Row CROPS, FORAGE,ANDSMALLGRAINS Fall Armyworm (: Noctuidae) and lineolata (Lepidoptera: Pyralidae): Impact of Larval Population Level and Temporal Occurrence on Yield in Nicaragua

ALLAN J. HRUSKAl ANDFRED GOULD Department of Entomology, North Carolina State University,Box 7634, Raleigh, NC 27695 Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021

J. Econ. Entomo!. 90(2): 611-622 (1997) ABSTRACT Four experiments were conducted in Nicaraguaover a 3-yrperiod to determine the effect of whorl-stage infestations of Spodoptera frugiperda (J. E. Smith) and Diatraea lineo/ata (Walker) on maize grain yields. The experiments separated the effects of period of infestation from level of infestation. Results from all experiments demonstrated that maize is more tolerant of lepidopteran infestation during early growth stages than at later stages. In- festation during mid-through late whorl caused yield losses of 15-73% when 55-100% of the plants were infested with S.frugiperda. D. lineolata infestations were highly correlated with S.frugiperda. D. /ineolata infestations were highly correlated with S.frugiperda infestation. Economic injury levels ranged from 23 to 63% of the plants infested with S.frugiperda and D. lineo/ata over a range of potential yields, using mean yield reduction from the 4 experi- ments, and an average 75% insecticide effectiveness.

KEY WORDS Spodoptera frugiperda, Diatraea lineolata, maize, Nicaragua,economicinjury level

Two OF THE more important biological constraints In Nicaragua, van Huis (1981) found a 33% in- to maize production throughout Latin America are crease in dry maize grain yield when plants were Spadoptera frugiperda (J. E. Smith) and Diatraea protected with an insecticide application to the linealata (Walker) (Andrews 1980, 1988; Rodri- whorl. The application decreased S.frugiperda in- guez-deI-Bosque et al. 1988; Evans and Stansly festation from 59 to 2% of the plants, and D. li- 1990). Subsistence fanners sometimes use multi- nealata infestation from 3.3 to 1.3 perforations per ple applications of insecticides to treat for these plant. In a study using cages to exclude S. frugi- pests, especially when governments subsidize ag- perda, van Huis (1981) found that D. linealata in- ricultural inputs. In 1989 on the Pacific plain of festation reduced maize grain yield from 3 to 6% Nicaragua, 99.6% of small fanners applied insec- per borer per plant. ticides to maize, with an average of 6.3 applications Obando (1976) did not find a significant corre- per season (A.J.H., unpublished data). lation between the presence of Diatraea spp. and Quantifying the effects of timing and level of yield, when other pests, including S. frugiperda, infestation of these 2 pests on maize yield is es- were not controlled. sential for developing economic thresholds and im- Other studies in North and Central America plementing rational pest management strategies. have found yield losses ranging from 7 to 35% at- Few studies have examined the impact of these 2 tributed to natural infestations of Diatraea spp., species on maize yield in Latin America, and fewer but none of these studies (Daniels and Chedester still have examined the separate effects of level and 1977, Alvarez and Morales 1979, Rodriguez-del- timing of infestation. Bosque et al. 1988) examined the effect of maize Hruska (1995) reviewed the data on maize yield growth-stage infestation on the extent of yield loss. losses from S. frugiperda infestation in 6 studies Throughout the Pacific plain of Nicaragua, S. done in Latin America. Linear regressions of the frugiperda and D. linealata occur together and in- relationship between S.frugiperda infestation and fest maize in similar patterns. van Huis (1981) yield loss predicted between 15 and 100% yield found a higWy significant correlation (,2 > 0.70) loss when all of the plants were infested during between D. linealata and S.frugiperda infestation mid- to late-whorl stage. in 3 studies carried out in 3 locations. Other stud- ies in Central America have found similar results

I Current address: Department of Crop Protection, Zamorano, from experiments with insecticide-treated and un- Box 93, Teguci~,tlpa, . treated plants (Fuentes 1969, Obando 1976, Leyva

0022-0493/97/0611-0622$02.00/0 © 1997 Entomological Society of America 612 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 2

1988). Both are found in the whorl of the were targeted at D. lineolata (van Huis et al. 1982). maize plant as early instars, especially during mid- In 1989, only 5% of Nicaraguan maize farmers in to late whorl stage. There are no insecticides that the Pacific region mentioned D. lineolata as 1 of are selective for one but not the other; thus, com- the 3 more important pests in maize, whereas mercial and experimental chemical control of one 100% mentioned S. frugiperda (A.J.H., unpub- species results in control of both, when applied lished data). from mid- to late-whorl. Economic threshold recommendations for S. Eighty percent of D. lineolata eggs are laid dur- frugiperda in maize vary from 11% (Evans and ing late whorl stage in Nicaragua (van Huis 1981). Stansly 1990) to 40% of the plants infested (An- S.frugiperda begins oviposition earlier than D. li- drews and Rueda 1986, Hruska et al. 1988), with neolata, with 80% of D. lineolata oviposition oc- most recommendations in the 20-30% range (Sar- curring during mid-through late-whorl stages (van miento and Casanova 1975, Obando 1976, van Huis 1981). Both species move quickly into the Huis 1981). Although economic thresholds are giv- Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 whorl on eelosion, with D. lineolata burrowing en in terms of S.frugiperda infestation, it is likely deeper than S.frugiperda. Damage to maize leaves that the recommendations are for the combined by early instars of 2 pests, a skeletonization of leaf infestation of S.frugiperda and D. lineolata, in ar- areas, cannot be distinguis,hed. Seven days after eas where the ranges of 2 pests overlap. eelosion, D. lineolata causes distinctive damage as This study was carried out to determine the im- it burrows into the unwhorled leaves, leaving a row pact of S. frugiperda and D. lineolata infestation of holes when the leaf expands. By 4th instar, D. on maize grain yields in Nicaragua. In contrast to lineolata begins to burrow into the stem, where it previous studies, the effects of timing of infestation is no longer accessible by foliar insecticides. As lar- and level of infestation were separated to deter- vae burrow downward, they make small holes in mine the impact of level of infestation during dif- the stem. Before entering diapause or pupating, ferent periods of infestation, leading to recommen- and exit hole is made in the stem, leaving a thin dations for both timing of control and economic layer of epidermis intact. . injury levels. Most S. frugiperda larvae stay in the whorl, feeding on new leaves (Morrill and Greene 1973). Materials and Methods The damage to newly expanding leaves and frass is easily observed by visual examination of the Four experiments were carried out at 2 field sta- whorls. Late instars are cannibalistic, usually re- tions within 5 km of each other on the eastern sulting in only 1 large S. frugiperda per plant edge of Managua, Nicaragua. The sites are on the (Wiseman and McMillian 1969). Pacific plain of Nicaragua at 70-100 m above sea The effect of timing of S.frugiperda infestation level and are characterized by distinct wet and dry on maize yield has been examined in 2 studies. seasons. An average of 1,030 mm of rain falls dur- Gross et al. (1982) found that S. frugiperda infes- ing the wet season, from May through November, tation during early- and late-whorl stages caused when both 1st and 2nd planting cyeles occur, and yield reduction, but not during midwhorl stage. an average 80 mm falls during the dry season from van Huis (1981) conducted an artificial defoliation December through April. These conditions are experiment, simulating S.frugiperda damage from similar to much of the Pacific plain, where annual early-whorl stage to tasseling. He found signifi- rainfall ranges from 600 to 1,400 mm. Maize also cantly greater yield reductions when defoliation may be produced during the dry season, using ir- occurred during mid- to late-whorl stages, com- rigation. Two experiments were carried out during pared with early-whorl or tasseling. the 1st rainy season, 1 during the 2 rainy season, The critical period of protection for D. lineolata and 1 under irrigation. has been studied for some growth stages of maize. Experiment 1: First Rainy Season 1986. The van Huis (1981) worked with the 1st half of the 1st experiment was planted in June 1986 at the whorl stage and found that in a long-cyele variety, "Las Mercedes" Experimental Station of the Univ- application of chlorpyrifos of the whorl at 15 d af- ersidad Nacional Agraria, located at Kilometer 10 ter planting did not affect infestation by D. lineo- of the Carretera Norte, Managua, Nicaragua. lata. There was a tendency toward reduced infes- Maize variety NB-lOO, a locally developed open tation with application at 30 d after planting. pollinated inbred line, with maturity at 90 d, was Another experiment (van Huis 1981) showed that planted by hand. Distance between rows was 92 insecticide application in tassel stage maize signif- cm and between plants 20 cm, for a planting den- icantly lowered infestation by D. lineolata. sity of 54,000 plants per hectare. Final plant den- Despite the fact that D. lineolata can cause sig- sity was =42,000 plants per hectare. nificant yield losses in maize, it is not recognized At planting, fertilizer (12:30:10, N:P:K) was ap- as an important pest by most farmers. Although plied (130 kglha) and chlorpyrifos (Lorsban 480 E) 72% of the maize farmers in the Pacific region of was applied at 2.06 kg (AI)/ha to the soil to control Nicaragua recognized D. lineolata larvae, only 16% Phyllophaga spp. detected in sampling. Metola- considered it an important pest, and only 1% of chlor (Dual 480 E) was applied before planting for the foliar insecticide applications used in maize Cyperus spp. control, and pendimethalin (Prowl April 1997 HRUSKA AND GOULD: ARMYWORM AND Diatraea EFFECT ON MAIZE YIELD 613

