributed to losses of $3.2–6.4 billion for that year alone. In developing countries, the losses can be much higher (Haines, 2000).

The arsenal of traditional synthetic chemical controls used by the cereal industry is rapidly being depleted because of increased regulatory constraints and insect resistance. Most of the current grain protectants are organophosphates, under scrutiny by the Environmental Protection Agency (EPA) because of the 1996 Food Quality Protection Act. Methyl bromide, a fumigant used for stored products and milling structures, was banned under the Clean Air Act and the Montreal Protocol, with a few exceptions noted in exemptions by the EPA. Other fumigants, such as sulfuryl fluoride, must be custom applied and are not generally considered compatible with IPM. In addition, many fumigants carry significant human health risks and are regulated by exposure and ventilation restrictions.

Because of all of these constraints, new insecticidal treatments are needed for integrated pest management (IPM) of raw grains, mills, and food storage facilities. We seek treatments that are effective against target pests, safe to the environment and non-target organisms (including insect predators/parasites), while also minimizing the development of resistant insect populations. The insecticidal toxins from the common soil bacterium Bacillus thuringiensis Berliner (Bt) are the most successful and widely used biopesticides to date and provide many of these use and safety attributes.

Transgenic corn hybrids expressing the insecticidal crystal (Cry) protein Cry1Ab from Bt have been available for commercial planting since the mid 1990’s. Transgenic grain and processed grain fractions were found to be less susceptible to attack and damage by lepidopteran stored-grain pests (Sedlacek et al., 2001). However, resistance to Bt was first noted in a major stored grain pest, Plodia interpunctella (Hübner) (Indian mealmoth) (McGaughey, 1985), and resistance to Cry1A toxins has been observed in many lepidopteran insects (Ferré and Van Rie, 2002). Therefore, there is a need to evaluate and understand the spectrum and efficacy of novel Bt proteins against stored-grain insects to predict and prolong the efficacy of transgenic grain to reduce damage by storage pests.

Cry1F (Event TC1507) was commercially available in hybrid corn in the United States in 2003 under the trade name Herculex™1 I Insect Protection. This product controls several destructive foliar and kernel feeding Lepidoptera. Cry1F transgenic corn negatively impacted the growth and development of both Bt-susceptible and -resistant populations of P. interpunctella selected with Dipel1 (http://www.reeis.usda.gov/web/crisprojectpages/190198.html). Cry34/35Ab1 (Herculex® RW Rootworm Protection) are Bt proteins expressed simultaneously in corn to protect against damage to the roots by the larvae of the coleopteran western corn rootworm (Diabrotica virgifera virgifera LeConte) and northern corn rootworm (Diabrotica barberi Smith and Lawrence) (Moellenbeck et al., 2001). Neither Cry1F nor Cry34/35Ab1 has been widely evaluated for the potential to control stored-grain insects. The research detailed here provides some insight into the activity of these proteins for the management of destructive coleopteran and lepidopteran stored-grain insects.

Section snippets

Insect colonies and bioassays

Insects were obtained from stock cultures maintained at the Grain Marketing and Production Research Center in Manhattan KS and included: the Angoumois grain moth, Sitrotroga cerealella (Olivier) (Lepidoptera: Gelechiidae); the red flour beetle, Tribolium castaneum (Herbst) and yellow mealworm, Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae); the Indian mealmoth, Plodia interpunctella (Lepidoptera: Pyralidae); the warehouse beetle, Trogoderma variabile Ballion (Coleoptera: Dermestidae);

Results

The efficacy of Cry1F and Cry34/35Ab1 was evaluated in a number of stored-products pests. Various parameters were measured to determine the effects of toxins on different lepidopteran and coleopteran pests, such as mortality, growth inhibition, and/or development time. Appropriate parameters were selected for each stored-grain pest based on the feeding behavior and basic biology of each insect, i.e., some insects are amenable to weighing, whereas others are internal feeders and/or more

Discussion

With the loss of fumigants and sprays due to resistance or increasing regulatory scrutiny, the development of new control products for storage pests is critical. The efficacy of Cry1A toxins against lepidopterans, including major moth pests in stored products, has been well-documented (reviewed in Ferré and Van Rie, 2002). However, the ecology of storage pests, found in confined spaces, suggests a propensity for resistance. In fact, the first reported case of resistance to Bt products was from

Acknowledgements

We thank Carlos Blanco for his thoughtful comments and suggestions. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.

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