THE RELATIONSHIP OF SVI'EET SORGHUM PLANT FIBER AND SURVIVAL OF THE SUGARCANE BORER, DlATRAEA SACCHARALIS (F.) (LEPIDOPTERA: PYRALIOAE)

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1 THE RELATIONSHIP OF SVI'EET SORGHUM PLANT FIBER AND SURVIVAL OF THE SUGARCANE BORER, DlATRAEA SACCHARALIS (F.) (LEPIDOPTERA: PYRALIOAE) B. W. Fuller' and T. E. Reagan' (Accepted for publication 25 January 1989) ABSTRACT Increased plant.ing density of 'Wray' sweet. sorghum, Sorghum bicolor (1...) Moench, resulted in decreased stalk barrel size and increased fiber content. Planting slhnds were cswblishcd at. 3.6, 5.3, 8.6 and 14,3 plon!s per metel" of row to provide sweet: soi'i-(htlm plots with discrete fiber levels. Sugarcane borer, Diatroco sncc!lal'll/is (1".), larval llloi'tlllit.y after stalk entry, moth emergence and other damage indices indicated no significnnl. reduction in survival of t.his pest in relation to increased fiber content of sweet. sorghum. ContrOl'ily. 2-fold increases in total molh emergence occurred in plots with higher libel' levels. Key Words: Plant fiber, Diatmea s(lccllllm!is, Sorghum bicolor. J. Agric. Entomol. 6(2): 1l3 118 (April 1989) Sweet sorghum, Sorghum bicolor (L,) Moench, 'Wray' is cultivated pl'imarially as a forage crop in many areas of the United Stales, However, sweet sorghum use in making fuel alcohol may lead to greatly increased acreage of this crop in the future. This crop is highly susceptible to attack by the sugarcane borer, Diatraea saccharalis (F,), (Reagan and Flynn 1986; Ii'uller and Reagan 1988) which is also regarded as the key pest of sugarcane in the western hemisphere. Use of resistant varieties in controlling this pest in sugarcane is considered the most important n:>m practice in Louisiana (Reagan and Martin J988). Some mechanisms of resistance reported have included leafsheath appl'ession, rind hardness, and high fiber content (Coburn and Hensley 1972; Martin et al. 1975; Reagan and Martin 1988). Research involving sugarcane borer resistance for sweet sorghum has been limited to varietal studies conducted in two trials at San Paulo, Brazil (Lara and Perussi 1984). In the first trial, a significant (r = 0.52, P < 0.05) positive correlation was found between percent infestation and stalk diameter at the median internode of sweet sorghum plants. However, no significant correlation between these characters was obselved in the second trial. The authors declined to speculate on the possible resistance mechanism. Our research assumes that the decreased level of infestation in sweet sorghum varieties with small stalk diameter may have been related to increased liber content which occurs as stalk barrel size decreases. This would coincide \\-'ilh obsel"vations by Mathes and Charpentier (1962) that high fiber conteni in sugarcane generally results in increased sugarcane borer mortality. Stalk barrel size can be manipulated by changing plant densities, as high densities result in narrow stalk burrel size with increased fiber content. The I Plnnl, Science Department, (Entomology Section), South DlJkola State University, Brookings, SO 5illOG. ~ Department of Entomology, Louisiana Agricultural EXJlerimenl Station, Louisiana Slule University Agricultural Center, Balon Houge, LA

