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Home , Bacillus thuringiensis, Pesticide resistance

miracle, myth…or marketing Bt

onventional such as Th e strains of Bt characterized so far CDDT and have aff ect members of three insect orders: not only become less eff ective as target (butterfl ies and ), insect populations develop resistance, Diptera (mosquitoes and biting fl ies), but have killed nontarget predators and and Coleoptera (). Commercially- Linda Chalker-Scott, Ph.D. parasites that otherwise keep insects available, Environmental Protection MasterGardener WSU editor in balance. Th us, in some conditions, pest Agency-registered Bt products include: Extension Urban Horticulturist populations have exploded to uncontrol- B.t. aizawai (Lepidoptera)—used for and Associate Professor, lable levels, decimating whatever crops or wax larvae in honeycombs Puyallup Research and Extension Center, gardens they happen to feed upon. B.t. israelensis (Diptera)—frequently Washington State University Puyallup, Washington Furthermore, these persistent used for mosquitoes (see sidebar) www.puyallup.wsu.edu/~Linda%20Chalker- pesticides have accumulated throughout B.t. kurstaki (Lepidoptera)— Scott aquatic and terrestrial food webs or frequently used for gypsy moth, chains, creating ecological imbalances spruce budworm, and many and impairing human health. Global pests concerns regarding resistance, B.t. san diego and tenebrionis environmental degradation, and (Coleoptera)—frequently used human health problems have led to for elm leaf , Colorado the development of biologically based, beetle narrow-spectrum pesticides with fewer B.t. kurstaki is the most commonly long-term hazards. used Bt formulation, as it will kill many Over the last several decades, more leaf-feeding larvae on , shrubs, e Informed farmers, forest and landscape managers, fruit trees, and conifers. Th ere is abundant Gardener and gardeners have turned to these more scientifi c literature on this biopesticidal by Linda Chalker-Scott environmentally friendly methods of pest organism. In this introduction to control, including Bt. Other Bt isolates have been character- sustainable landscaping ized but not yet registered by the EPA. practices, Dr. Chalker-Scott An introduction to Th ese include: addresses some of the thuringiensis B.t. galleriae (Coleoptera)—used on THE INFORMED common myths and GARDENER Bt is an acronym for Bacillus Japanese beetles LINDA CHALKER-SCOTT misconceptions that plague thuringiensis, a naturally occurring species B.t. japonensis and kumamotoensis home gardeners. The book is of found worldwide in the soil (Coleoptera)—used on several available in March 2008 from and on plants. Nearly 100 years ago, turf beetle species the University of Washington this bacterium was discovered to have Local isolates of Bt could represent Press or directly from the pesticidal properties if consumed by the an underutilized, yet powerful, biological author herself. larvae of specifi c insects. control resource. In China, 30 new Many subspecies, varieties, and strains strains of Bt were isolated from drylands, of Bt have been identifi ed since then, and gardens, and rice fi elds; from these, one NOTE: it’s likely others have yet to be discovered. highly toxic strain was able to kill 100 Dr. Chalker-Scott researched the safety Th ough from Bt have also been percent of treated of Bt as a as a result of an used in genetically modifi ed organisms, larvae (Plutella xylostella). Similarly high inquiry of one of our readers. If you have concerns about products or practices, this article will focus only on the use of toxicities were found in ten new Bt strains consider contacting MasterGardener. the microbe as an applied . isolated from leaf and soil samples in

SPRING 2008 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 33 Poland and in four new strains discovered in Mexico. Local bacterial populations have the advantage of being adapted to local insect hosts; thus, it is logical to expect to find powerful biocontrol agents in the pest’s backyard. Mode of action strains produce crystalline (called δ-endotoxins) from the University of California that, when consumed by particular insect larvae, have a poisonous effect upon the gut lining. While some of the toxico- The California Master Gardener logical details are still a topic of scientific Handbook debate, we do know that the crystalline 3382 ISBN 978-1-879906-54-9 $30.00 proteins manufactured by the bacteria are toxic, causing the cell membranes in the gut to split open and thus kill the larvae. Pests of Landscape Trees The specificity of these for insect & Shrubs physiology means that other animals are An Integrated Pest Management not affected by the bacteria. Guide, 2nd Edition Bt found naturally on or applied 3359 ISBN 978-1-879906-61-7 $42.00 to leaf surfaces must be ingested by the feeding form or an insect (the larvae) to have an effect; in other words, Bt has Abiotic Disorders of no effect on adult insects. Susceptible Landscape Plants larvae that ingest the are not killed A Diagnostic Guide immediately, but die over the next few days. They do stop feeding, however, 3420 ISBN 978-1-879906-58-7 $35.00 and thus plant damage is halted. Larvae that survive the toxin may be more Pests of the Garden and susceptible to other environmental Small Farm stresses, such as cold temperatures or A Grower’s Guide to Using Less low levels of botanical . This Pesticide, 2nd Edition type of synergistic effect underscores the