Tobl•• 1. Exp••rin•••nt 1: S. frugiperda ond D. lineolata meosured infestotion levels ond yield (meoD ± SE)

Targ ••t and measured S. frugiperda infestation Trt'atm~nt D. /illeo/ata Yield 20--33 DAP 33 DAP 33-45 DAP (n'plicat •• plots) infestation index (grams per plant) Target Actual Target 1 (12) 100 74.4:!: 3.9 100 110.0:!: 13.0 55.5:!: 3.H 2 (12) 40 22.0:!: 3.1 40 16.7 :!:4.4 71.H :!:3.3 3 (12) 20 22.7:!: 6.2 20 11.0 :!:4.3 70.9 :!:2.4 4 (44) 0 4.7:!: 0.89 0 4.5:!: 1.2 75.] :!: 2.0 5 (12) 0 10.3 :!:2.1 100 19.5 :!:4.1 65.4 :+c 4.3 6 (12) 0 3.2:!: 1.1 40 7.3:!: 2.2 73.2:!: 3.2 7 (12) 0 12.9 :!:5.6 20 16.6 :!:7.4 76.0::!: 5.5 H(H) 100 67.0:!: 6.1 0 15.6:!: 4.3 64.1 :!: 3.4 Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 9(H) 40 17.7:!: 4.2 0 5.5:!: 3.4 74.5 :!:4.4 10 (8) 20 10.9 :!:2.8 0 2.2:!: 1.1 70.6:!: 5.2

DAP, days aftt'r planting.

500 E) 1 d after plant emergence for grass and determined by nondestructive visual inspection of broadleaf control. They were applied at 2.06 kg the whorls. Presence of S.frugiperda larvae, fresh (AI)/ha and 2.15 kg (AI)/ha, respectively. At 33 d plant damage, or recent S. frugiperda frass indi- after planting, urea (45% N) was applied at 130 cated S.frugiperda infestation. Because of earlier kg/ha and the plots were manually weeded. than anticipated tasseling, the effect of S. frngi- Natural S. frngiperda infestation was expected perda infestation during 33-45 d after planting to be 100%, so different infestation levels were could not be measured. As a proxy measure of S. created by applying insecticide to protect plants frugiperda infestation during that period, the per- from naturally occurring infestation. Protection centage of plants in each plot that received chlor- was provided by applying chlorpyrifos at 0.336 kg pyrifos application was calculated, and 1 minus this (AI)/ha. The liquid insecticide was mixed with value was used as a variable in the analysis. moist sawdust and applied directly to the whorls At harvest (113 d after planting), D. lineolata of the plants. This is a common application method exit holes in the 3 most basal internodes were in Nicaragua. To match the target levels of infes- counted in plants in the 2 central rows and the tation, tlle percentage of plants infested in each mean calculated (e). An infestation index (1.1.)was plot was determined 1 d before application, and calculated for each plot as follows: the number of plants to receive application was 1.1. = % plants infested per plot X E X 100 determined. Plants that received application were chosen at random. At 113 d after planting the 2 center rows were The experimental design involved 4 completely harvested. The number of plants harvested per row randomized blocks, with patterns of chlorpyrifos was counted. Ears were weighed, and grain weight application resulting in 10 distinct targeted S.fru- at 15% moisture was determined per plant. giperda infestations (Table 1). The 10 treatments Experiment 2: Second Rainy Season 1986. A consisted of combinations of 2 factors: period and 2nd cycle experiment was planted in September level of infestation by S. frugiperda and D. lineo- 1986. Open-pollinated maize variety NB-6 (matu- lata. There were 2 periods of infestation: 20-33 rity at 110 d) was planted at a density of 80 cm and 33-45 d after planting. These periods corre- between rows and 20 cm between plants, for a spond to mid- to late-whorl and late-whorl to tas- density of 62,500 plants per hectare. Final density seling stages. Each period was used alone, and in was 46,000 plants per hectare because of seedling combination with the other period, making 3 pe- mortality caused by birds. Fertilizer (12:24:12, N: riod combinations. Chlorpyrifos provided protec- P:K) was applied at planting at a rate of 130 km/ha, tion from S.frngiperda for 12-15 d. Within each and urea 45% was applied at 33 d after planting at period of infestation there were 3 target levels of a rate of 130 kglha. Soil and weed control S.frngiperda infestation: 20, 40, and 100% of the were identical to the 1st experiment except that plants infested. In addition, there was 1 control the herbicide pendimethalin was not applied. treatment \vith a target infestation of 0% target for A randomized complete block design was used all periods (2 or 3 insecticide applications). Each with 5 treatments and 5 blocks. Insecticide appli- plot consisted of four 4-m rows. There was a I-m cations (chlorpyrifos, applied at the same rate and border of maize plants at the ends of sampled manner as in experiment 1) to 100% of the plants rows, and 2 rows of plant border were at the ends per treatment were made at the following 4 treat- of the experimental area. ment schedules: (1) 8 d, (2) 8 and 18 d, (3) 18 d, At 8, 20, and 33 d after planting the percentage (4) 8, 18, 33, and 48 d after planting; and (5) with- of plants infested by S.frngiperda in each plot was out application (Table 2). The 4 application dates determined by examining 20 plants in each of the correspond to early-, mid-, late-midwhorl, late- 2 center rows chosen at random. Infestation was whorl, and tasseling stages. 614 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 2