2 114 J. Agric. Entomol. Vol. 6, No.2 (1989) primary objective of our study was to investigate the relationship of sweet sorghum plant fiber content and the sugarcane borer through manipulation of plant density. MATERIALS AND METHODS Sweet sorghum cv. 'Wray', the principal vartiety grown in Louisiana, was planted in four row plots with 0.91 m spacing (13.72 In long, ha plots) on a commerce silt loam soil on 2 June 1984 and t5 June 1986 at the St. Gabriel Research Station, Iberville Parish, Louisiana. A randomized complete block with a split plot experimental design was used with five and four replications for 1984 and 1986, respectively. Whole-plot treatments consisted of untreated plots and those plots without sugarcane borers (suppression achieved by using monocrotophos 0.85 kg IAII per hal. Sub-plot treatments of four plant densities of 3.6 (39, 353 per hal, 5.3 (59, 936 per hal, 8.6 (94, 016 per hal and 14.3 (156, 320 per hal plants per meter of row were established by hand thinning higher plant populhtions to the desired density levels to ensure even spacing within treatments. Liquid chlordane (1 kg [All per hal was applied to the soil stu face to remove soil inhabiting predatory arthopods (Fuller and Reagan 1988), particularly the red imported fire ant, Solenopsis invicta Buren. Thus, mortality factors were limited to those associated with the O eatmcnt. parameters. Ot.her possible mortality factors (e.g. parasites, diseases and weather stress) were considered to have affected all treatments equally. Insecticide applications were initiated when sugarcane borer larvae were found infesting sweet sorghum leaf sheaths. Routine applications followed at " week intervals to remove any infestations in those plots where sugarcane borers were suppressed. Five plants per plot were taken [rom the two outside rows approximately every 4 weeks in 1984 and examined for infestations of sugarcane borer larvae. Sampling efforts were intensified in 1986 to provide \veekly larval counts throughout the growing season. Sample size was restricted to five plants to maintain plant stand integrity. Equal number of stalks sampled across density treatments resulted in a larger percentage of plants per area sampled in the lower planting densities. Therefore, sugarcane borer infestation estimates were converted to number of larvae per m 2. Irrespective of these samples, we used damage,'ecords (entrance and exit holes) of stalks at harvest to serve as our primary index of sugarcane borer population levels. Two center rows of each plot were cut and weighed in August in ]984 and Leaves were removed from 50 plants in each plot to examine the internodes for sugarcane borer larval entrance or moth exit holes. Percent survival of sugarcane borer l81vae after stalk entry was determined from moth exit holes divided by the number of entrance hotes (Fuller and Reagan 1988). Moth exit holes are distinctively lurger (6 to 8 mm diu.) and more oval in shape than the small (2 to 3 mm dia.) round larval entry holes. Percent bored internodes were calculated by division of the number of damaged internodes by the total number of internodes. Sugarcane borer adult emergence levels were determined by converting moth exit holes into per ha units. Two 25 stalk bundles per plot were randomly chosen for sugar analysis. Diameter of the third internode from the base of each stalk was recorded.,juice extraction and percent fiber estimates were conducted using the press method as described by Tanimoto (1967). Total sugars (all fermentable sugars)

3 FULLER and REAGAN: Plant Fiber and Sugarcane Borer Survival 115 estimates were derived from solu ble solid measurements (Brix) according to methods by Ricaud and Arceneaux (J984). Arcsin square root transforma.tions were performed on percentage values. Data were subjected to the General Linear Model Analysis (SAS Institute 1985) and mean separations were by the F test or Duncan's multiple range test where appropriate. RESULTS AND DISCUSSION Altering sweet sorghum plant. density pl ovided the four discrete levels of fiber content (Table 1). Differences in percent bored internodes per plant were not detected. Similar mean damage levels per plant reveal that the number of larvae increased per m 2 pl'opoitionally as the number of plants increased; thus the number of larvae per stalk remains constant among the different plant density treatments. Also, a significant IF = 2.96; df = 3, 48; P = increase in total sugarcane borer IBlvae was found in plots with higher sweet sorghum stands. Data fol' individual sampling dates varied considerably (Table 2); however on most dates the 14.3 and 8.6 plunts/rn density level had higher larval density estimates pel' m 2 than those found in lower plant stand plots. No clear evidence was found to indicate why greater sugarcane borer l81val densities were l>resent in higher plant stands. Significant. differences in moth emergence pel' ha were not detected among treatments without sugarcane borer suppression (Table 1), however, there was an approximate 2-fold increase in moth production in H.3 and 8.6 plantslm when compared to that found in plots with 5.3 and 3.6 plants/m stands. Damage indices, exit. holes and larval counts all rel1ecl larger sugarcane borer population densities found in the plots with increased plant stands. Thus, the plants which possessed high fiber levels and reduced stalk diameter were harboring a greater number of sugarcane borer larvae per m 2. Lmval sujvival after stalk entry in untreated (no monocrotophos) plots was similar in all treatment densities. Although morc lalvae pci' area were present in the higher density stands, the proportion reaching the adult stage per stalk remained constant among the different stands. Irrespective of sugarcane borer infestations, sugar yield was greatly reduced in lower plant density plots (see Table I). In untreated areas (no monocrotophosl, the natural sugarcane borer infestations caused damage which reached, but did not greatly exceed the LO% bored internode economic injury level (Fuller et al. 1988). Comparing untreated and monocrotophos treated plots, yield losses attributable to sugarcane borer infestation were observed in 3.6, 5.3 and 14.3 plantim plots, however, this situation did not appeal' in the 8.6 plants/ill plot. Mathes and Charpentier (1962) reported high fiber as one of several resistance mechanisms responsible for reducing sugarcane borer survival. This relationship between fiber and larval survival after stalk entry was not evidenced in '\VI'8Y' sweet sorghum. Renewed intel'cst in sweet sorghum as a feeder stock for alcohol production and the need for non-chemical control techniques may lead to udditional studies to provide effective cultural control practices for the sugarcane borer. However, high libel' content as a result of increased plant stand should not be considered as a cultural control strategy. Other possible resistance mechanisms such as rind hardness and leafshcath appression may be more valuable.