3332 ISBN 978-1-879906-40-2 $35.00 importance of utilizing Bt as part of an integrated pest management plan. Some insects have already developed Pest Control around resistance to Bt, most importantly the Gardens and Homes diamondback moth (Plutella xylostella), 2nd Edition regarded as one of the most destructive crop pests worldwide. It is a particularly 21385 ISBN 978-1-879906-67-9 $25.00 resiliant species, reported to be the first insect to develop resistance to DDT and Healthy Roses almost every other synthetic insecticide. Environmentally Friendly Ways to Bt-resistant insects apparently are able to Manage Pests and Disorders in Your detoxify the bacterial proteins quickly and Garden and Landscape thus survive. Interestingly, Bt resistance 21589 ISBN 978-1-879906-49-5 $10.00 appears to harm the insect’s fitness when Bt is not present; in other words, resistant individuals do not reproduce well so that resistance is quickly lost in the larger Call: population when Bt is not applied. This 800-994-8849 “resistance instability” may explain why Click: anrcatalog.ucdavis.edu Bt resistance is uncommon in pest insect populations.

PAGE 34 • MASTERGARDENER • www.MasterGardenerOnline.com SPRING 2008 consumes the Bt (diagram 1) and crystalline toxin-treated leaf. The Bt crystalline toxin (diamond shapes in diagram 2) binds to gut wall receptors, and the caterpillar stops feeding. Within 1 hours, the gut wall breaks down, al- lowing (oval tube shapes) and normal gut bacteria (circular shapes) to enter body cavity, where the toxin dissolves. The caterpillar dies in 24 to 48 hours from septicemia, as spores and gut bacteria proliferate in its blood (diagram 3).

used Human health and safety decades All strains, subspecies, and varieties of ago. Bt used as pesticides must be extensively Likewise, tested for both human and environmental granular 2 safety. Regulatory agencies, such as the and U.S. Environmental Protection Agency, microcapsule require thorough evaluations of the formulations active microbial ingredient before they reduce the inhalation hazard. 3 can be registered as pesticides. Bacillus Volatile agents associated with some Bt thuringiensis has been used extensively for formulations do not appear to constitute a formulations, four decades in biopesticidal formulations signifi cant health hazard. including spiders due to its environmental and human In the 50 years that Bt has been (Araneae), ladybugs (Coleoptera), health records. used for insect control, there have been true bugs (Hemiptera), and Bt is considered to be “practically few reports of human pathogenicity, (). nontoxic” to humans and other suggesting that the commercially available Th ere are few reports of Bt vertebrates. It can cause a “very slight products are free from non-Bt microbes lethality upon nontarget organisms, irritation” if inhaled, and can cause and can be safely used around people. such as leaf-feeding . eye irritation. Th ese acute eff ects are In contrast, there are oft en signifi cant Another researcher has suggested considered to be minor; there are no human health risks associated with the that clay soils may bind the bacterial chronic toxicities. Bt is not carcinogenic, pest insects themselves. toxin, increasing its environmental mutagenic, or teratogenic: in other persistence and possible toxicity words, it does not cause cancer, induce Impacts to nontarget species. Th ough the chromosomal mutations, or lead to birth In addition to their excellent record preponderance of the evidence does defects in exposed animals. on human health, Bt products are not agree with these reports, all Bt does not persist in the brains, lungs, globally recognized by researchers in researchers concur that Bt monitoring or digestive systems of animals, including many disciplines as an environmentally must continue to explore these humans. While Bt has been found in safe means of controlling pest insects. exceptions and to modify Bt usage as fecal samples of exposed greenhouse An extensive and reliable body of science needed. workers, no gastrointestinal symptoms demonstrates the environmental safety Increasingly, land managers were associated with its presence. In fact, of Bts, allowing governmental and health are recognizing the environmental Bt appears to be a normal component in organizations to recommend its use on a advantages of reduced chemical the feces of vegetable-consuming animals, variety of landscapes worldwide. treatments in urban areas. Given where it apparently causes no problem. Specifi cally, no danger has been found their extraordinary record in human Since Bt is a common, naturally occurring to aquatic communities accidentally and environmental health and safety, bacteria oft en found on leaf surfaces, this exposed to Bt (but see sidebar) or to Bt products are increasingly applied should not be a surprise or a cause for nontarget organisms including benefi cial to urban parks and landscapes to concern. insects, amphibians, fi sh, and mammals. A control gypsy moth (Lymantria Like the active bacterial ingredient, the number of researchers have demonstrated dispar) and other nuisance insects. inert ingredients in Bt formulations have the general safety of Bt formulations to Th ese insects are of particular concern also been studied and modifi ed for safety. natural predators of pest insects. By and because of their abilities to denude Newer formulations employ preservatives, large, these predators belong to diff erent trees; invade woodpiles, houses, and like sorbitol, that are safer than the xylene orders than those aff ected by most Bt vehicles in search of pupation sites