Tabl" 2. Muitil'l" regrtJssion annlystJs ttJsting tbtJ ef- were used. Each block included a control (target fects of S. frugiperda infestation and D. lineolata infes- of 0% of plants infested) for a total of 16 treat- tation on yield (grams per plant) for experiments 1-4 ments. D. lineolata infestation was measured at Source F df p harvest, as described in experiment 1. Experiment 4: Irrigated Maize 1987. Maize Experiment 1 variety NB-6 was planted at the National Center Block 7.99 3 0.0001 S.fmgil'erda 33-45 DAP 5.04 1 0.0264 for Basic Grain Research in Managua in March D. linealata 17.32 1 0.0001 1987. Planting density was 67,500 plants per hect- Error 134 are. Final density was 52,000 plants per hectare. Experiment 2 A randomized complete block design with 19 Block 0.57 4 0.6904 treatments and 8 blocks was used. Treatments S. flll{!.il'crda infestation 4] DAP 4.32 1 0.0524 were 6 combinations of 3 periods of infestation and D. lincalata infestation 0.04 1 0.8352 3 levels, plus a control. The periods (5-17, 17-31, Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 Error 18 31-45 d after planting) correspond to early to mid- Experiment 3 whorl, midwhorl, and midwhorl to tasseling. Insect Block 3.66 5 0.0048 control methods were the same as experiments 1- S.fl1lgil'crda 32 DAP 6.60 1 0.0120 D. lineolata 46 DAP 10.39 1 0.0018 3. Four target levels of infestation (0, 40, 70, and D. linl'Olata infestation 5.50 1 0.02.'38 100%) of the plants infested were used. The per- Error 84 centage of plants infested was determined by vi- Experiment 4 sually inspecting the whorls at 10, 23, 30, and 39 Block 5.41 7 0.0001 d after planting. At harvest, infestation level of D. S. fmgiperda 10 DAP 4.51 1 0.0360 lineolata was determined. Yield was determined at S. fmgil'crda 23 DAP 7.92 1 0.0058 harvest (115 d after planting) for grain at 15% D. lineolata 1.03 1 0.3174 Error 108 moisture. Statistical Analysis. The effect of block, level, DAP, days after planting. and period of infestation was analyzed using a fac- torial analysis of variance (ANOVA), with the level and period treated as fixed effects. All interactions Each plot consisted of six 5-m rows, and borders were tested, and significant interactions were re- were the same in the 1st-cycle experiment. Infes- tained in the models. Data were tested for nor- tation by D. lineolata was measured in the 4 cen- mality before conducting ANOVAs. Planned or- tral meters of the 4 central rows of each plot using thogonal contrasts were carried out to compare the the same method as in the 1st experiment. effect of target level of infestation and period of At 41 d after planting, plants in the 2 central infestation on yield (SAS Institute 1989). rows of each plot were examined for infestation by Regressions were carried out using yield as the S. frugiperda. The percentage of plants infested dependent variable, and actual S. fmgiperda and was determined by visual inspection, and the se- D..lineolata infestation levels as independent vari- verity of damage by S.fmgiperda in the whorl and ables. Both linear and quadratic models were test- first 2 expanded leaves was estimated with a visual ed. Type III sums of squares and significance levels rating scale from 1 to 5, where 1 = no leaf damage, are reported in all cases. 2 = small «1 cm2) areas of skeletonized leaves, 3 The relationship between S. fmgiperda infesta- = medium (> 1 but <2 cm2) areas of skeletonized tion and yield was estimated by multiple regression leaves and small «1 cm2) holes, 4 = medium (> 1 analysis. To avoid confounding timing of infesta- but <2 cm2) holes, and 5 = large (>2 cm2) holes. tion with level of infestation, regressions examining Experiment 3: First Rainy Season 1987. The the impact of early dates of infestation on yield 3rd experiment was planted at the San Cristobal used treatments that were infested during the early National Center for Basic Grains Research in June periods only. Regressions examining the impact of 1987. Maize variety NB-6 was planted at 57,700 later dates of infestation on yield used treatments plants per hectare. Final plant density was that had infestation only during later periods, or ""42,500 plants per hectare. At planting, fertilizer that were infested throughout all periods. Both lin- (12:30:10, N:P:K) was incorporated into the soil at ear and quadratic regressions were fit. The corre- 195 kglha. Two days after planting, atrazine was lation between S. fmgiperda infestation at differ- applied at 0.254 kg (AI)/ha. At 33 and 40 dafter ent dates and D. lineolata infestation at harvest was planting the field was cultivated, and nitrogen determined. (urea, 46%) was applied on each date (65 kglha). Economic injury levels (ElL) (expressed as per- Two factors, level and period of infestation of S. centage of plants infested) were calculated using fmgiperda infestation, were arranged in 6 random- the following equation: ELL = CIPDK., where C is ized complete blocks. The 3 target levels of S. fm- the cost (U.S.$/ha) of chemical insecticide appli- giperda infestation were 100, 70, and 40% of the cation (U.S.$ 15.50) in 1995, P is the price the plants infested. Five combinations of 3 periods farmer receives for maize (U.S.$0.0803/kg) in (11-24,24-39,39-46 d after planting) correspond- 1995, D is the slope of the linear regression of S. ing to midwhorl and late-whorl and tasseling stages fmgiperda infestation against yield (kglha per per- April 1997 HRUSKA AND GOULD: ARMYWORM AND Diatraea EFFECT ON MAIZE YIELD 615

Table 3. ANOVA testing the effects of block, level Table 4. Experiment 1: Mean yields for 4 levels of S. (per("eutullieof plants infested) and period (timing of in- frugiperda infestation during 3 IlirowthI,criods and 3 I,e- festation) of S. frugiperda, and D. lineolata infestation riods of infestation (0 = 0% target infestation; I, natural and treatm ••nt (timiulliof infestation) on yield infestation)

S()urct~ F df P Mean Yield infes- Yield ± SE Period of (grams [wr Expl'riment 1 tation (g per plant) infestation plaut) Rlnek 8.13 3 0.0001 (% (5E) L,·v,·1 8.49 2 0.0001 plants) P"riod 1.24 2 0.2927 Error 88 74 61.6 ± 2.25a I-I 08.0 ± 3.57a 23 72.5 ± 2.25b 0-1 72.6 ± 3.57a Experim ••nt 2 22 73.2 ± 2.25h 1-0 70.7 ± 4.38a Rloek 0.40 4 0.8034 7 75.2 ± 3.54b

Pl'riod 5.02 4 0.0081 Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 Error 16 Means followed by th£' same lettf'r are not significantly differl'nt Experiment 3 (P> 0.05; LSMEANS [SAS Institute 1989]). Block 3.73 5 0.0049 L,'v,,1 1.10 2 0.3391 P"riod 2.75 4 0.0352 Plants infested with the maximum S.frugiperda Level X Period 1.14 8 0.3474 infestation (74%) had significantly lower yield than Error 67 treatments with levels of 7, 22, and 23%. Yields Experiment 4 were 18% lower in treatments with an average of Rlock 4.34 7 0.0004 74% S.frugiperda infestation than treatments with Lew·1 2.33 2 0.1036 7% S.frugiperda infestation (Table 4). Yields were p,'riod 3.43 5 0.0071 not significantly different among the levels of 7, L"vl'l X Period 1.75 10 0.0808 Error 89 22, and 23% S.frugiperda infestation. Orthogonal contrasts confirmed the significant differences be- All intt'raetions w,'re test ••d and wer •• not significant (P > 0.05). tween 74% infestation levels and both 7 and 22% All ,·fTt·ds W,'rt· t,·st ••d 'L< fixed t'ff£'cts. target infestation on yield (F = 21.86; df = 1, 127; P = 0.0001 and F = 13.38; df = 1, 127; P = centage of plants infested), and K is the efficacy of 0.0004), and there was no significant difference be- the pesticide application. This model of a single tween 7 and 22% S.frugiperda infestation on yield application to achieve insect control fits the S.fru- (F = 0.45; df = 1, 127; P = 0.51). giperda-maize situation in Nicaragua, when appli- Experim.ent 2: Second Rainy Season 1986. cation occurs during mid- to late-whorl stages. The To allow comparison with the other experiments, economic injury levels over a range of potential S. frugiperda damage ratings were converted into yields and efficiency of insecticide applications was measures of percentage of plants infested. The calculated. correlation between these 2 measures is highly sig- nificant (r = 0.97, P < 0.0001) (A.J.H., unpub- lished data). As in experiment 1, the maximum S. Results frugiperda infestation did not reach 100%, and was Experiment 1: First Rainy Season 1986. S. 77% (as predicted from damage rating) (Table 5). frugiperda infestation did not reach 100% of the D. lineolata infestations were similar to experiment plants infested, even in plots that were not treated 1. Yields were much lower than experiment 1 be- with insecticide. The maximum recorded infesta- cause of a severe drought in the later stages of tion was 74% (Table 1). Because of the low natural plant development. Only the treatment of com- infestation levels, targeted S. frugiperda infesta- plete protection had a significantly higher yield tions were not matched closely, as measured at 33 than other treatments (Table 5). d after planting (Table 1). The low target level Spodoptera frugiperda infestation at 41 dafter treatments (20 and 40%) had infestations of be- planting was significantly positively correlated with tween 3 and 23% at 33 d after planting. D. lineolata infestation (r = 0.768, P < 0.0001) and Spodoptera frugiperda infestation at 33 dafter negatively correlated with yield (r = 0.526, P = planting was positively correlated with D. lineolata 0.007). D. lineolata infestation was marginally neg- infestation at harvest (P < 0.0001, r = 0.677). atively correlated with yield (r = 0.384, P = 0.058). Estimated S. frugiperda infestation during 33- Neither block nor D. lineolata was significant in 45 d after planting (as measured by the percentage explaining variance in yield (Table 2). S.frugiperda of plants receiving insecticide application at 33 d infestation at 41 d after planting was marginally after planting, D. lineolata infestation and block significant (P = 0.0524) (Table 2). Timing of infes- had significant effects on yield (Table 2). Factorial tation had a significant effect on yield, but block ANOVA with block, level, and period of S. frugi- did not (Table 3). perda infestation showed a significant effect of Experim.ent 3: First Rainy Season 1987. The block and level of infestation, but not of period of maximum S. frugiperda infestation in the 3rd ex- infestation (Table 3). Interaction terms were not periment was 64%. Measured infestations were significant (P > 0.05). closer to target infestation levels than in experi- 616 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 2

Table 5. Experiment 2: S. frugiperda target periods of infestation, actual infestation at 41 d after plantin~ (DAP) (dama!(c rating and predicted equivalent percentage of infested), D. lineolata infestation, and yield

S. fn'giperda D. lineolata Tarw't inf"station periods Predicted % Yi"ld ± SE damage rating infestation (% plants infested) plants infested (grams p"r plot) at 41 DAP ± SE index ± SE 0-100-100-100 2.51 ± 0.38a 76.7 82.7 ± 22.2a 168.1 ± 39.0a 0-0-100-100 1.24 ± 0.29b 37.9 45.5 ± 20.8ab 231.2 ± 66.3a 0-100-0-0 1.04 ± 0.19bc 31.8 40.8 ± 5.80ab 270.8 ± 64.0a 0-0·0-0 0.16 ± 0.05c 4.8 2.68 ± 0.74b 601.9 ± 86.3h lOO-100-100-100 2.04 ± 0.19ab 62.5 64.1 ± 15.7lab 240.7 ± 9Ua

Mt'ans followed by the same letter are not significantly different (P > 0.05; LSMEANS [SAS Institute 1989]). Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 ment 1 (Table 6). D. linealata infestation was sig- at 30 d after planting; and r = 0.80, P < 0.0001 at nificantly correlated with S. fru.giperda infestation 39 d after planting). (r = 0.22, P = 0.036 at 24 d after planting; r = Block and period of S. fru.giperda infestation 0.>72, P < 0.0001 at 39 d after planting. were significant predictors of yield, while D. linea- Spadoptera fru.giperda infestation at 32 and 46 lata infestation was not (Table 2). d after planting, D. lineolata infestation and block Block and period of S. fru.giperda infestation wcre significant predictors of yield (Table 2). had significant effect on yield. Level of S. fru.gi- Block and period of infestation had significant penZa infestation was not significant, whereas, the impact on yield, whereas level did not. There was level X period interaction was marginally signifi- no significant level by period interaction (Table 3). cant (P = 0.08) (Table 3). Treatments that had infestation in both of the Plots with 94% infestation had significantly low- last 2 periods had lower yields than the other 3 er yields than treatments with 78, 37, or 7% (Table treatments (orthogonal contrast of the last 2 peri- 10). There were significantly lower yields when ods versus others: F = 7.43; df = 1, 72; P = 0.008). treatments were infested in either 2 periods or the Although treatments with 60% infestation yielded last, as compared with either of the first 2 periods 25% less than treatments with 0% target infesta- of infestation (Table 11). Orthogonal contrasts tion in all periods, the difference was not signifi- showed that when only 1 period was infested, the cant (Tables 7 and 8). 3rd period had significantly (P = 0.029) lower yield Experiment 4: Irrigated Maize 1987. S. fru.- than the 1st, the 2nd had marginally significant giperda infestations in experiment 4 were the high- lower yield than the 1st, the 2nd had marginally est of all 4 experiments, reaching 100% of the significant lower yield than the 1st period (P = plants infested. The measured S.fru.giperda infes- 0.096), and the 2nd period was not significantly tations also most closely matched the target infes- different from the 3rd (P = 0.542). Treatments tation levels (Table 9). that were infested in the last 2 periods had signif- Spadoptera fru.giperda infestation at 23, 30, and icantly lower yields than treatments that were not 39 d after planting was significantly positively cor- infested during the last 2 periods (P = 0.004). related with D. lineolata infestation (r = 0.69, P < Regression Analysis of the Impact of Infesta- 0.0001 at 23 d after planting; r = 0.67, P < 0.0001 tion on Yield. S. fru.giperda infestation before

Table 6. Experiment 3: S. frugiperda target and actual infestations at 24, 39, and 46 dafter plantin!( (DAP), D. lineolata IDfestation at harvest, and yield (grams pcr plot) for 16 treatments :t SE

Target and measured S.frugiperda infestation

11-24 DAP 24 DAP 24-39 DAP 39 DAP 39-46 DAP 46 DAP D. lineolata Yield tar~et actual target actual target actual infestation (~rams per plot) 0 6.0 ± 2.7 100 46.2 ± 10.6 100 54.2 ± 10.9 56.4 :t 5.6 1,085 :t 167 0 2.3 :t 1.7 70 63.8 ± 3.9 70 53.8:!: 5.0 54.0 ± 4.9 1,l1l6 ± 204 0 2.2:!: 1.4 40 36.0 ± 10.6 40 17.7 :!: 7.5 42.6 ± 7.1 1,121 ± 105 0 2.3:!: 1.1 100 58.0 ± 12.2 0 O±O 32.2 ± 5.3 1,499 ± 1113 0 6.8 ± 4.1 70 59.0:!: 10.9 0 1.7 ± 1.7 27.7 ± 6.4 1,435:!: 240 0 4.5 ± 4.5 40 35.0:!: 9.3 0 1.8 ± 1.2 24.7 ± 5.4 1,251l :!: 154 100 17.8 ± 8.5 100 50.7:!: 11.3 0 2 :!: 1.4 28.4 ± 8.1 1,137 ± 190 70 14.2 ± 4.8 70 59.2:!: 6.5 0 O:!:O 43.8 ± 3.6 1.368 ± 121l 40 29.3 ± 12.4 40 26.8 ± 3.3 0 1.8 :!: 1.2 30.0 ± 2.3 1,624 :t 151 0 3.2 ± 1.4 0 0.7 ± 0.7 100 2.7:!: 2.0 3.3 ± 2.6 I.471l ± 204 0 2.3 ± 1.5 0 O±O 70 O:!:O 2.0 ± 1.0 1,200:+: 256 0 5.0 ± 3.4 0 O:!:O 40 O:!:O 0.5 ± 0.5 1,514 ± 219 100 30.8:!: 11.2 100 58.5 :t 5.6 100 63.2 ± 13.4 56.7 ± 6.2 1,049 ± 221 70 22.0 ± 5.7 70 53.8 ± 5.8 70 61.2 ± 5.4 50.5 ± 5.6 1,083 ± 58 40 19.2 ± 7.4 40 34.3 ± 6.3 40 55.0 ± 9.3 42.3 ± 3.6 1,348:!: 137 0 8.5:!: 3.2 0 O±O 0 O:!:O 0.5 ± 0.5 1,557:+: 83 April 1997 HRUSKA AND GOULD: ARMYWORM AND Diatraea EFFECT ON MAIZE YIELD 617

Tab!.- 7. Experiment 3: Yields of 5 periods and mean Table 8. Experiment 3: Yields for 4 mean leyels of S. leyels of S. frugiperda infestation during mid- to late frugiperda infestation during mid- to late whorl (0 = 0% whorl (0 = 0'70 targeted infestation; I, natural infestation) targeted infestation; I, natural infestation)

Pt'riod Yield:!: SE Yield Infestation (grams per plot) (% plants) 0-1-1 1,131.9 :!:I01.3a (SE) 0-1-0 1,397.4 :!:94.7ab 1-1-0 1,387.6:!: 97.7ab 60 1,173.3 :!:223.3a 0-0-1 1,417.4 :!:94.7b 58 1,254.0 :!:240.0a 1-1-1 1,159.9 :!:94.7a 45 1,335.6 :!:205.5a 0 1,556.9 :!:82.6a Mt'lllls followl'd by the san'" lettt'r are not significantly different (P > 0.05; LSMEANS [SAS Institute 1989]). Means followed by the same letter are not significantly different (P > 0.05; LSMEANS [SAS Institute 1989]). Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 midwhorl stage did not explain a statistically sig- nificant amount of variation in maize yield in the but D. lineolata damage cannot. Because early- 2 experiments that examined the effect, whereas whorl infestation had little impact on maize yield, in all 4 experiments, infestation during mid- to economic injury levels were calculated from yield- late-whorl stage, or during all periods did (Fig. 1- infestation regressions from mid- to late-whorl pe- 4), riods (Figs. 1-4). Using 75% effectiveness of in- The regressions between season-long S. frugi- secticide application and the actual yields and perda infestation and yield are all significant, are slope, the economic injury levels for the 4 experi- fit well with linear regressions, and explain be- ments were 66, 26, 31, and 36% of the plants in- tween 42 and 62% of the variation in yield (Figs. fested, using the mean slope from the 4 experi- IB, 2, 3B, and 4C). The slopes differ significantly, ments. Economic injury levels ranged from 16 to with 100% infestation predicting between 30 and 94% of the plants infested for combinations of a 77% yield reduction. Infestation caused the great- range of potential yields and insecticide effective- est yield reduction (77% reduction predicted at ness (Table 12). 100% S. frugiperda infestation) in experiment 2. Experiment 2 had the lowest yields (one-third of Discussion yield in the other studies) because of severe drought-stress. Spodoptera frugiperda and D. lineolata are im- Economic Injury Levels. Economic injury lev- portant biological constraints to maize production els were calculated for the combined effects of S. throughout Latin America, where maize plays a frugiperda and D. lineolata, based on S.frugiperda central role in both the diets and farming produc- infestation levels for the 4 experiments. This was tion. In Nicaragua, most maize is grown on small justified because of the correlation between the fields of 1-3 ha. Approximately 200,000 ha are pro- abundances of these 2 pests. S.frugiperda damage duced annually. Maize is grown throughout the c~m be monitored by a farmer nondestructively, country, with the greatest concentration in the cen-

Table 9, Experiment 4: S. frllgiperda target and measured infestation leyels at 10, 23, 30, and 39 d after planting (DAP), D. lineolata infestation at huryest, and yield (grants per plant) :!: SE

S.fmgiperd8 infestation: target and measured Trt'at- 5-17 10 18-30 23 30 31-45 39 D. lineolata Yield lIlt'nt DAP DAP OAF OAF OAF OAF DAP infestation (glplant) target actual target actual actual target actual 1 100 50.8:!: 8.0 0 1.5:!: 1.0 1O.0:!: 4.0 0 6.5:!: 3.2 13.8 :!:4.0 69.8 :!:4.8 2 70 43.4 :!:8.5 0 3.7:!: 2.1 37.3:!: 6.8 0 5.6:!: 2.1 21.8 :!:5.7 71.4 :!:9.2 3 40 34.2:!: 8.0 0 0.8:!: 0.8 16.5 :!:2.9 0 3.1 :!:2.2 13.2 :!:5.0 68.8 :!:7.1 4 0 15.2 :!: 4.3 100 87.5 :!:11.7 99.4 :!:0.6 0 19.9 :!:2.4 43.4:!: 5.6 65.0 :!:7.4 5 0 18.7:!: 6.0 70 68.3 :!:1.7 79.1 :!:5.3 0 11.4 :!:3.3 46.2 :!:6.7 76.0:!: 10.6 6 0 29.0:!: 12.0 40 33.3 :!:4.7 53.0:!: 6.3 0 10.1 :!:4.5 39.2:!: 4.3 53.3 :!:7.4 7 0 21.0:!: H.6 0 1.4 :!:1.4 12.3 :!:5.9 100 74.9:!: 8.0 67.0:!: 5.9 58.7:!: 6.9 8 0 16.7:!: 5.0 0 1.0 :!:1.0 11.1 :!:3.7 70 50.9:!: 10.2 43.3:!: 8.0 62.3:!: 5.6 9 0 12.1 :!:3.2 0 21.5 :!:15.9 29.5:!: 15.3 40 21.4 :!:4.7 27.3:!: 6.2 57.0 :!:8.3 10 100 55.0:!: 8.8 100 l00:!: 0 lOO:!: 0 0 8.4 :!: 1.5 56.5:!: 14.5 51.7 :!:4.4 H 70 29.8:!: 4.6 70 63.9:!: 3.7 79.5:!: 3.0 0 17.9 :!:7.1 48.8:!: 7.1 62.4 :!:5.8 ]2 40 29.4 :!:4.3 40 30.5:!: 3.1 57.9:!: 3.1 0 8.7:!: 2.0 39.0 :!:3.7 66.4 :!:8.2 13 0 14.0 :!:6.5 100 98.1 :!:1.9 100:!: 0 100 100:!:0 84.2:!: 3.5 41.1 :!:4.7 ]4 0 17.6 :!:4.3 70 62.3:!: 4.2 75.3:!: 3.3 70 72.7:!: 1.6 75.5 :!: 3.4 55.9:!: 4.9 ]5 0 17.1 :!:4.7 40 30.4 :!:2.4 49.3:!: 3.0 40 45.2:!: 1.6 47.6:!: 2.0 68.1 :!:6.4 ]6 100 53.9:!: 6.8 100 97.4 :!:1.7 l00:!: 0 100 91.3 :!:7.3 85.1 :!:2.8 38.7 :!:4.7 17 70 39.4 :!:6.6 70 67.5 :!:1.9 77.8 :!:3.2 70 75.1 :!:4.3 72.4 :!:3.9 49.6:!: 6.5 18 40 22.2 :!: 3.7 40 40.0 :!:4.7 54.5:!: 8.2 40 34.2:!: 6.5 45.0:!: 4.2 63.5:!: 5.5 19 0 26.3:!: 6.9 0 1.5 :!:1.0 24.2:!: 4.5 0 12.0 :!:4.3 20.7:!: 3.6 67.5:!: 7.8 618 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 2

Table 10. Experiment 4, Yields of 3 levels:!: SE of S. Table ll. Experiment 4, Yields of 6 periods:!: SE of frugiperda infestation S. frugiperda infestation

Infestation Infestation period Yield (glplant) Yield per plant (% plants) 1-0-0 70.98 :!:3.93a 94 38.7:!: 4.7a 0-1-0 62.22:!: 3.70ab 78 49.6:!: 6.5b 0-0-1 58.77 :!:3.95bc 37 63.5 :!:5.5c 1-1-0 59.47 :t 3.8Ibc 7 67.5 :!:7.8c 0-1-1 56.30 :!:3.52bc 1-1-1 50.91 :!:3.19c Means followed by the same letter are not significantly different (P > 0.05; LSMEANS [SAS Institute 1989]). Means followed by the same letter are not significantly different (P > 0.05), according to LSMEANS (SAS Institute 1989). tral region and the Pacific coast. Depending on Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 rainfall patterns and crop rotations throughout the festations were most important in reducing yield country, maize is planted in May-June (66%), Sep- in both grain and sweet com yield. Data from tember-October (10%), or November-December these 3 studies and from experiments presented (24%). Maize can also be produced during the dry here indicate that high levels of S. frugiperda in- season (January-April) under irrigation. Most pro- festation can be tolerated during the early to mid- ducers use traditional planting methods, including whorl stages, without Significantly reducing yield. land preparation with oxen and use of a planting stick. Average yields are =1,200 kglha, which are among the lowest in Central America (MAG 1993). Experiment 1 Yields in the studies· presented here range from A 1.6 423 to 3,175 kglha. iii

'tl There are reports of contradictory results and "il 1.4 recommendations about the effect of timing of in- ;;: 0 festation of S. frugiperda on yield and economic E injury levels. Morrill and Greene (1974) reported l!Ill .5" 0 0 that infestation of S.frugiperda was related to yield 1.2 0 " :tj od'! 0 reduction when infestation occurred at early and :." 0 ... .0 0 midwhorl stages, but not with infestation at late- 0 1.0 whorl and tasseling stages. Harrison (1984) report- '" ed that the greatest yield losses occurred when ~.. 0 Co 0.8 plants were infested with S. frugiperda in the 1st 0 0 wk after emergence. Linduska and Harrsion (1986) c.... I1i\ reported that plants infested during early growth 0.6 stages were more susceptible to S.frugiperda dam- o 20 40 60 80 100 age than later stages. These studies led Buntin Infestation at JJ DAP for Treatments (1986) to conclude that late-whorl maize is less Infested only in First Period sensitive to S. frugiperda injury than younger B Experiment 1 plants. 1.2 Evans and Stansly (1990) studied the yield im- ... "il pact of S.frugiperda by comparing protected and ;;: 1.1 unprotected plants, with protection beginning from 2 wk until 6 wk after germination. They E 1.0 "E found the greatest yield reduction when plants ';, 0.9 were infested earlier, and calculated weekly eco- ~ 0.8 nomic thresholds, which are lowest at week 2 (11% ...o of the plants infested) and increase over time to c 42% of the plants infested at week 6. ~ o.. Three of the 4 experiments presented in this ar- Clo o ticle indicate that infestation during late vegetative c.... growth, from midwhorl onward, are the stages 0.5 o 20 40 60 80 100 when S. frugiperda and D. lineolata infestation Infestation at 33 DAP for Treatments cause the greatest yield loss. Early infestation lev- Infested in Both Periods els, when subsequently controlled, did not always cause any significant yield reduction. Fig. 1. Experimentl.(A) Relationshipbetween S.fru- giperda infestationat 33 d after planting(DAP) and per- The results presented here concur with other centage of maximumyield (y = 1.0018-O.0019x, r2 = studies, which have separated the effects of period 0.0052, P = 0.061) for treatments that had 33 DAP and of infestation from duration of infestation. Gross (B)percentage maximumyield (y = 0.967-O.003Ox, r2 = et al. (1982), Foster (1989), and Marenco et al. 0.421, P < 0.0001) for treatments that had infestation (1992) all found that late-whorl S. frugiperda in- during both periods. April 1997 HRUSKA AND GOULD: ARMYWORM AND Diatraea EFFECT ON MAIZE YIELD 619

Experiment 2 A Experiment 3 ... 1.2

" 0.6 III I!I ...::a: •• ... III I!I 0.4 •• I!I I!I I!I I!I " I!I ~ ~" 0.5 I!I .. 0.2 •• .. I!I Cl.•• ••"".. ••.. Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 =- =- 0.0 +--.----.-~.-...... ,....__,-..--..,_-..-__,-..-_, 2 3 4 o 20 40 60 80 100 120 % Plants Infested at 41 DAP During All Periods % Plants Infested at 33 DAP for Treatments Infested in the First Two Periods Fig. 2. Experiment 2. Relationship between S. frugi- penZa infestation at 41 days after planting (DAP) and per- B Experiment 3 centage of maximum yield (I} = 0.971-O.0083x, r'l = 1.2 0.605, P = 0.007) for treatments that had infestation in all 4 periods. 1.0' I!I

