The fall armyworm prefer species in the grass

Transcription

1 The Journl of Cotton Science 3:35-44 (1999) The Cotton Foundtion ARTHROPOD MANAGEMENT Dmge to Cotton Fruiting Structures by the Fll Armyworm, Spodopter frugiperd (Lepidopter: Noctuide) R. G. Luttrell nd J. S. Mink INTERPRETIVE SUMMARY Fll rmyworm is spordic pest of cotton tht feeds on leves nd fruiting structures. Reserch ws conducted t Mississippi Stte Univ. to mesure the mount of fruit feeding by fll rmyworm t different stges of cotton crop development nd to develop estimtes of fruit dmge rtes for economic thresholds. Results indicted tht fll rmyworm lrve hve the potentil to dmge cotton fruiting structures t rtes comprble to those of the cotton bollworm nd tobcco budworm, ssuming similr survivl rtes. Development of economic thresholds for fll rmyworm will require dditionl studies to relte current or revised smpling procedures to expected fruit dmge. Reltionships between insect density nd fruit dmge in this study should be helpful in the design of these studies. ABSTRACT Cotton t three stges of crop phenologicl development ws infested with eggs nd third instr lrve of the fll rmyworm, Spodopter frugiperd (J.E. Smith), to determine the effect of lrvl feeding on fruit dmge nd yield. Regression nlyses indicted tht numbers of dmged squres nd bolls were significntly 0.05) influenced by the number of egg msses nd third instrs plced on the plnts. More dmge resulted with infesttions of third instrs thn with eggs. Fruiting structures tht were penetrted tended to hve lower probbilities of survivl to hrvest thn did non-penetrted or R. G. Luttrell, Deprtment of Entomology nd Plnt Pthology, Mississippi Agriculturl nd Forestry Experiment Sttion, Box 9775 Mississippi Stte University, Mississippi Stte, Mississippi 39762; nd J. S. Mink, Zenec Ag. Products, Rt. 1, Box 65, Lelnd, MS Received 2 July Correspondence - K. Knighten, Deprtment of Entomology nd Plnt Pthology, Box 9775, Mississippi Stte, MS 39762, (ksk2@r.msstte.edu). undmged fruit. In subsequent study with individul lrve confined for 48 h by cotton bgs on fruiting structures, third instrs dmged 0.63 squres, 0.72 smll bolls, nd 0.40 lrge bolls; fourth instrs dmged 0.71 squres, 0.76 smll bolls, nd 0.63 lrge bolls; nd fifth instrs dmged 0.83 squres, 0.81 smll bolls, nd 0.66 lrge bolls. Dmge to squres by ll instrs resulted in significnt 0.05) reduction in survivl of fruit to hrvest. Feeding on smll bolls by fourth nd fifth instr lrve, but not third instr lrve, resulted in significnt 0.05) reductions in probbility of hrvest. Feeding on lrge bolls did not reduce the probbility of survivl of fruit to hrvest. The fll rmyworm prefer species in the grss fmily s hosts (Luginbill, 1928), but dmging popultions cn occur on other economiclly importnt plnts, such s cotton nd soyben (Bss, 1978; Pitre, 1979; Young, 1979; Pitre nd Hogg, 1983). Studies hve shown tht one fll rmyworm strin the corn-cotton strin prefers cotton s host, while nother strin prefers grsses (Pshley nd Mrtin, 1987; Pshley et l., 1992). The fll rmyworm is spordic pest tht does not overwinter in most res of the USA (Luginbill, 1950). Popultion densities vrytremendously from yer to yer nd plce to plce, but dmging popultions infrequently develop on cotton in the Midsouth. As corn crege increses in trditionl cottonproduction regions such s the Midsouth, there is concern tht the fll rmyworm will become pest of incresing importnce on cotton. Another concern, s it reltes to fll rmyworm, is the growing populrity of trnsgenic cottons, which express endotoxin proteins of Bcillus thuringiensis kurstki. The fll rmyworm is one of the lest susceptible lepidopterns to the endotoxin proteins expressed in cotton (McIntosh et l., 1990; Wn, 1994). Dmging densities of fll rmyworm on these trnsgenic cottons will likely need to be treted with conventionl insecticides.

2 R. G. LUTTRELL AND J. S. MINK: DAMAGE TO COTTON FRUITING STRUCTURES 36 The fll rmyworm does not routinely infest cotton in the Midsouth, nd little reserch hs been conducted to quntittively describe feeding nd dmge of this pest on cotton. Quntifible economic thresholds re not vilble nd control recommendtions re bsed lrgely on subjective opinions nd personl experiences. Severl ppers on the biology nd ecology of fll rmyworm on cotton hve been published during the pst decde. They included studies on the distribution of fll rmyworm egg msses on cotton (Ali et l., 1989), survivl of fll rmyworm immtures on cotton (Ali nd Luttrell, 1990), distribution of fll rmyworm lrve within the cotton cnopy (Ali et l., 1990), nd developmentl rtes of fll rmyworm feeding on cotton (Ali et l., 1990b). Mink nd Luttrell (1989) nd Smith et l. (1993) hve reported on commonly used insecticides for control of fll rmyworm. Chndler (1995) reported on the potentil use of insect growth regultors for Spodopter spp. Erlier literture contins numerous reports of fll rmyworm dmging cotton (Dew, 1913; Wlton nd Luginbill, 1916; Luginbill, 1928; Vickery, 1929; Pitre, 1979; Clower, 1984; Smith, 1985), but no quntittive informtion exists on the mount nd type of dmge cused by fll rmyworm feeding on this crop. This type of informtion is vilble for fll rmyworm feeding on corn nd sorghum (Henderson et l., 1966; Morrill nd Greene, 1974; Cruz nd Turpin, 1983). A considerble mount of informtion cn be found in the literture describing dmge to cotton by other lepidoptern pests, prticulrly bollworm (Helicoverp ze Boddie) nd tobcco budworm (Heliothis virescens F.) (Adkisson et l., 1964; Kincde et l., 1967; Hrtstck et l., 1978). These studies were conducted to mesure the effects of feeding of fll rmyworm lrve on dmge nd retention of cotton fruiting structures. The results should be useful to those interested in the development of economic thresholds (Stern et l., 1959) for this pest on cotton. MATERIALS AND METHODS Fll rmyworm lrve were confined on individul cotton fruiting structures to mesure fruit retention from different levels of feeding dmge. All studies were conducted in field of 'Stoneville 506' cotton plnted during erly My 1986 nd 1987 nd mintined under norml growing conditions on the Plnt Science Reserch Frm t Mississippi Stte Univ., Mississippi Stte, MS. Presquring cotton ws thinned to 10 plnts m -1 plnt density (74,100 98,800 plnts h -1 ). Additionl thinning to 5 plnts m -1 (49,500 plnts h -1 ) ws done 1 wk prior to insect infesttion in order to fcilitte plnt hndling during rtificil infesttions. Applictions of the insecticides cypermethrin (0.32 kg [AI] h -1 ) nd zinphosmethyl (1.35 kg [AI] h -1 ) were mde before nd fter infesttions when necessry to protect the cotton ginst bollworm, tobcco budworm, nd boll weevils (Anthonomous grndis grndis Bohemn). The study plots were spryed with non-persistent insecticide (mevinphos 1.35 kg [AI] h -1 ) 1 d prior to infesttion to reduce the number of predtors present. Egg msses were obtined from fll rmyworm colony mintined t the USDA-ARS Crop Science Reserch Lbortory, Mississippi Stte, MS. This colony ws initited with insects collected from corn. Subsequent studies, tht involved infesting cotton with fll rmyworm lrve, were conducted with insects originlly collected from cotton. This ws to ensure tht the corn-cotton strin of fll rmyworm (Pshley nd Mrtin, 1987; Pshley et l., 1992) ws used in the studies. For ll studies, insects were rered by the procedures described by Dvis et l. (1985) nd fed whet germ-csein diet (Dvis, 1989). Infesttion of Cotton with Egg Msses In 1986 egg msses s uniform in size s possible (45 ± 9 eggs mss -1 ) were obtined by plcing wire screen ( cm) onto wx pper before oviposition nd visully selecting the pproprite msses. Four egg mss densities (0 per plnt,3per10plnts,nd1nd5perplnt)were pplied by pinning the egg mss through the border of the wx pper to the midribs on the bxil lef surfce in the middle portion of the plnt. Cotton ws infested t three stges of crop phenologicl development (squring, flowering, nd boll mturtion). During the squring stge, the fruit lod on the plnt verged 12.8 squres, <1 bloom, nd <1 boll per plnt. At the flowering stge, the plnts hd n verge of 14.6 squres, 1.9 blooms,

3 JOURNAL OF COTTON SCIENCE, Volume 3, Issue 2, nd 9.1 bolls per plnt. Fruit on plnts t the boll mturtion stge were minly bolls (verge of 1.9 squres, <1 bloom, nd 16.0 bolls per plnt). To llow the fll rmyworm lrve to disperse nd behve s if n entire field ws infested t uniform densities, we lso infested 10 plnts on ech side of the plots with egg msses. Plots contined five plnts, were t lest 1 m long, nd were replicted 10 times. The plots were monitored weekly fter infesttion by mking whole plnt observtions until no fll rmyworm lrve were found. Informtion ws recorded on fll rmyworm density nd sizes (estimted lrvl instr), plnt dmge, nd fruit production. Plnts were hnd hrvested during the lst week of September, nd the hrvested seed cotton ws weighed to determine yield differences mongtretments. The experiment ws conducted in split plot design with whole plots nd sub-plots rrnged in rndomized complete block design. Our whole plots were the stges of plnt phenologicl development; egg mss densities were the sub-plots. Dt were nlyzed using nlysis of vrince nd regression nlysis (MSTAT, 1986) t Infesttion of Cotton with Lrve High mortlity of egg msses from unknown cuses occurred in our 1986 studies. Therefore, dditionl studies were conducted during 1987 with plnts infested with third instr fll rmyworm. During three phenologicl stges of plnt development (squring, flowering, nd boll mturtion), four densities of lrve (0, 2, 6, nd 18 per plnt) were plced on cotton-plnt fruiting structures using low-tension forceps. Ech phenologicl stge ws treted s seprte study. The fruiting structures on the plnts during the squring stge verged 27.4 squres, <1 bloom, nd <1 boll per plnt. During the flowering stge, n verge of 19.2 squres, 2.9 flowers, nd 12.9 bolls per plnt were observed in the plots. Fruit on the plnts t the boll mturtion stge of crop development consisted minly of bolls (verging 1.9 squres, <1 bloom, nd 16.0 bolls per plnt). If plnt did not contin dequte fruiting structures in which to plce the lrve (1 lrv per fruiting structure), lrve were plced on leves in rndomized complete block design. Ech plot ws 3 rows wide nd 6 m long. All plnts within ech plot were infested, but observtions were restricted to 10 consecutive plnts in the center row. Infesttion during the squring stge ws replicted seven times in nturl uncged environments. During the flowering stge, four replictes of nturl uncged environments nd four replictes of cged environments using m field cges (32-mesh screen) were infested. During the boll mturtion stge, only four replictions in cged environments were infested. Plnts were mpped by recording the min stem nd brnch node position of ll fruit prior to infesttion, t 3 nd 7 d post-infesttion, nd t hrvest. Dt were lso recorded for ech selected fruit clss (smll squre = squre with brct width <0.5 cm, medium squre = squre with brct width 0.6 cm to 1.2 cm, lrge squre = squre with brct width >1.2 cm, flowers, smll bolls = boll dimeter 1.5 cm to 2.5 cm nd lrge bolls = boll dimeter >2.5 cm) including the totl number of fruit, dmge to the fruit, fll rmyworm lrve density, nd type of fll rmyworm injury. The types of dmge cused by fll rmyworm re described in detil by Mink (1988) nd include clyx feeding, penetrtion (feeding hd resulted in hole in the coroll of squres or crpel wll of bolls, brct plus clyx feeding, brct feeding plus penetrtion, clyx feeding plus penetrtion, brct plus clyx feeding plus penetrtion, nd totl destruction (extensive internl feeding nd hollowing out of fruiting structure). Mpping the plnts prior to infesttion provided sufficient informtion to follow ll fruiting structures from infesttion until hrvest. Fruit survivl to hrvest lso ws recorded for ech dmge clss. The plots were hnd hrvested on 1 nd 8 October when bout 75 nd 95% of the totl bolls were open in the uninfested plots, respectively. The dt were nlyzed using nlysis of vrince (ANOVA) nd regression techniques (MSTAT, 1986). To determine the impct of the different types of fll rmyworm injury on ech cotton fruiting structure, the types of dmge within fruiting structure clsses were grouped cross lrvl density tretments in ech experiment, nd the probbility of hrvest ws determined. The types of dmge were ctegorized into two groups, vegettive

4 R. G. LUTTRELL AND J. S. MINK: DAMAGE TO COTTON FRUITING STRUCTURES 38 (feeding restricted to brct nd/or clyx) nd penetrtive (feeding hd resulted in hole in the coroll of squres or crpel wll of bolls). The undmged fruit within ech ge clss were used s stndrd with which to compre mong the dmge ctegories. An dditionl ctegory, totl dmge (totl number of fll rmyworm dmged fruit within fruit clss), ws included in the nlysis for comprison purposes. These individul comprisons were performed using K-smple binomil test for equl proportions (Mrscuilo nd McSweeney, 1975). This pir-wise comprison procedure djusts lph levels for ech comprison to control experiment-wise Type I error 0.05). Cging Lrve on Individul Fruit Third, fourth, nd fifth instr lrve were cged individully on cotton fruiting structures in 5.5 x 7.0 cm cotton cloth bgs. Four seprte groups of lrve were used in order to void removing bg more thn once in 24h. The observtion periods for the four groups were 6 nd 30, 12 nd 36, 18 nd 42, nd 48h. Dmge to young fruiting structures lte in the growing seson is generlly considered to hve little effect on yield. Thus, the ge clsses of fruit included in the present experiments were lrge squres (1 to 1.5 cm brct width), smll bolls (c. 1.5 to 2.5 cm in dimeter), nd lrge bolls (>2.5 cm in dimeter). Lrve were plced on undmged fruiting structures by trnsferring them from their rtificil diet with low-tension forceps. Preliminry studies indicted tht fll rmyworm lrve trnsferred from rtificil diet survived nd dmged cotton eqully s when they hd been llowed to feed on cotton fruiting structures 48 h prior to infesttion (Mink, 1988). After lrve were plced on the fruit, the cotton bgs were plced round the fruiting structures nd closed with drwstring. A cloth lundry tg (2 x4cm)ws ttched round the stem of the fruiting structure to id in post tretment observtions. All infesttions were mde either erly in the morning or lte in the fternoon to reduce the effects of high temperture on survivl of lrve. Seprte experiments were conducted for ech ge clss of fruit. All of the experiments were conducted in rndomized complete block design with three tretments (lrvl instr) nd four replictions. Ech tretment included 25 individully cged lrve on plnts in 20-m section of row ( 200 plnts per plot). A single plnt generlly hd only one cged insect. All dt were nlyzed by ANOVA nd mens were seprted by Duncn's (1955) multiple rnge test. Mens were significntly different t The experiment with squres ws conducted in July (14 nd 25) when the plnts were in the squring stge nd the fruit on the plnt consisted of minly squres nd few blooms. The experiment on bolls ws conducted during the first week of September when the plnts contined few squres nd flowers but minly smll nd lrge bolls. Bgs were removed t either 24 or 48 h to determine the sttus of the insect nd to note the types of dmge to the fruiting structure. Dmge ws ctegorized s vegettive, penetrtion, nd totl destruction. Another ctegory, no dmge, ws lso included. Observtions were gin mde t hrvest (13 nd 14 October) by removing nd counting the tgs tht were ssocited with hrvestble cotton bolls when c. 90% of the fruit in the untreted check (bgged fruit without n insect) were mture. RESULTS Infesttion of Cotton with Egg Msses Ech egg mss contined 45 ± 9 eggs. Percent survivl for egg msses ws determined s follows: number of lrve observed / (45 x number of egg msses per 10 plnts). Fll rmyworm survivl ws only 0.07 % (1587 lrve found fter 22,680 lrve infested) 3 d fter the egg msses were plced on the cotton plnts. Regression nlyses indicted tht severl plnt-dmge vribles in both the squring nd flowering stges of crop development were significntly influenced by number of egg msses plced on the plnts (Tble 1). The number of dmged squres, dmged bolls, nd totl dmged plnt structures were relted significntly to egg mss density in both the squring nd flowering stges of plnt phenologicl development. The number of leves dmged by fll rmyworm lrve ws significntly influenced by density of egg msses in the squring stge, s ws the number of flowers dmged in the flowering stge.

5 JOURNAL OF COTTON SCIENCE, Volume 3, Issue 2, Tble 1. Cotton plnt vribles found to be significntly relted 0.05) to S. frugiperd egg mss density. Vrible leves squres bolls Totl dmge Regression slope (SE) Regression intercept Correltion coefficient Infesttion t squring stge of development (0.225) (0.179) (0.024) (0.217) Infesttion t flowering stte of development (0.312) squres (0.057) flowers (0.216) bolls Totl dmge (0.435) Regression eqution is in the form of y = + bx where y = vrible observed (number per 10 plnts), = regression intercept, b=regression slope, nd x=number of egg msses (verge of 45 ± 9 eggs) plced on ech plnt. Totl dmge includes squres nd few smll bolls. Totl dmge t the flowering stge included squres, flowers, nd bolls. Infesttion of Cotton with Lrve Infesting cotton plnts with third instr lrve produced more dmge thn when plnts were infested with egg msses. Fll rmyworm dmge 7 d fter infesttion did not differ from tht recorded 3 d fter infesttion. After 3 d, survivl of fll rmyworm lrve ws 2.5 % (123 lrve found of 4940 infested) nd no lrve survived to 7 d postinfesttion. Fctors responsible for this low survivl were not identified. During the squring stge of crop development, fll rmyworm lrve cused significnt levels of dmge to smll, medium, nd lrge squres (Tble 2). Smll nd medium squres exhibited severl types of dmge lthough brct feeding ws the most common type of dmge observed. The totl number of fruit dmged ws significntly influenced by fll rmyworm density in ll three fruit clsses (Tble 1). As the density of lrve incresed, more dmge ws observed. Infesttions of fll rmyworm lrve t the flowering stge lso produced severl plnt dmge vribles tht were significntly ffected by density of fll rmyworm lrve (Tble 2). The fruit ge P Tble 2. Cotton plnt vribles found to be significntly relted 0.05) to S. frugiperd lrvl density. Vrible smll squres medium squres lrge squres lrge squres smll bolls lrge bolls Regression slope (SE) Regression intercept Correltion coefficient Infesttion t squring stge of development (0.090) (0.299) (0.008) Infesttion t flowering stge of development (0.017) (0.102) (0.102) Infesttion t mturtion stge of development (0.056) lrge bolls Regression eqution is in the form of y = + bx where y = vrible observed (number per 10 plnts), = regression intercept, b = regression slope nd x = number of lrve plced on ech plnt. Dmge includes vegettive feeding nd penetrtion. Vegettive dmge ws feeding injury to the surfce of the fruit. Penetrtion dmge resulted in hole in the crpel wlls of the fruit. clsses primrily ffected were smll nd lrge bolls (bsed on regression slopes). Medium squres nd blooms were not influenced by fll rmyworm density. During the boll mturing stge, the mjority of fruit on the plnts ws lrge bolls. Feeding dmge on lrge bolls ws incresed significntly by density of fll rmyworm lrve. However, this incresed level of dmge did not result in reductions in hrvestble fruit (Fig. 1). Yield ws not significntly influenced by number of lrve plced on plnts t 0.05 in ny experiment, lthough the slopes of regressions tended to be negtive between fll rmyworm lrve nd the mount of cotton hrvested s number of fll rmyworm lrve incresed (Mink, 1988). Regression of numbers of lrve plced on plnts ginst yield showed no significnt effect [Totl yield = (number of lrve plced on plnts) + 592, P = 0.07]. Anlysis of vrince P

6 R. G. LUTTRELL AND J. S. MINK: DAMAGE TO COTTON FRUITING STRUCTURES 40 Figure 1. Percent of cotton squres nd bolls dmged by S. frugiperd tht survived to hrvest. *Number of squres dmged t the flowering stge were insufficient to perform individul comprisons. indicted no significnt differences in % survivl of fruit to hrvest for the three types of fll rmyworm feeding dmge during the flowering stge of plnt phenologicl development in cged nd uncged plots (dt shown only for uncged plots in this report refer to Mink (1988) for more detil). As might be expected during plnt phenologicl development, feeding dmge by lrve ws limited to lrge squres during squring nd to lrge bolls during boll mturtion becuse these were the primry fruiting structures vilble during those stges (Fig. 1). During the flowering stge, lrve dmged both squres nd bolls, but more bolls thn squres were dmged during this stge. The number of dmged squres ws insufficient to perform the K-smple binomil test to determine differences in probbilities of hrvest. However, the probbilities of hrvest clculted for squres could be useful in estimting economic thresholds on cotton. Cged Lrve on Individul Fruit Mortlity of fll rmyworm lrve when cged individully on cotton fruiting structures ws low (<10%, 816 of 900 survived to 48 h). No significnt differences existed in mortlity mong instrs, so the effects of dmge re not djusted for mortlity. Lrge lrve cused more dmge thn did smll Figure 2. Type nd mount of dmge resulting from feeding of S. frugiperd lrve individully cged for 24 nd 48 h on cotton squres nd lrge bolls. lrve except for lrge bolls (Fig. 2). The percentge of squres dmged incresed significntly from 21 ± 19.7% (men ± stndrd devition) to 63 ± 11.9% for third instrs, 46 ±

7 JOURNAL OF COTTON SCIENCE, Volume 3, Issue 2, % to 71 ± 5.0% for fourth instrs, nd 52 ± 3.3% to 86 ± 5.2% for fifth instrs, respectively, from 6 to 48 h of exposure (dt not shown for 6 hr of exposure in Fig. 2). Third instrs penetrted the squres in less thn 6 h, but did not completely destroy ny squres until 36 h post-infesttion. Penetrtion of squres by fourth instrs lso occurredwithin6h,nddestructionofsqures occurred within 18 h. Fifth instrs were ble to destroysquresinlessthn6hofexposure. Fifth instrs destroyed significntly more fruit thn the other instrs t 18, 24, 30, 42 nd 48-h postinfesttion (dt shown only for 24 nd 48 h in Fig. 2). However, there were no significnt differences t hrvest in the survivl of squres exposed to fourth (25.3 ± 14.8%) nd fifth (22.5 ± 13.6%) instrs (Fig. 3). Significntly more squres dmged by third instrs were hrvested (43.5 ± 23.6%) thn those dmged by fourth nd fifth instrs. All instrs significntly reduced the survivl of squres to hrvest, compred with those tht were not exposed to lrve (83.5 ± 12.0% survivl of undmged squres). Since it is probble tht "destroyed" nd "penetrted" squres did not contribute to hrvest, the effect of "vegettively" dmged squres on hrvest cn be estimted. Of the squres "vegettively" dmged by third instrs nd fourth instrs, 54 nd 81% were not hrvested, respectively. However, only 24% of those vegettively dmged by fifth instrs were not hrvested. As with the squres, significntly more smll bolls were destroyed by fifth instrs t both post tretment mesurements (6.0 ± 4.0% t 24 h, nd 15.0 ± 5.0% t 48h.). Third instrs showed no destroyed bolls (0.0 ± 0.0%) t both mesurement times (Fig. 2). This dmge ws reflected in the number of smll bolls surviving to hrvest (Fig. 3), s well s in the overll trend for fifth instrs to dmge more fruiting structures. Only 34 ± 10.0% of the smll bolls infested with fifth instrs survived to hrvest (Fig. 3). This ws significntly lower thn tht observed for survivl of smll bolls dmged by fourth (50.0 ± 10.0%) nd third (73.0 ± 8.4%) instrs. The percentge of smll bolls exposed to third instrs nd surviving to hrvest ws not significntly different from the untreted check (65.0 ± 4.0%). It ppered tht "penetrtion" nd "destruction" of smll bolls by fourth nd fifth instrs resulted in the fruit not being hrvested, % Survivl to Hrvest b Not Infested Fourth Instr c c Third Instr Fifth Instr Squres Smll Bolls Lrge Bolls Figure 3. Effect of feeding by third, fourth nd fifth instr S. frugiperd cged individully for 48 hr on cotton fruiting structures on number of fruit surviving to hrvest. while "vegettive" dmge did not reduce the probbility of hrvest. "Penetrtion" dmge by third instrs ppered to hve less effect in reducing the probbility of hrvest. There my hve been difference in the depth of penetrtion by the different instrs, but this ws not recorded. As observed with smll bolls nd squres, the fourth nd fifth instrs significntly dmged more lrge bolls thn third instrs t both 24 nd 48 h post-infesttion (Fig. 2). However, the impct of this dmge ws not reflected in the percentge of bolls surviving to hrvest since none of the instrs cused dmge which significntly reduced the percentge of lrge bolls hrvested (77.0 ± 6.0) s compred to tht for the untreted check (80.0 ± 7.2) (Fig. 3). Neither "penetrtion" nor "vegettive" dmge of lrge boll ppered to reduce the probbility of hrvest of lrge bolls. DISCUSSION Although survivl of fll rmyworm on cotton ws low in ll studies, this insect exhibited the potentil to cuse dmge to cotton fruiting structures. This ws illustrted in tht dmge ws found to ll ge clsses of fruit when the cotton plnts were infested with the egg msses nd resulting lrve dispersed on the plnt normlly. b c

8 R. G. LUTTRELL AND J. S. MINK: DAMAGE TO COTTON FRUITING STRUCTURES 42 In the infesttion of cotton with lrve, preferences for vrious ge clsses of fruit by third instrs my hve been msked by the procedures used to infest the plnts. After infesting the plnts, however, the lrve were free to move normlly. In these studies, dmge ws gin observed on virtully ll ge clsses of fruit present on the plnt. Ali et l. (1990) found tht first nd second instrs of the fll rmyworm feed primrily on leves, but third instrs grvitte to cotton fruiting structures. Lter instrs feed predomintely on cotton fruiting structures. Fll rmyworm fed on ll ge clsses of cotton fruiting structures, but the dmge did not lwys hve n effect on survivl of fruit to hrvest. Although penetrtion of bolls did not lwys reduce probbility of hrvest, it my cuse decrese in qulity nd quntity of lint or increse the probbility of disese under different environmentl conditions. These effects were not recorded in this study. Informtion resulting from these studies cn be used to construct simple thresholds (Poston et l., 1983) for fll rmyworm on cotton. Our present study determined informtion t ech phenologicl stge of development, on the number of fll rmyworm lrve present, the number nd type of dmged fruit in prticulr ge clss, nd the probbilities of ech dmge ctegory within fruit ge clss being hrvested. These dt re n importnt first step in clculting simple thresholds for fll rmyworm on cotton. Fll rmyworm lrvl dmge to cotton fruiting structures my be put into perspective by comprison with lrvl dmge of H. virescens nd H. ze. Nicholson (1975) reported tht H. virescens lrve dmged 0.51 squres d -1 lrv -1 nd 0.1 bolls d -1 lrv -1 during their totl development on the cotton plnt, nd H. ze lrve dmged 0.55 squres d -1 lrv -1 nd 0.15 bolls -1 d -1 lrv -1. These dt indicte tht H. virescens nd H. ze lrve dmge more squres d -1 lrv -1 thn fll rmyworm lrve. Fll rmyworm lrve cused dmge to squres when egg msses nd third instrs were plced on plnts t the squring nd flowering stges of crop development (Tbles 1 nd 2), but the mount of dmge ws low. Low survivl rtes of eggs nd smll lrve contributed to the low dmge rtes mesured in regressions with infesttion densities. Clculted number of dmged squres per fll rmyworm egg mss infested t the squring stge ws less thn 0.2 for the entire observtion period (3 wk of observtions). Higher survivl rtes would hve incresed this estimted dmge potentil. The dmge rtes for H. virescens nd H. ze reported by Nicholson (1975) re conservtive estimtes reltive to those mesured for the individul cged fll rmyworm lrve on cotton fruiting structures in our study. In Nicholson s studies, lrve were llowed to move freely nd select feeding sites on the plnt, nd the verge vlues were clculted cross ll lrvl instrs. Erly instr H. virescens nd H. ze prefer squres to bolls nd thus spend more time feeding on squres thn on bolls. If H. virescens nd H. ze lrve hd been cged on the fruiting structures, s were the fll rmyworm lrve in our study, there might hve been more dmge to bolls. The estimted dmge to bolls by feeding of fll rmyworm lrve in the studies reported here rnged from 0.40 bolls per 2 d per infested lrv for third instrs feeding on lrge bolls to 0.81 bolls per 2 d per infested lrv for fifth instrs feeding on smll bolls. A reltive comprison of boll dmge my be postulted if one ssumes tht first nd second instrs of both species do not feed on bolls nd tht the verge length of the lrvl stge is 15 d with 3 d in ech of five instrs. Bsed on these ssumptions, fll rmyworm lrve would dmge 0.23 bolls d -1 lrv -1 throughout the entire lrvl stge if feeding rtes for lrge bolls re used. This estimted dmge rte is ctully higher thn those reported by Nicholson (1975) for H. virescens (0.1 bolls -1 d -1 lrv -1 )ndh. ze (0.15 bolls -1 d -1 lrv -1 ). These estimtes suggest tht feeding by fll rmyworm lrve on bolls my be s dmging s tht by H. virescens nd H. ze. A more detiled exmintion of thresholds for fll rmyworm on cotton is found in Mink (1988). The current recommended threshold for H. virescens nd H. ze on cotton in Mississippi is four lrve per 100 plnts (Lyton, 1997). This ction level is bsed on detection of smll lrve in the terminls. Survivl of lrve in the bsence of insecticide nd expected levels of control re importnt considertions in the development of thresholds. Studies by Mink nd Luttrell (1989) indicted tht fll rmyworm lrve would be more

9 JOURNAL OF COTTON SCIENCE, Volume 3, Issue 2, difficult to control thn H. virescens nd H. ze becuse they re found lower within the plnt cnopy (Ali et l., 1990). Fll rmyworm survivl on cotton is low nd the detection of egg msses nd smll lrve requires more ttention tht detection of H. virescens nd H. ze eggs nd lrve (Ali nd Luttrell, 1990). Most often, fll rmyworm infesttions re recognized fter the lrve hve developed to lte instrs nd hve begun to feed on fruiting structures. The need to control lte instrs of the fll rmyworm lrve when lrge popultions re present on cotton seems to be just s gret s the need to control lte instrs of H. virescens nd H. ze. Current recommendtions for detection of fll rmyworm in Mississippi re bsed on exmintions of blooms nd bolls. Feeding on the brcts of bolls in the lower prt of the plnt nd detection of lrve in blooms will trigger insecticide pplictions. These smpling procedures re expensive nd time consuming but re wrrnted when high popultion densities of fll rmyworm re present nd when cotton is n ttrctive crop for fll rmyworm oviposition (Pitre, 1979; Ali et l., 1989). ACKNOWLEDGMENTS The uthors thnk Frnk Dvis, Blke Lyton, Henry Pitre, nd Mike Willims for reviews of erly drfts of this mnuscript. The Cotton Foundtion nd Mississippi Agriculturl nd Forestry Experiment Sttion provided funds for this reserch. This is mnuscript J93-07 in the journl series of the Mississippi Agriculturl nd Forestry Experiment Sttion. REFERENCES Adkisson, P.L., C.F. Biley nd R.L. Hnn Effect of the bollworm, Heliothis ze, on yield nd qulity of cotton. J. Econ. Entomol. 57: Ali, A., R.G. Luttrell, H.N. Pitre nd F.M. Dvis Distribution of fll rmyworm (Lepidopter: Noctuide) egg msses on cotton. Environ. Entomolg. 18: Ali, A. nd R.G. Luttrell Survivl of fll rmyworm (Lepidopter: Noctuide) immtures on cotton. Fl. Entomol. 73: Ali, A., R.G. Luttrell nd H.N. Pitre Feeding sites nd distribution of fll rmyworm (Lepidopter: Noctuide) lrve on cotton. Environ. Entomolg. 19: Ali, A., R.G. Luttrell nd J.C. Schneider. 1990b. Effects of temperture nd lrvl diet on development of the fll rmyworm (Lepidopter: Noctuide). Ann. Entomol. Soc. Am. 83: Bss, M.H Fll rmyworm: Evlution of insecticides for control. Auburn Univ. Agric. Exp. Stn., Leflet 93:7. Chndler, L.D Comptibility of insect growth regultors nd bcteril insecticides for mngement of Spodopter spp. p In D.A. Richter nd J. Armour (ed.) Proc. Beltwide Cotton Conf., Sn Antonio, TX. 5 8 Jn Ntl Cotton Council Am., Memphis, TN. Clower, D.F Thrips, whiteflies nd Spodopter problems in the Midsouth. p In J.M. Bown nd T.C. Nelson (ed.). Proc. Beltwide Cotton Conf., Atlnt, GA Jn Ntl Cotton Council Am., Memphis, TN. Cruz, I. nd F.T. Turpin Yield impct of lrvl infesttions of the fll rmyworm(lepidopter:noctuide) to midwhorl growth stge of corn. J. Econ. Entomol. 76: Dvis, F.M., T.G. Oswlt nd S.S. Ng Improved oviposition nd egg collection system for the fll rmyworm (Lepidopter:Noctuide). J. Econ. Entomol. 78: Dvis, F.M Rering the southwestern corn borer nd fll rmyworm t Mississippi Stte University. p In Towrd insect resistnt mize for the third world: Proceedings of the interntionl symposium on methodologies for developing host plnt resistnce to mize insects. CIMMYT. Mexico City, Mexico. Dew, J.A Fll rmyworm, Lphygm frugiperd. J. Econ. Entomol. 6: Duncn, D.B Multiple rnge nd multiple F test. Biometrics 11:1-41. Hrtstck, A.W., R.L. Ridgwy nd S.L. Jones Dmge to cotton by the bollworm nd tobcco budworm. J. Econ. Entomol. 71: Henderson, C.F., H.G. Kinzer nd E.G. Thompson Growth nd yield of grin sorghum infested in the whorl with fll rmyworm. J. Econ. Entomol. 59: Kincde, R.T., M.L. Lster nd J.R. Brzzel Dmgeto cotton by the tobcco budworm. J. Econ. Entomol. 60: Lyton, M.B Cotton insect control guide, Publ. 343, Mississippi Stte Univ. Luginbill, P.A The fll rmyworm. USDA Tech. Bull. 34. U.S. Gov. Print Office. Luginbill, P.A Hbits nd control of the fll rmyworm. USDA Frmers' Bull U.S. Gov. Print Office.

10 R. G. LUTTRELL AND J. S. MINK: DAMAGE TO COTTON FRUITING STRUCTURES 44 McIntosh, S.C., T.G. Stone, S.R. Sims, P.L. Hurst, J.T. Greenplte, P.G. Mrrone, F.J. Perlk nd R.L. Fuchs Specificity nd efficcy of purified Bcillus thuringiensis proteins ginst gronomiclly importnt insects. J. Invertebr. Pthol. 56: Mrscuilo, L.A. nd M.McSweeney Nonprmetric nd distribution-free methods for the socil sciences. Wdsworth Publ Co., Inc. Belmont, CA. Mink, J.S Dmge potentil nd chemicl control of the fll rmyworm, Spodopter frugiperd (J. E. Smith), on cotton. M.S. Thesis, Mississippi Stte Univ. Mink, J.S. nd R.G. Luttrell Mortlity of fll rmyworm, Spodopter frugiperd (Lepidopter: Noctuide) eggs, lrve nd dults exposed to severl insecticides on cotton. J. Entomol. Sci. 24: Morrill, W.L. nd G.L. Greene Survivl of fll rmyworm lrve nd yields of field corn fter rtificil infesttions. J. Econ. Entomol. 67: MSTAT Microcomputer sttisticl progrm: User s guide. Michign Stte Univ. Est Lnsing, MI. Nicholson, W.F Feeding of Heliothis virescens (F.) nd H. ze (Boddie) on cotton with emphsis on development of simultion model of lrvl feeding. Ph. D. Diss., Mississippi Stte Univ., Mississippi Stte, MS. Pshley, D.P. nd J.A. Mrtin Reproductive incomptibility between host strins of the fll rmyworm (Lepidopter:Noctuide). Ann. Entomol. Soc. Am. 80: Pshley, D.P., A.M. Hmmond, nd T.N. Hrdy Reproductive isolting mechnisms in fll rmyworm host strins (Lepidopter: Noctuide). Ann. Entomol. Soc. Am. 85: Pitre, H.N. nd D.B. Hogg Development of the fll rmyworm on cotton, soyben nd corn. J. Georgi Entomol. Soc. 18: Poston, F.L., L.P. Pedigo nd S.M. Welch Economic injury levels: relity nd prcticlity. Bull. Entomol. Soc. Am. 29: Smith, R.H Fll nd beet rmyworm control. p In J.M. Brown nd T.C. Nelson (ed.) Proc. Beltwide Cotton Conf., New Orlens, LA Jn Ntl Cotton Council Am., Memphis, TN. Smith, T.W., M.J. Sullivn, nd S.G. Turnipseed Control of fll rmyworm in South Crolin cotton producer/consultnt dilemm. p In D.J. Herber nd D.A. Richter (ed.) Proc. Beltwide Cotton Conf., New Orlens, LA Jn Ntl Cotton Council Am., Memphis, TN. Stern, V.M., R.E. Smith, R. vn den Bosch nd K.S. Hgen The integrted control concept. Hilgrdi 29: Vickery, R.A Studies of the fll rmyworm in the Gulf Cost district of Texs. Tech. Bull USDA, Wshington, DC. Wlton, W.R. nd P. Luginbill The fll rmyworm or grss worm nd its control. Frmers Bull USDA. Wshington, DC. Wn, L Susceptibility nd voidnce to Bcillus thuringiensis endotoxin in noctuid lrve ttcking cotton nd soyben. M.S. Thesis, Mississippi Stte Univ. Mississippi Stte, MS. Young, J.R Fll rmyworm: Control with insecticides. Fl. Entomol. 62: Pitre, H.N Fll rmyworm on sorghum: other hosts. Bull Mississippi Agric. For. Exp. Stn. Mississippi Stte, MS.