SPRING 2008 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 35 (fall webworm); and cause human health New hope for problems as mentioned earlier. Bt use in urban areas requires a control significant public education effort. This was illustrated several years ago when Perhaps nowhere has Bt usage had such dramatic effects as in fighting citizens and environmental groups mosquitoes and the illnesses they carry. Historically, mosquitoes have been became upset with the aerial spraying implicated as transmitters for , encephalitis, and dengue fever. More done in parts of Oregon and Washington recently they have been recognized as the carriers of West Nile and many to control invasions of gypsy moths and other viruses, pathogens, and parasites. Mosquitoes of the genera Aedes, attempted to get an injunction to halt , Culex, Psorophora, and Stegomyia cause much human misery and have spraying. The presiding judge found no high societal costs associated with them. scientific proof that Bt was hazardous Conventional mosquito treatment has usually consisted of DDT, a highly toxic, broad-spectrum pesticide whose residues persist throughout food chains to people and that the eradication of decades after their application. Though banned in the U.S. since 1973, DDT is the moth populations was of greater still legally applied in many regions of the world where malaria is a problem. environmental concern. Obviously, Less devastating are the synthetic , which still kill about 150 to close communication with the public is 200 non-target organisms for each adult mosquito killed. In comparison, Bt critical when Bt products are used, so that products represent a much gentler approach to mosquito management. environmental and health concerns can Bacillus thuringiensis israelensis, or Bti, has been used effectively to kill many be addressed. In addition, governmental species of mosquitoes within these genera, as well as other biting in the organizations must be proactive in order Diptera worldwide. This has been demonstrated repeatedly through enforcing safety regulations. field studies in Africa, Asia, Australia, Eastern and Western Europe, India, and Unfortunately, some gray literature North America. Though not registered by the U.S. Environmental Protection (i.e., not peer-reviewed) ignores the Agency, Bacillus sphaericus (Bs) also has activity against mosquito larvae, as does decades of scientific research on Bt and B.t. jegathesan. Field tests have shown significant reductions in both mosquito instead uses scare tactics against it. These numbers and associated malarial cases. unbalanced articles serve only to upset the Formulations are important with Bt products applied to aquatic systems. general public and do not advance either Dry preparations tend to be less successful, as the spores settle to the bottom the research, or the discussion, that must and are not eaten by larvae, which tend to be near the surface of the water. continue to take place regarding Bt and Biofilms, fizzy tablets, and slow-release floating rings are more effective in this other biopesticides. regard. The latter two formulations are readily available, inexpensive, and can It is naïve to assume that growers and be easily handled and applied by volunteers. They should be used anywhere landscape managers will give up Bt and that standing water—and mosquito larvae—accumulate. Applications often simply sit back to watch their livelihood need to be repeated to treat subsequent hatchings. collapse, if safe and practical alternatives Though some mosquitoes have developed resistance to some Bacillus are not available, they will resort to species, applying these biopesticides in rotation has overcome resistance. Use conventional methods. This is not the of other IPM choices, such as predatory fish, can help reduce larval numbers. direction we need to move. Finally, new strains of Bti and Bs are constantly being discovered in rice fields, plantations, gardens, and other habitats with standing water. Bt as part of an IPM program Bti is generally considered safe for nontarget aquatic organisms, such as Like any other pest control method, dragonflies, damsel flies, notonectid bugs, fish, frogs, and birds, according Bt works best as part of an integrated to the majority of studies that have been performed. Conflicting management plan. The philosophy of information comes from two studies in Minnesota: one over such a plan is to reduce pests to acceptable three years and the second over six years. The shorter levels, not to eliminate them completely. of the studies reported severe declines in Diptera species, As we’ve discovered—much to our causing the authors to question the environmental safety detriment—attempts to exterminate pests of Bti. However, the longer study found no negative result in resistant pest populations and effects on zooplankton or bird populations resulting environmental degradation. from insect decreases. These authors Bt has become a cornerstone of IPM noted that the ecological complexity systems, accounting for more than 90 of wetland food webs and/or other percent of the biological insecticides environmental factors could nullify the impact of reduced currently used. Though Bt has been used insect numbers. Indeed, the first authors acknowledged successfully by itself, the practice of IPM that drought years would cause a similar decline in insect populations, a natural situation from which one generally incorporates Bt with other biological, cultural, mechanical, and would expect the system to recover. n chemical controls. A great deal of research