4 Table 1. Sweet sorghum planting stand effects on D. saccharalis larval survival, damage indices, and yield of total sugars at St. Gabriel, LA, in 1984 and Whole Plant Stalkt Internodes Larval, D. saccharalis Total plot density diameter bored Fiber swvival moth emergence sugars treatmen~ (plants/m) (em) (%) (%) (%) (lo'lha) (kg/ba) Monocrotophos d 3.4 b 11.4 ab 4.2 b 8,584.9 a e 4.0 b 11.6 a 8.8 ab 6,139.7 cd b 3.3 b 10.8 be 5.0 b 5,620.0 d a 3.2 b loa e 2.9 b 4,337.5 e No Monocrotophos d 10.5 a 12.1 a 9.4 a 17.7 a 7,340.6 b e 10.9 a 11.4 ab 10.7 a 17.4 a 6,734.6 be Z b 8.8 a 10.2 e 11.6 a 9.3 ab 5,537.4 d a 9.7 a 10.8 be 11.0 a 8.0 ab 4,045.1 e ~ MeanJl within columns followed by the same letter are not significantly different (P> 0.05; Ducnn's multiple range test. with a weighted whole-subplot. interaction ~ term). S Treatments to allow for natural or insecticide suppressed D. saccharafis populations. t Diameter wajl measured in the middle of the third internode (rom the bose of the plant. 1: Based on D. sacchamfi... survival after entry into ~weet sorghum stalks.

5 Table 2. Mean weekly D. saccharalis larval infestation estimates for differing plant densities of sweet sorghum in untreated (no c: monoerotophos) plots at St. Gahriel, La., r Mean no. larvae/m 2.., '" ;;;l Plant density (plant/m) 24 July August Septemher II a 0.5 a 1.0 ab 1.3 a 0.7 ab 0.0 e 0.7 a 0.9 a 1.1 a 3.9 a 2.2 abc 2.7 ab 1.2 be 2.6 a 1.0 a 0.8 a 1.1 a 4.3 a 1.7 a 0.8 a 2.9 a 4.2 a 2.4 a 2.6 a 0.7 a 1.3 a 1.1 a 0.4 a 2.9 a 2.1 ab 3.4 a 0.5 b Means within columns followed by the same letter are not significantly different (P> 0.05); Duncan's multiple range test).

6 118 J. Agric. Entomol. Vol. 6, No. 2 (1989) ACKNOWLEDGMENTS We wish to express our appreciation to Alton N. Sparks, Jr, Sharron S. Quisenberry, Freddie A. Martin, Jerry B. Gmves, and Elvis A. Heinrichs for editorial assistance with this manuscript. John H. Holcomb of Progreso, TX, graciously donated the sweet sorghum seeds used in this study. The assistance of Ricardo T. Bessin and Jeff L. Flynn in collecting data is greatly apprccillted. Approved for publication by the Director of the Louisiana Agricultural Experiment Stnlion as manuscript number LlTERATUHE CITED Coburn, G. E., and S. D. Hensley Differential sulvival of Diatraea sacchnmlis (F.) Imvae on 2 varieties of sugarcane. ProC. Tntl. Soc. Sugar Cane Techno!. 14: Fuller, B. \V., T. E. Reagrln, Ilnd J. L. Flynn Economic injury level of the sugarcane borer (Lepidoptera: Pyralidae) on sweet sorghum, Sorghum bic:olor (L.) Moench. J. Eeon. EnWmol. 81: Fuller, B. W., and 1'. E. Reagan Comparative predat.ion of the sugarcane borer (Lepidoptera: Pyralidae) in sweet sorghum and adjacent sugarcane. J. Econ. Ent.omol. 81: Lara, F. 1\,1., and E. M. Pel'llssi Resistance of sweet. sorghum genotypes to the sugarcane borer. So. Cienc. Cult. (Sao Paulo). 36: Martin, F. A., C. A. Richard, and S. D. Hensley Host resistance to Dialraea saccharalis (F.): relationship of sugarcane internode hardness to larval damage. Environ. Entomol. 4: Mathes, R, and L. J. Charpentier Some techniques and observations in st.udying the resistance of sugarcane varieties to the sugarcane borer in Louisiana. Proc. Inti. Soc. Sugarcane 1'eclmol. 11: 59, Reagan, T. E., and F. A. Martin Breeding for resistance to Diatmea saccjwralis (F.). Proc. Intl Symposium on Sugarcane Valictal Improvement. - Present. Status and Future Options. Coimbatore, Indio. SepL 3 ll, Reagan, T. E., lind J. L. Flynn Insect pest management of sweet sorghum in sugarcane production systems of Louisiana: problems and integration, pp III W. H. Smith IEd.1 Biomass Energy Development. Plenum Pub., N.Y. Ricaud, R., and A. Arceneaux Sweet. sorghum for biomass and sugar production in Louisiana. In. Report of Projects for Department of Agronomy, Louisiana State Universit.y Agricultural Center. pp SAS Institute SAS User's Guide: Statistics, 5th ed. SAS Institute, Cary N.C. Tanimoto, T The press method of cune analysis. Hawaiian Planter's Record 57: