Biological Control 26 (2003) 333–340 www.elsevier.com/locate/ybcon

Field observations of the effects of fenitrothion and Metarhizium anisopliae var. acridum on non-target ground dwelling in the Sahel

Steven Arthurs,* Matthew B. Thomas, and Juergen Langewald1

Leverhulme Unit for Population Biology and Biological Control, NERC Centre for Population Biology and CABIBioscience, Imperial College, Silwood Park, Ascot, Berks SL5 7PY, UK

Received 30 May 2002; accepted 22 October 2002

Abstract

The effect of the chemical , fenitrothion, and a mycoinsecticide based on Metarhizium anisopliae var. acridum on the activity of non-target epigeal scavengers was investigated in areas of open savannah in southeast Republic, West Africa. Both were applied as full cover sprays to unreplicated 800 ha plots to assess their season-long control of Sahelian . Compared with control plots, fenitrothion caused an immediate but temporary reduction in numbers, whereas M. anisopliae var. acridum provided delayed but prolonged control. Scavenging rates of pyrethroid-killed grasshoppers placed along transects in unsprayed plots and those treated with fenitrothion and M. anisopliae var. acridum at various intervals after spraying were assessed. In the fenitrothion plot, an immediate reduction in scavenging activity occurred that was still apparent after 40 days at the plot center, although recovery at the plot edges was more rapid. By contrast scavenging rates remained high over equivalent areas in the M. anisopliae var. acridum and two untreated plots. Concurrent to the scavenging study, counts of grass- hopper cadavers resulting from the spray treatments were conducted. These counts revealed that the density of grasshopper cadavers remained low throughout the M. anisopliae var. acridum plot and explained <1% of the reduction in live grasshoppers resulting from treatment, compared with >20% in the fenitrothion plot. This shortfall in grasshopper cadavers resulting from the spray treatment in the M. anisopliae var. acridum plot was unexpected because in a monitoring study, fungus-killed (unlike pyrethroid-killed) grasshoppers were unattractive to scavengers and readily persisted in this plot, and thus should have become apparent. Given we did not observe significant grasshopper dispersal, the scarcity of cadavers generated in the M. anisopliae var. acridum plot, together with unquantified visual observations, suggests that predation of infected but living grasshoppers was high. Our data provide circum- stantial evidence that the different effects of chemical and biological grasshopper control on grasshopper natural enemies may in- fluence the efficacy of large-scale treatments. Ó 2002 Elsevier Science (USA). All rights reserved.

Keywords: ; Metarhizium anisopliae; Sahelian grasshoppers; Mycoinsecticide; Fenitrothion; Intraguild predation

1. Introduction in the development of alternative control methods using mycoinsecticides based on entomopathogenic fungi in Concerns over the environmental and human health the genera Metarhizium and Beauveria (Deuteromyco- impacts of chemical control of and grasshoppers tina: Hyphomycetes) (Bateman, 1997; Jaronski and (: ) have led to considerable interest Goettel, 1997; Milner, 1997). One such product com- prising an oil formulation of conidia of Metarhizium anisopliae (Metschnikoff) Sorokin var. acridum, a natu- * Corresponding author. Present address: Biological Control Lab- rally occurring fungal pathogen of and grass- oratory, Department of Entomology, Texas A&M University, College hoppers, has provided effective control of a range of Station, TX 77843-2475, USA. Fax: +979-845-7977. economically important acridid species throughout Af- E-mail address: [email protected] (S. Arthurs). 1 IITA Plant Health Management Division, 08BP 0932, Cotonou, rica (Lomer et al., 2001). A similar isolate of M. ani- . sopliae var. acridum has also proved successful in tests