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worldwide has explored the use of Bt in concert with these associated methods: Cultural—; minimum tillage; shelter strips Mechanical—Removal of pest (eggs and larvae); removal of infested materials Biological—Parasitoids; pathogens, including Bt, fungi, granulosis virus, and Local Growers Providing Local Plants nucleopolyhedrosis virus; predators Many years of Lower Valley experience with McMahan Chemical—Botanical insecticides Nursery and A.T. James Bark & Tree Company such as neem; pheromone baiting/mating disruption; and pyrethroids. Bookstore: Bestsellers, Regional and Garden Titles Cards • Chocolate • Jewelry • Gifts • Fishing Gear! Practical considerations Garden Supplies (Yes, we have gloves!) As with other living organism, Bt activity is affected by environmental factors including temperature, rainfall, pH, and sunlight. Bt applied to leaf surfaces, for example, can be degraded by solar ultraviolet or washed off by irrigation or rainfall. Many of these limitations have been addressed through the development of new Bt formulations that protect the organism from deactivation. Still, there are other Next to I-82 at EXIT 52 APRIL-MAY: factors that influence effectiveness of this 804 Zillah West Road Mon.-Sat. 9-6, Sunday 10-4 509-829-6014 biocontrol agent. Zillah, Washington (Rest of year: Monday-Saturday 9-6) Alvin and Cindy James Because only the feeding larval stage is susceptible to Bt, timing of application is paramount. While this may be during the spring for many leptidopteran pest species, for coleopteran pests in turf, HEIRLOOMHEIRLOOM ROSESROSES application is effective only in the fall. Cold weather decreases effectiveness, ‘America’s Most Romantic perhaps because larval feeding activity is Rose Garden’ reduced. Revel in thousands of fragrant Location of the target insect also blooms while walking our theme influences Bt effectiveness. Boring insects, gardens. See why our customers though susceptible to Bt in laboratory bring their families and friends to trials, can escape Bt exposure if feeding their favorite place to in protected sites. Likewise, it is difficult ‘shop and smell the roses’! to spray the crowns of tall trees from the 24062 Riverside Dr. NE ground. In such cases, a cherry-picker St. Paul, OR could be used for spraying individual trees, but larger areas are more effectively 503-538-1576 managed through aerial spraying. Five miles south of Newberg off Hwy 219 As with any pesticide, Bt must be Peak Bloom late May through July. considered as an option, not a magic Gardens open dawn til dusk bullet, for pest management. Consumer seven days a week. education is critical to avoid improper or overapplication of Bt. Misapplication Sales Cottage open of Bt at the wrong time or on the wrong 9:00 a.m. - 5:00 p.m. May - Sept. species can lead to pest resistance. 9:00 a.m. - 4:00 p.m. Oct. - April In its infancy, use of Bt was costly Closed Sundays in Nov., Dec., Jan. to produce and to apply; while it is still expensive, new production techniques www.heirloomroses.com

SPRING 2008 www.MasterGardenerOnline.com • MASTERGARDENER • PAGE 37 have been developed that promise to greater because of reduced insecticide only is the pest killed, but so too are the lower the cost in developing countries. costs. Though not included in these beneficial predators and parasites, leading Bt is more cost effective to use now, studies, the more intangible benefits to future outbreaks of resistant pest since application costs have decreased. associated with Bt-treated fields—such populations. The negative, long-lasting Proponents hoped that the environmental as reduction in , less effects of these pesticides on human and and human health benefits would more environmental damage, fewer human environmental health should not be than offset the economic costs. health risks—cannot be ignored and must ignored or considered collateral damage. Indeed, the economic comparisons be emphasized. Once insects become resistant to between conventionally managed chemical pesticides, the usefulness of and IPM (including Bt) fields have The big picture that compound is finished, at least demonstrated that not only was insect There is no question that broad- temporarily. The elegance of biocontrol damage reduction approximately the spectrum, conventional pesticides can systems, like Bt, is that the pesticide is same, but that IPM net profits were cause more problems than they solve. Not a living organism—one that can evolve as its host becomes resistant. New strains of Bt and related species are GREENHOUSES discovered routinely. These specifically targeted control agents are considered ŏ Largest Selection in the Northwest by the scientific community to be ŏ Full Accessory Line environmentally friendly, with little or no ŏ Building Materials effect on humans, wildlife, , ŏ Cold Frames and most other beneficial insects. We ŏ Propagation Aids ŏ Call toll-free for FREE catalog continue to discover what we’ve always (800) 322-4707 known—that it’s easier to work with Charley’s nature than against it. n GREENHOUSE & GARDEN For references, see the Web site: 17979 State Route 536, Mount Vernon, WA www.puyallup.wsu.edu/~Linda%20 charleysgreenhouse.com Chalker-Scott/Bt%20references.htm

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