0.8 In contrast to the studies described above, some published reports indicate that early stage damage 0.6 is very important. The contrasting results are ex- ... •• 0.4 plained by different experimental methods. In the c ~ experiments presented here, the effects of period .. 0.2 Cl.•• of infestation were separated from the total dura- ..•• tion of infestation, whereas the contrasting studies c.. 0.0 did not test the effect of early infestation alone. o 20 40 60 80 100 Instead, they tested whether early infestation plus % Plants Infested at 33 DAP for Treatments late infestation decreased yield more than late in- Infested in all Three Periods festation alone. Experiment 3 The damage functions generated from the 4 ex- 1.1 C periments reported here, actual yields, and 75% insecticide effectiveness, indicate that economic 1.0 injury levels range from 22 to 58% of the plants infested. The lowest level was produced under 0.9 drought conditions, where S.frugiperda and D. li- 0.8 nealata apparently cause greater yield loss. ... 0.7 The results and recommendations from the CUf- •• rent study are valid only when there is a correlation .~c 0.6 between S.fmgiperda and D. linealata infestation. 1:: •• 0.5 In cases where S.fmgiperda occurs in the absence ••.."" of D. linealata, other damage functions and eco- =- 0.4 nomic injury level recommendations would most o 20 40 60 80 likely be found, because the additive impact is % Plants Infested at 46 DAP for Treatments probably greater than the impact of just 1 insect. Infested in the Last Two Periods Although it is possible to conduct such studies, by Fig. 3. Experiment 3. Relationship between (A) S. using cage experiments, the utility of the results to frugipercla infestation at 33 days after planting (DAP) and farmers throughout Latin America would not be percentage of maximum yield (I} = 0.957-0.001.'52;r, r'l = great because the correlation between abundance 0.030, P = 0.439) for treatments infested during the flrst of these 2 pests is common (e.g., van Huis 1981). 2 periods. (B) S. frugiperda infestation at 33 OAP and It would, however, be useful to understand in what percentage of maximum yield (I} = 0.954--0.00615x, r'l = areas or under what conditions S. frugiperda and 0.493, P = 0.009) for treatments infested during all 3 periods, and (e) S.frugiperda infestation at 46 DAP and D. linealata occurrence is most and least correlat- percentage of maximum yield (I} = 0.981-O.006Ox, r'l = ed. 0.673, P = 0.049) for treatments that were infested dur- The economic injury level recommendations ing the last 2 periods. also vary with changing insecticide prices. It is in- teresting to note that when these experiments were conducted originally, Nicaraguan pesticides were subsidized heavily by the government, which also 620 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 2

A Experiment 4 C Experiment 4 1.2 III 1.2 III 'I:l ll!I ";j III ;;:: 1.0 IIItIED e 5= 0.8 'j; :E" ...c 0.6 ...c c c .:: 0.6 1:: 0.4 ~ c o.. Cl. Cl. c o

.. .. Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 ~ 0.2 ~ 0.4 o 20 40 60 80 100 120 o 20 40 60 80 100 0/0 Plants Infested at 10 DAP for Treatments % Plants Infested at 39 DAP for Treatments Infested Onlv in First Period Infested in Only Third Period Experiment 4 Experiment 4 1.1 B 1.2 D 'I:l 'I:l ";j ";j ;;:: 1.0 Iil Iil Iil Iil ;;:: 5 e 0.9 e= e= 'j; 'j; 0.8 :E" ::E" ... 0.7 c ...c c 0.6 .::c ~.. 1: c o III .•. 0.5 Cl. o o III .. 1!I .. ~ 0.4 ~ 0.2 o 20 40 60 80 100 120 o 20 40 60 80 100 120 % Plants Infested at 30 DAP for Treatments % Plants Infested at 39 DAP for Treatments Infested in Only the Second Period Infested in All Three Periods