1049-9644/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. doi:10.1016/S1049-9644(02)00166-4 334 S. Arthurs et al. / Biological Control 26 (2003) 333–340 against a number of important locust and grasshopper sible. Nevertheless differences among treatments were pests in Australia, including largely successful trials up dramatic and consistent over a wide area and thus, in- to 30,000 ha (Milner, 2000). terpreted sensibly, we believe still yield valuable insights Although the products mentioned above can provide into the different effects of the chemical and biological effective control, a number of ecological studies con- treatments. ducted with M. anisopliae var. acridum have indicated that both biotic and abiotic factors influence the overall efficacy of spray applications across time and/or space. 2. Materials and methods For example, the speed of kill can be highly variable and depend strongly on environmental temperatures and 2.1. Spray trial host thermal biology (Blanford and Thomas, 1999a,b, 2000). In addition, M. anisopliae var. acridum may alter Aerial spray treatments were applied towards the host feeding, fecundity, and mobility (Arthurs and beginning of the rainy season in August 1997 to two Thomas, 2000; Seyoum et al., 1994; Thomas et al., 800 ha plots near the Ma€iine Soroa district in southeast 1997). There is also potential for recycling of the fungus Niger Republic, West Africa, using a single Cessna Ag. to new generations of acridids through horizontal Truck 188 employing 4 Micronair AU5000 atomizer transmission (Thomas et al., 1995). nozzles attached to the trailing wings. Full details can be Recently, there has been a growing recognition of the found in Langewald et al. (1999). One plot was treated importance of higher-level interactions between com- with an oil miscible flowable formulation of M. ani- peting natural enemies. Such interest has been stimulated sopliae var. acridum (strain IMI 330189) at 0.5 L/ha, by a number of well-documented cases of intraguild delivering 2:5 1012 conidia/ha. Conidia were produced predation (i.e., when different groups of natural enemies in vitro at a pilot production plant in Benin, West Africa that compete for the same host/prey are themselves (Cherry et al., 1999). An equivalent plot was treated engaged in predator–prey type interactions) amongst with technical fenitrothion, an organophosphate widely natural and introduced biological control agents. Both used for acridid control (FAO, 1998), at 0.225 L/ha. theoretical and empirical evidence suggests that this type Two similar untreated areas served as a control for each of competitive interference can have important but of the treated plots with all plots separated by at least variable consequences for the population dynamics of 5 km. biological control agents and target pests, and hence Assessments of live grasshopper densities within food webs and community structure (Polis and Holt, treated and control plots were made every 3 days from 1992; Rosenheim et al., 1995). However, most studies of 10 days pre-treatment until 22 days post-treatment the interactions between arthropod control agents are (Fig. 1 shows efficacy data from Langewald et al., 1999). laboratory based, while examples from the field tend Grasshopper density within untreated plots remained to be manipulative and rarely focus on a population high (>10 m2) throughout the assessment period. level. Treatment with fenitrothion caused a dramatic fall in In this study we examine the effects of an operational density within 24 h of spraying. However, in this plot scale application of M. anisopliae var. acridum on non- recolonization was relatively swift, and from day 7 after target natural enemies in the sub-Saharian Sahel, spraying onwards, there was no significant difference in exploring both direct effects (i.e., direct impact on grasshopper natural enemies) and indirect effects (i.e., the effect on the grasshopper–natural enemy interaction) compared with a conventional chemical insecticide treatment. Grasshoppers in this region of the Sahel frequently cause significant damage during the short rainy season by migrating into seasonally cultivated crops [mainly rain-fed pearl millet, Pennisetum glacum (L.)] and require frequent control measures from the national crop protection services (Stonehouse et al., 1998). Full details of the large-scale spray trial alongside which the studies presented here were conducted are given in Langewald et al. (1999), although to provide the necessary background, information particular to the current study and the basic results obtained in the spray Fig. 1. Mean counts (SE) of grasshoppers per m2 in 800 ha plots trial are included in the methods. Because of the scale treated with an oil-based formulation of Metarhizium anisopliae var. and cost involved, the spray trial was not replicated and acridum and fenitrothion in the Sahel. Taken from Langewald et al. hence rigorous statistical testing of our data is impos- (1999). S. Arthurs et al. / Biological Control 26 (2003) 333–340 335 grasshopper density between fenitrothion-treated and 150 m transect lying parallel to the sprayed plot edge. control plots. In contrast, a slower but more prolonged All cadavers comprised of adult A. blondeli, P. cognata, reduction in live grasshoppers was observed within the and O. senegalensis that were collected in an unsprayed M. anisopliae var. acridum plot. In this plot, grasshopper area (i.e., uninfected) and killed with a pyrethroid densities differed significantly from control plots by day insecticide the day prior to use. The pyrethroid was non- 16, and populations remained low (90% reduction) residual and did not appear to deter scavengers com- with no recolonization observed within the assessment pared with freeze-killed . Ten long-term transects period. Generally, similar patterns were observed in in the M. anisopliae var. acridum and 12 in the fenitro- replicated 50 ha plots in 1996 (Langewald et al., 1999). thion plot were set up. Parallel transects were located at 300 m intervals running centrally through the plot. In 2.2. Field site the M. anisopliae var. acridum plot, transects ran along a perpendicular northeast to southwest line, whereas in The studies described below were conducted the fenitrothion plot, transects originated from an throughout the 1997 rainy season (August–October). equivalent northeasterly point until the central area but The study areas comprised lightly wooded open savan- continued in southeasterly direction until the plot nah within widely spaced mobile and fixed dunes. Veg- boundary. The different arrangement in the latter case etation was principally annual, ranging from 2 to 20 cm was to prevent disturbance to a grasshopper sampling at the time of spraying with ground coverage estimates area. Transect locations were within the accuracy given between 20 and 60%, and consisted of mixed forbes and by using hand-held GPS equipment. grasses (mainly Cenchrus biflorus Roxb. together with For sampling, all transects were baited with intact members of the gramineous genera Aristida, Eragrostis cadavers (equal ratios of three grasshopper species) that Chloris, Dactyloctenium, and Pergularia). The dominant were placed in a slight depression (5 mm) on an area of grasshoppers present were Acrotylus blondeli (Saussure), bare ground and marked with a small flag to enable re- Oedaleus senegalensis (Krauss), and Pyrgomorpha cog- location. Each cadaver was revisited in the same se- nata (Krauss), although A. longipes (Charpentier), A. quence 48 h later and assessed for presence/absence. paturalis (Herrich–Schaeffer), Diabolocanantops axillaris Because not all scavenged cadavers were completely re- (Thunberg), Cryptocatantops haemorrhoidalis (Krauss), moved, partial scavenging was included, whereby ca- Poekilocerus bufonius hieroglyphicus (Klug), Heteracris daver remains were initially ranked as either Ô3,Õ intact annulosa (Walker), and Acorypha clara (Walker) were excluding appendages; Ô2,Õ damage to one body section; also present at lower densities. Climatic data were Ô1,Õ damage >1 body section/hollow or Ô0,Õ removal. A measured with a Squirrel 1000 logger (Grant Instru- scavenging rate was obtained for each sampling location ments, Cambridge, UK) and ranged from 20 to 40 °C; 20 by calculating as the overall proportion of whole/partial to >95% relative humidity (day/night, respectively). cadavers (combined) remaining at the location after 48 h. Eight major and 16 minor (<10 mm) rainfall events were Preliminary assessments from three untreated areas recorded over the study period. showed that consistently high rates of scavenging (>95% removal) could be expected using this baiting technique. 2.3. Effects of treatments on non-target epigeal arthro- The cadaver baiting method was repeated at exactly the pods same locations on six occasions between days 2 and 42 and eight occasions between days 3 and 40 after spray- A field bioassay was conducted at various locations ing within the M. anisopliae var. acridum and fenitro- within treated and untreated plots to assess the effect of thion plots, respectively. On each occasion, cadavers the treatments on non-target ground-dwelling (epigeal) were baited at equivalent co-ordinates within control arthropods. In preliminary studies, we observed that plots. Due to the time taken to visit all locations, each ground-dwelling ants and beetles rapidly located (within treated plot together with its associated control location 24 h) and scavenged cadavers placed on the were baited/assessed on consecutive days. Differences in ground. Previous studies in the Ma€iine Soroa district scavenging rates between and within treated and control showed that that such epigeal arthropods are very plots were determined by one- and two-way ANOVA on abundant, widespread and susceptible to fenitrothion at the average scavenging data (normalized via arcsine- operational rates (Peveling et al., 1999). Transects of transformation) for each sampling location. This ap- grasshopper cadavers were set up at known distances proach seems valid due to the arrangement of cadavers from sprayed plot edges and scavenging rates measured. (spaced widely over a 150 m transect) and mobility of It was anticipated that this approach would allow scavengers (i.e., we were effectively subsampling within a comparisons of the functional effect of such non-target bigger pool of scavengers on each occasion). Lines of groups over a wide area. least variance were fitted to the overall data for central Each sampling replicate consisted of 30 grasshopper and border locations within each treated plot (SPSS, cadavers placed individually at 5 m intervals along a 1997). 336 S. Arthurs et al. / Biological Control 26 (2003) 333–340

2.4. Estimating grasshopper mortality through cadaver fungal mycosis), 108 fungus- and insecticide-killed counts grasshoppers in equal proportions were placed individ- ually on bare ground in a random stratified sequence at The densities of grasshopper cadavers resulting from 4-m intervals within a 50 50 m area. In addition, to treatment were periodically assessed and their decom- estimate whether strictly epigeal arthropods were the position/scavenging rates measured. It was anticipated main cause of scavenging as opposed to non-epigeal that this approach would help identify the proximate arthropods and vertebrates (birds, lizards, etc.), we causes of grasshopper mortality in treated plots. To es- compared the scavenging rate on the ground with an timate cadaver density, dead grasshoppers were scouted adjacent paired sample maintained 10 cm above the for at different locations on seven occasions between ground. For this second study (effect of microsite), 72 days 7 and 42 and on three occasions between days 6 insecticide-killed grasshoppers were either placed indi- and 36 after spraying within the fenitrothion and M. vidually on bare ground or were loosely tied to an ad- anisopliae var. acridum plots, respectively. On each jacent stick placed upright in the ground (paired sampling occasion, counts were made at four locations sample), using a similar layout to the previous study. In that appeared representative of the area, two at central both studies, small flags were used to mark the position locations and two at outer locations of treated plots; of each cadaver, which was revisited daily for 30 days to >1.2 km and within 300 m of the treatment border, re- check for presence/absence. The cumulative rate of ca- spectively. A thrown stick was used to randomly select a daver disappearance over this period was assessed using position and all visible cadavers (regardless of minor Kaplan–Meier survival analysis (SPSS, 1997). Differ- damage) within a 1-m2 quadrat were counted. Cadavers ences in the mean persistence among categories of ca- were not apparently aggregated. Thirty quadrats were davers were tested using a log rank test with significance counted at each of the four sampling locations and the levels of multiple comparisons adjusted using a Bon- same process repeated at equivalent locations in control ferroni correction. plots. Due to the low numbers in most cases (mode ¼ 0), the means from each sampling location (n ¼ 30) were transformed via a Poisson distribution with a log-link 3. Results function and change in deviance attributable to a factor tested against values of v2 using GLIM 3.77 (Crawley, 3.1. Effects of treatments on non-target epigeal arthro- 1993). pods

2.5. Persistence of grasshopper cadavers Beetles and ants were by far the most abundant ground-dwelling arthropods observed and evident To estimate decomposition/scavenging rates of scavengers in all plots. The tenebrionids (notably Zo- grasshopper cadavers, the persistence of fungus-killed phosis posticalis (Deyrolle) and Erodius laevigatus), and and insecticide-killed grasshoppers was assessed. Fun- ants (predominantly Monomorium spp. but also Messor gus-killed insects were infected adult A. blondeli and P. galla (Mayr), Pheidole spp., and Cataglyphis spp.) were cognata that were collected alive within the M. anisop- the most common species recovered from grasshopper liae var. acridum plot and allowed to die from fungal cadavers during the day and carabid beetles (Anthia mycosis within laboratory cages. Both freshly killed sexmaculata (F.) and an unidentified Scarites sp.) were cadavers with non-sporulating fungus and those that also observed scavenging at dusk. All these non-target had sporulating fungus were used. ÔNon-sporulating ground-dwelling species were abundant in the same re- cadaversÕ exhibited characteristic red (mycotoxin) col- gion in August 1996, when together the four insect oration but were not allowed to develop saprophytically, families, Carabidae, Tenebrionidae, Formicidae, and whilst Ôsporulating cadaversÕ were incubated on damp Ephydridae made up about 75% of the total pitfall catch filter paper in Petri dishes for 72 h and exhibited both (Peveling et al., 1999). internal and external fungal growth. Insecticide-killed Scavenging rates in the fenitrothion plot were initially grasshoppers originated from equivalent insects col- significantly depressed but recovered to levels found in lected from an untreated site and killed with a non- the control and M. anisopliae var. acridum plots, which residual pyrethroid within 24 h of use. remained high (>95%) throughout the assessment pe- The procedure for estimating decomposition/scav- riod. A one-way ANOVA repeated across equivalent enging rates of fungus and insecticide-killed grasshop- sampling periods showed that overall, scavenging rates pers in epigeal locations follows that of Arthurs et al. within the fenitrothion-sprayed plot remained signifi- (2001), except that measurements were made within a cantly depressed (df ¼ 1, 22; P < 0:05) compared with Metarhizium-treated area, as opposed to an unsprayed control plots until 44 days post-spraying, after which site, and thus scavenging rates would reflect any effect of time no differences were observed. No difference the Metarhizium treatment. For the first study (effect of ðP > 0:05Þ was detected for any equivalent sampling S. Arthurs et al. / Biological Control 26 (2003) 333–340 337 periods between the M. anisopliae var. acridum and acridum and control plots, the overall mean density of untreated plots. cadavers remained low (<0.25 m2) throughout the as- The results from the fenitrothion plot suggest there sessment period. Statistical analysis revealed no signifi- were spatial differences in the recovery of scavenger cant difference in cadaver density between the M. activity. Fig. 2 compares within plot scavenging rates anisopliae var. acridum and either control plots at any between the four transects nearest the spray border with equivalent sampling period. In contrast, cadaver density the four in the most central locations in both treated in the fenitrothion plot was significantly higher (2m2) plots. In the M. anisopliae var. acridum plot fitted lines compared with both M. anisopliae var. acridum describing these data are linear and show no difference (v2 ¼ 9:2; P < 0:01) and control plots (v2 ¼ 8:7; P < between central and border locations. By contrast, 0:01) at the equivalent first sampling period 6–7 days after central transects within the fenitrothion plot recorded spraying, although fell to a similar low level by the end of lower removal rates compared with border transects at the assessment period. We suspect cadaver density was each equivalent sampling period. One-way ANOVA highest shortly after the fenitrothion application, al- showed that the decrease in scavenging rate at central though counts were not made before day 7. Fig. 3 com- areas was significant on day 7 (F1;6 ¼ 7:15; P < 0:05) pares cadaver density between the two sampling locations and the final three sampling periods after spraying (day nearest the spray boundary with the two in central loca- 28 onwards) (F1;6 ¼ 13:35; P < 0:01, F1;6 ¼ 10:07; P ¼ tions in both treated plots. Within plot comparisons re- 0:05, F1;6 ¼ 12:97; P < 0:01). Scavenging rates from vealed no differences between border and central border transects in the fenitrothion plot were no longer sampling locations at equivalent periods in either treated significantly different (df ¼ 1, 6; P < 0:001) to equivalent plot, although comparisons were restricted by the small locations in M. anisopliae var. acridum and untreated number of sampling locations. However, differences are plots after 35 days while those from central transects apparent between border and central locations across remained significantly lower on all equivalent dates until plots. Here, the difference in the density of grasshopper the end of the assessment period. cadavers observed between the M. anisopliae var. acridum and fenitrothion plots at 6–7 days post-spraying was 3.2. Estimating grasshopper mortality through cadaver significant between central locations (v2 ¼ 6:01; P < counts 0:025) but not for border locations (v2 ¼ 3:3; P > 0:05).

There were differences in densities of grasshopper 3.3. Persistence of grasshopper cadavers cadavers between treatments. In the M. anisopliae var. In the cadaver persistence study, the numbers of ca- davers used were relatively small, but the results were

Fig. 2. Comparison of cadaver scavenging rates from 4 central1 ðNÞ and border1 ðÞ locations within plots sprayed with a mycoinsecticide Fig. 3. Comparison of cadaver density from 2 central1 ðNÞ and border1 (grey) and fenitrothion (black). [1 Equivalent to (>900 m; >1.2 km) and ðÞ locations within plots sprayed with a mycoinsecticide (grey) and (<300 m) from plot edges, respectively]. Mean removal rates (SE) are fenitrothion (black). [1 Equivalent to (>1.2 km and <300 m) from plot shown with time post-spraying and least variance models fitted. My- edges, respectively.] Mean densities (SE) are shown with time post- copesticide plot border and central locations are described linearly spraying and least variance models fitted; mycopesticide plot border (broken line) by y ¼À0:22x þ 101:57 (R2 ¼ 0.71) and y ¼À0:22x þ and central locations are described linearly (broken line) by 101:69 (R2 ¼ 0.86), respectively. Fenitrothion plot border and central y ¼ 0:007x þ 0:007 (R2 ¼ 0:55) and y ¼ 0:006x À 0:018 (R2 ¼ 0:99), locations are described logarithmically (solid line) by y ¼ 12:78 lnðxÞþ respectively. Fenitrothion plot border and central locations are de- 50 (R2 ¼ 0:63) and y ¼ 15:91 lnðxÞþ20:58 (R2 ¼ 0:54), respectively. scribed (solid line) by y ¼ 1:593eÀ0:064x (R2 ¼ 0:78) and y ¼À1:084 Data from equivalent untreated plots are not shown but did not differ lnðxÞþ4:102 (R2 ¼ 0:93), respectively. Equivalent assessments from from the mycopesticide plot (see Section 3). control plots (not shown) recorded low densities (<0.2/m2). 338 S. Arthurs et al. / Biological Control 26 (2003) 333–340

