6 85 Field Evaluation of Granular Starch Formulations of Bacillus thuringiensis Against Ostrinia nubilalis (Lepidoptera: Pyralidae) MICHAEL R. !\'lcGUIRE, BARUCH S. SHASHA, LESLIE C. LEWIS,l ROBERT J. BARTELT, AND KARL KINNEY2 VSDA-ARS, Plant Polymer Research, Northern Regional Research Center, Peoria, Illinois 61604 J. Econ. Entomol. 83(6): 2207-2210 (1990) ABSTRACT Bacillus thuringiensis subsp. kurstaki Berliner was encapsulated within corn­ starch granules with the feeding stimulant Coax or the VV screen Congo red and tested at two field sites against European corn borer, Ostrinia nubilalis (Hubner), feeding in whorl­ stage corn. These tests were done to determine the relative effect of these additives on efficacy of starch-encapsulated B. thuringiensis. At both sites, all treatments with B. thuringiensis significantly reduced tunneling by O. nubilalis. At one site, significant effects of addition of the phagostimulant were observed. When Coax was added at 1 or 10% of starch dry weight with 400 international units (IV) B. thuringiensis per mg dry granule weight, response of O. nubilalis was equivalent to that obtained with granules containing no feeding stimulant and 1,600 IV/mg. Also, granules with Coax and 400 IV/mg gave a response similar to that obtained from the commercial product Dipel lOG formulated at 1,600 IV/mg. At the other site, the effect of phagostimulant was not significant, primarily because O. nubilalis infestation levels were too low for precise discrimination among treatments. KEY WORDS Insecta, Ostrinia nubilalis, starch encapsulation, feeding stimulant FOR APPROXIMATELY 20 YEARS, Bacillus thuTin­ ringiensis could be reduced from 1,600 interna­ giensis subsp. kUTstaki Berliner has been used as tional units (IU)/ mg to 400 IU/ mg in the presence an agent to control the European corn borer, Os­ of Coax without loss of insecticidal activity (Bartelt trinia nubilalis (Hubner) selectively. During that et al. 1990). time, much research was conducted to determine Here, we report the results of field trials designed efficacy of B. thuringiensis in comparison with to test the effects of a phagostimulant and a UV broad-spectrum chemical insecticides (e.g., protectant on the activity of B. thuringiensis in a McWhorter et al. 1972, Lynch et al. 1977). This starch matrix. These effects were examined by research demonstrated that results were often vari­ measuring the response of O. nubilalis feeding in able with B. thuringiensis and that the variability whorl-stage corn treated with the various formu­ may be due to formulation (Lynch et al. 1980). lations. Dunkle & Shasha (1988) tested this hypothesis by encapsulating B. thuringiensis \vithin cornstarch Materials and Methods granules. Preliminary evidence suggested that their technique should permit the incorporation of Formulation Composition and Preparation. screens to protect spores and crystals from ultra­ Starch granules were prepared to test the effects violet (UV) light degradation and extend residual of various additives and levels of B. thuringiensis activity (Dunkle & Shasha 1989). Bartelt et al. (1990) on O. nubilalis survival as measured by stalk tun­ used the encapsulation technique to demonstrate neling. Additives tested were a wettable powder the response of O. nubilalis to various phagostimu­ formulation of Coax (CCT Corporation, Litchfield lants. In laboratory tests, a range of responses was Park, Ariz.), a commercial product shown to be a noted when, for example, corn oil (low response) highly preferred phagostimulant (Bartelt et al. or Coax (high response) was incorporated into the 1990), at two levels (lor 10% of starch, dry wt), granule. In glasshouse tests, the amount of B. thu- and Congo red (Aldrich Chemical Company, Mil­ waukee) (1% of starch, dry wt), a common labo­ ratory dye with UV-absorbing properties (Shapiro The mention of firm names or trade products does not imply that they are endorsed or recommended by the USDA over other 1989, Dunkle & Shasha 1989). In addition, granules firms or similar products not mentioned. were prepared without either of the above addi­ I USDA-ARS, Corn Insects Research Laboratorv, RR Box 45B, tives. Each type of granule (no additive, 1% Coax, Ankeny, Iowa 50021. , Illinois Natural History Survey, 607 E. Peabody, Champaign, 10% Coax, or 1% Congo red) was made \vith three Ill. 61820. Current address: Department of Entomology, 237 Rus­ different rates of B. thuringiensis (provided by sell Laboratories, University of Wisconsin, Madison, Wis. 53706. Abbott Laboratories, North Chicago, Ill., as tech- 2208 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 83, no. 6 Table 1. Starch granule evaluation site information domized complete block with 14 treatments (12 starch treatments, Dipel lOG, and an untreated Site Characteristic control) and six blocks. To stabilize the variance, Urbana, Ill. Ankeny, Iowa the response variable (mean length of tunneling Corn varietv Pioneer 3379 Garst 8555 per plant in each plot) was transformed to the Planting date 17 Mav 17 Mav square root scale before analysis. Analysis of vari­ Infestation date 26 Jun~ 28 Jun'e ance (ANOVA) on the whole experiment indicated Approximate no. larvae 50 50 per plant a significant site-by-treatment interaction (proba­ Source of larvae Funk Seed Co'" USDA-ARS, blv related to the low level of infestation at Ur­ Ankenv b;na). Therefore, the treatment effects were ana­ Plant growth stage at Middle to late Middle t~ late lyzed separately for the two sites (two-way infestation whorl whorl ANOVA). All means were compared with the least Granule application date 30 June 30 June Evaluation date lQ-.ll August 10 August significant difference procedure (Lund 1988) so that the Dipel lOG and controls (untreated and a Bloomington, Ill. starch granules lacking B. thuringiensis) could be compared with any other treatment. Because any reduction in tunneling was dependent on the pres­ nical powder lot #24-830-cd, 77,721 IV/mg): 0, ence of a toxicant, it was important biologically to 400, and 1,600 IU/ mg dry granule for a total of examine the effects due to additive and additive­ 12 granule batches. by-level of B. thuringiensis interaction only with Granules were prepared using procedures mod­ those granules containing B. thuringiensis. There­ ified from Dunkle & Shasha (1988). Pregelatinized fore, the treatment sums of squares were parti­ cornstarch (2.3 kg) (Mira-gel, A. E. Staley, Inc., tioned to test for differences among levels of B. Decatur, Ill.) was mixed at room temperature with thuringiensis (df = 1), for differences among ad­ 2.3 liters of deionized water containing B. thurin­ ditives (df = 3), and for interaction between level giensis or Congo red (or both) and 0.2% (of starch, of B. thuringiensis and additive (df = 3). dry wt) Benomyl 50 WP (methyl 1 (butylcarba­ moyl)-2-benzimidazole-carbamate; Miller Chemi­ Results cal & Fertilizer Corporation, Hanover, Pa.) to re­ duce fungal growth on the granules. If Coax \vas At both field sites, significantly less tunneling to be added, it was mixed with the starch before (i.e., better control of O. nubilalis larvae) occurred being mixed with water. The mass was then al­ in all plots that received granules formulated with lowed to incubate and harden for 2-4 h at 4°C. B. thuringiensis compared with plots that received The hardened mass was broken apart by hand, control granules (Table 2). In addition, plots re­ passed through a single-disc mill, allowed to dry ceiving starch control granules did not differ sig­ at room temperature, then ground in a laboratory nificantly in response from the untreated control blender until the particles passed through 16-mesh plot, indicating no effect due to formulation ma­ screening. Half of each batch was sent to each of terials. two field sites. In addition, an untreated control At Ankeny, significant effects were observed be­ and the commercial standard Dipel lOG (Abbott cause of additive (F = 7.94; df = 3,65; P < 0.001) Laboratories) formulated at 1,600 IU/mg were ex­ and level of B. thuringiensis (F = 15.94; df = 1, amined at each site. 65; P < 0.001). The additive by level interaction Field Site Information. Table 1 lists information was not significant (F = 2.27; df = 3, 65; P > 0.05). pertinent to each field site. Soil preparation, plant­ Data presented in Fig. 1 illustrate the differences ing, weed control, and other cultivation techniques in response among additives. Plants that received were done in accordance with practices that v..ere granules containing 10% Coax had significantly re­ standard for each location. Plots were 15 m long duced tunneling compared with plants receiving by 4 rows wide (0.75-m centers); the middle two granules without Coax. Granules with no additives rows received treatments. Plants were infested \vith provided the least amount of control. For all ad­ O. nubilalis larvae as described by Ortega et al. ditives tested, granules with 1,600 IU B. thurin­ (1980). Larvae were applied to 20 (Urbana) or 25 giensis/mg (f = 1.53 cm) provided better control (Ankeny) plants within the two ro\vs. Granules were than granules formulated with 400 IV/mg (f = applied over the rows in 18-cm bands with metered 2.75 cm). Other differences were observed among applicators calibrated to deliver 11.2 kg/ha (10 lbs/ individual treatment means (Table 2). When Coax a) mounted on a high-clearance vehicle. Evaluation was present at either the 1 or 10% rate, no signif­ \vas conducted about 6 wk later (after O. nubilalis icant difference was observed between the 1,600 pupated). All plants infested with larvae were split IU/mg and 400 IU/mg levels of B. thuringiensis. from base to tassel, length of vertical tunneling was Thus, the level of B. thuringiensis could be re­ measured with a ruler, and average centimeters of duced from 1,600 IV/mg to 400 IV/mg without tunneling per plant were calculated for each plot.