Fig. 4. Experiment 4. Relationship between (A) S. fru.giperda infestation at 10 days after planting (DAP) and percentage of maximum yield (y = 0.969-0.0045x, r- = 0.161, P = 0.886) for treatments infested during only the 1st period. (B) S.fru.giperda infestation at 30 DAP and percentage of maximum yield (y = 0.948-0.0023x, r'! = 0.154, P = 0.887) for treatments infested during only the 2 period, (C) S.fru.giperda infestation at 39 DAP and percentage of maximum yield (y = 1.024--0.003Ox, r- = 0.258, P = 0.129) for treatments \\~th infestation during only the 3rd period, and (D) between S.fru.giperda infestation at 39 DAP and percentage of maximum yield (I} = l.014--0.0049x, r- = 0.618, P = 0.0015) for treatments with infestation during all 3 periods. guaranteed prices for basic grains. Using 1987 than details of plant response to infestation (Hms- prices, the economic injury level was calculated to ka 1990). be 2% of the plants infested with S. frugiperda Economic injury levels also vary with yield po- (Hruska and Gladstone 1988). Government poli- tential and effectiveness of insecticide application. cies can have much more of an impact in deter- The economic injury levels reported in this article mining "rational" pest management for a farmer are based on the use of granular formulations, ap- plied directly to the whorl, which are used by many Nicaraguan farmers and can be very effective. Table 12. Economic injury levels (percentage of Many resource-poor maize farmers in Nicaragua plants infested) calculated for combinations of potential yield (kg/ha) and effectiveness of insecticide use liquid insecticide formulations applied with backpack sprayers for S.frugiperda. The effective- Potential Effectiveness of insecticide ness of the insecticide has a direct negative impact yield on economic injury level (Table 12). It would be 0.5 0.75 0.9 (kg/ha) feasible to have 2 separate economic injury levels 750 93.6 62.4 52.0 46.8 for these 2 commonly used application methods. 1,000 70.2 46.8 39.0 35.1 Knowledge of potential maize yields in different 1,200 58.5 39.0 32.5 29.3 1.500 46.8 31.2 26.0 23.4 areas of the country could also be used to make 2.000 35.1 23.4 19.5 15.8 different economic injury level recommendations, based on historic yield information. Percentage of loss in yield caused by percentage of infestation Evans and Stansly (1990) suggest that economic of S.fmgiperM (0.55) is mean of 4 experiments. Costs of control ($15.50/ha) and price received by fanner ($0.0803/kg) in Nicara- injury levels are lowest for early maize stages, en- gua (1995 values). couraging earlier control tactics. This recommen- April 1997 HRUSKA AND GOULD: ARMYWORM AND Diatraea EFFECT ON MAIZE YIELD 621 dation is based on cumulative S.fmgiperda dam- Noctuidae) infestation of com in lowland Ecuador. J. age, lmd does not examine the effect of controlling Econ. Entomol. 83: 2452-2454. early infestations late. Early application allows Foster, R. E. 1989. Strategies for protecting sweet re-infestation by S.fmgiperda, possibly necessitat- com ears from damage by fall armyworm (Lepidop- ing an additional application. Based on these stud- tera: Noctuidae) in southern Florida. Fla. Entomol. 72: 146-151. ies we have recommended to farmers in Nicaragua Fuentes, G. 1969. Combate de Spodoptemfrugiperda that they not take control action against S. fmgi- y Zeadiatraea lineolata en mafz con insecticidas gran- pcrda until the midwhorl stage. This timing of con- ulados. Ingeniero Agr6nomo thesis, Universidad de trol prevents economic yield loss by S.frugiperda, Costa Rica, San Jose. controls D. lineolata, and in most cases eliminates Gross, H. R., J. R. Young, and B. R. Wiseman. 1982. 1 insecticide application completely. Relative susceptibility of a summer-planted dent and Saving just 1 spray per season in maize is sig- tropical flint com variety to whorl stage damage by nificant to the household economies of typical the fall armyworm (Lepidoptera: Noctuidae). J. Econ. Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021 maize farmers in Nicaragua. At a nationallevef, the Entomol. 75: 1153-1156. savings are also significant. A savings of $15.50 (the Harrison, F. P. 1984. The development of an econom- cost of insecticide plus application) per hectare on ic injury level for low populations of fall armyworm (Lepidoptera: Noctuidae) in grain com. Fla. Entomol. 200,000 ha of maize represents an annual savings 67: 335-339. of $3.4 million, of which $2.7 million would go to Hruska, A. J. 1990. Government pesticide policy in importing insecticides. This is a significant savings Nicaragua 1985-1989. Global Pesticide Monitor 1: 3-5. to a country, which imported $17 million of insec- 1995. Reducing insecticide use among resource-poor ticides in 1994. maize farmers in Nicaragua. Ph.D. dissertation, North Carolina State University, Raleigh. Hruska, A. J., and S. M. Gladstone. 1988. Effect of Acknowledgments period and level of infestation on the fall arm)'\vorm, Spodoptera frugiperda. on irrigated maize yield. Fla. We thank S. Gladstone for suggestions and support Entomol. 71: 249-254. throughout all phases of the experiments, A. Sediles for Hruska, A. J., S. M. Gladstone, and R. Lopez. 1988. his support at the Universidad Nacional Agraria, R. Perfodos crfticos de protecci6n y el efecto de infes- L6pez, C. Gutierrez, A. Gaitan, F. Leyva, L. Paraj6n, J. taci6n del gusano cogollero Spodoptera frugiperda (}. Padilla, and L. Perez for fiE'ldassistance. C. Brownie pro- E. Smith) (Lepidoptera: Noctuidae) en mafz de pri- \idt'd valuable statistical advise. G. G. Kennedy, J. R. mera. Instituto Superior de Ciencias Agropecuarias, BratUey, Jr., G. A. Carlson, and M. T. Johnson (North Carolina State Uniwrsity) provided valuable comments Managua, Nicaragua. and suggestions on a draft of the manuscript. This work Leyva, F. 1988. Determinaci6n de perfodos crfticos y was financially supported by the Norwegian Ministry of niveles de infestaci6n del cogollero, Spodoptera int- Forpj~l Cooperation, Conservation and Research Foun- giperda (}. E. Smith) en el cultivo del mafz (Zea 1TUlYS) dation, Department of Entomology, North Carolina State en epoca de siembra de primera. Ingeniero Agr6nomo UniVE'rsity,Instihlte for the Development of Agricultural thesis, Instituto Superior de Ciencias Agropecuarias, Alternatives, Carol Bernstein Ferry, and Kathleen D. Managua, Nicaragua. Gladstone. Lidnduska, J. J., and F. P. Harrison. 1986. Adult sampling as a means of predicting damage levels of fall armyworm (Lepidoptera: Noctuidae) in grain References Cited com. Fla. Entomol. 69: 487-491. [MAG] Ministerio de Agricultura. 1993. Estadfsticas Alvarez, J. F., and E. Morales. 1979. EI taladrador Basicas del Sector Agropecuario. Divisi6n de Infor- nt'otropkal del mafz )' su relaci6n con el rendimiento matica, Managua, Nicaragua. )' Ia calidad de la cosecha. Ministerio de Agricultura, Marenco, R. J., R. E. Fosler, and C. A. Sanchez. San Jose, Costa Rica. 1992. Sweet com response to fall armyworm (Lep- Andrews, K. L. 1980. The whorlworm, Spodoptera idoptera: Noctuidae) damage during vegetative fn/giperda. in Central America and neighboring areas. growth. J. Econ. Entomol. 85: 1285-1292. Fla. Entomol. 63: 456-467. Morrill, W. L., and G. L. Greene. 1973. Distribution 1988. Latin American research on Spodoptera frugi- of fall armyworm larvae. 1. Regions of field com pen/a (Lepidoptera: Noctuidae). Fla. Entomol. 71: plants infested by larvae. Environ. Entomol. 2: 195- 630-653. Andrews, K. L., and A. Rueda. 1986. EI Cogollero. 198. Publicaci6n 91. Departamento de Protecci6n Vegetal, 1974. Survival of fall armyworm larvae and yields of Escuda A!!:rfcolaPanamericana, EI Zamorano, F.M., field com after artificial infestations. J. Econ. Ento- Honduras. mol. 67: 199-123. Buntin, G. D. 1986. A review of plant response to fall Obando, R. 1976. Diatraea lineolata: dinamica de pob- arm)'\Vornl, Spodoptcra frugiperda (}. E. Smith), in- laciones y su dano en plantas de mafz, pp. 113-133. jury to sdected field and forage crops. Fla. Entomol. In Informe anual: cultivo del mafz. Instituto Nicara- 69: S49-5S9. gtiense de Tecnologfa Agropecuaria, Managua, Nica- Daniels, N. E., and L. D. Chedester. 1977. Insecti- ragua. cidal control and damage evaluation of the south- Rodriguez-del-Bosque, L. A., J. W. Smith, Jr., and H. westem com borer. Tex. Agric. Exp. Stn. PR-3431. W. Browning. 1988. Damage by stalkborers (Lep- Evans, D. C., and P. A. Stansly. 1990. Weeklyeco- idoptera: Pyralidae) to com in northeastern . nomic inju!)' levels for fall armyworm (Lepidoptera: J. Econ. Entomol. 81: 1775-1780. 622 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 90, no. 2

Sarmiento, J., and J. Casanova. 1975. Busqueda de van Uuis, A., R. S. Nauta, and M. E. Vullo. 1982. lfmites de aplicaci6n en el control del "cogollero del Traditional pest management in maize in Nicaragua: mafz", Spodoptera frugiperda S. & A. Rev. Peru. En- A survey.Meded. Landbouwhogesch.Wageningen82: 6. tomol. 18: 104-107. Wiseman, B. R., and W. W. McMillian. 1969. Com- SAS Institute. 1989. SAS user's guide: statistics. SAS petition and survival among the com earworm, tht' Institute, Cary, NC. tobacco budworm, and the fall armyworm. J. Econ. Entomol. 62: 734-735. van Uuis, A. 1981. Integrated pest management in the small farmer's maize crop in Nicaragua. Meded. Land- Received for publication 31 July 1995; accepted 4 Oc- bouwhogesch. Wageningen 81: 6. tober 1996. Downloaded from https://academic.oup.com/jee/article/90/2/611/806948 by guest on 01 October 2021