Table 1 Environmental persistence SE (days) of grasshopper cadavers within an area treated with a mycopesticide in the Sahel: (a) effect of fungal mycosis, control (insecticide) versus mycopesticide (Metarhizium), (b) effect of microsite (insecticide-killed insects only), control (epigeal) versus above ground (category n ¼ 36) Metarhizium-killed, initially showing No sporulation External sporulation (a) Control (pyrethroid-killed) 1:3 0:1aa (0)b 24:3 1:6 b (63.9) 15:8 1:3 c (19.4) (b) Control (epigeal) Above ground 1:5 0:7a(0)6:4 2:3b(0) a Different lower case letters represent differences between cadavers at the 1% level (log rank test). b The percentage of cadavers censored (not disappeared) after 30 days if not zero. striking (Table 1). As in the last study, pyrethroid-killed Homoptera (Ball et al., 1994; Peveling and Demba, grasshoppers placed on the ground were rapidly scav- 1997; Prior, 1997). The present field assessment also enged (within 48 h) by ground-dwelling beetles and ants. detected no reduction in scavenging activity of non- By contrast, these insects did not actively scavenge target epigeal arthropods in the M. anisopliae var. ac- equivalently placed Metarhizium-killed grasshoppers, ridum plot compared with untreated locations. This which on average, persisted in the field for several weeks. complements previous assessments in the same region in The persistence of cadavers that harbored Metarhizium 1996, which showed that the median effect of M. ani- appeared to be related to a gradual weathering. Ca- sopliae var. acridum on the abundance of the same non- davers had sporulating fungus tended to fragment and target groups within replicated 50 ha plots (via pitfall disintegrate during rainfall. Insecticide-killed grasshop- traps) was <25% and the product was classified as Ôlow pers were far less likely to be scavenged when held just riskÕ (Peveling et al., 1999). In the present study feni- above the ground, out of the reach of epigeal arthro- trothion caused an immediate and dramatic reduction in pods. the scavenging activity of these non-targets, which ap- peared to last >40 days at the plot center, although as might be expected recovery at the plot edges was more 4. Discussion rapid. This ÔfunctionalÕ measurement builds on the studies of Peveling et al. (1999), who measured a similar A potential criticism of these data concerns the reduction in the abundance of non-target groups within pseudo-replicated design imposed by the original spray replicated 50 ha plots treated with fenitrothion in 1996, trial. The decision to treat a single large area at the ex- although in the latter case most of the non-target fauna pense of true replication followed the spraying of had fully recovered after 31 days. Cadavers of all non- smaller (50 ha) replicated plots with M. anisopliae var. target species mentioned in Section 2 were evident acridum and fenitrothion in the previous year (1996) and throughout the fenitrothion plot in the days following was taken to allow the season-long population dynamics spraying, although only live specimens were observed of grasshoppers to be observed following an operational elsewhere. scale campaign (Langewald et al., 1999). Similarly, a Regrettably predation of grasshoppers per se was not large treated area may better reflect changes in the ac- quantified at sampling locations. However, the cadaver tivity of many mobile non-target epigeal arthropods density assessments coupled with the estimated persis- compared with smaller replicated plots which tend to tence of insecticide- and fungus-killed grasshoppers suffer from problems of dispersal from treated areas provides indirect evidence that grasshopper natural en- within a few days or weeks. Although the effect of non- emies played a role in the ultimate fate of grasshoppers independent sampling bias must be considered for the within the M. anisopliae var. acridum plot. In this plot, present data, it cannot reasonably explain the extent to although the population of live grasshoppers was re- which patterns of scavenging and apparent grasshopper duced by 92% after 22 days (Fig. 1), few signs of disease mortality differed between treated plots in the weeks could be observed; indeed >1% of the population re- following spraying. The chemical and biological treat- duction could be explained by the density of Metarhiz- ments appeared to affect non-target epigeal arthropods ium-killed grasshoppers accumulating throughout this differently. Moreover, as discussed below, such differ- plot (Fig. 3). This shortfall was despite the fact that ences (in selectivity) may arguably influence the efficacy Metarhizium-killed grasshoppers placed in the field in of such large-scale treatments. the presence of scavengers persisted for several weeks In direct exposure assays, M. anisopliae var. acridum within the M. anisopliae var. acridum plot (Table 1), and has low toxicity to the Hymenoptera, Coleoptera, and thus should have become apparent had grasshoppers S. Arthurs et al. / Biological Control 26 (2003) 333–340 339 been killed directly by the fungus. The scavengers, Z. hosts could be beneficial by enhancing the effective posticalis, E. laevigatus, and Monomorium spp., as well mortality rate. Moreover, it seems likely that the con- as active predators included mantids (Mantodea: Man- servation of and synergistic interactions with other tidae), robber flies (Diptera: Asilidae), and sphecid natural enemies might also enhance the persistence of a wasps (Hymenoptera: Sphecidae), were observed prey- large-scale mycopesticide treatment by enabling natural ing upon live but often apparently sluggish grasshoppers control to act as a buffer against the recolonization of in the M. anisopliae var. acridum plot. Interestingly, treated areas once pest populations have been reduced. both Monomorium spp. and tenebrionids are generally This is in contrast to current chemical pesticides, such as considered more as scavengers than predators (Peveling fenitrothion, which may cause grasshopper resurgences et al., 1999). Given we did not observe significant (Lockwood et al., 1988). However, because Hyph- grasshopper dispersal in the first weeks after spraying, omycete fungi can only initiate new infections following the shortfall in grasshopper cadavers in the M. anisop- the death of the host (Hajek and St. Leger, 1994), pre- liae var. acridum plot, together with unquantified visual dation of live infected hosts may reduce or eliminate the observations, suggests that predation of diseased but potential for further cycles of the disease and so reduce still living grasshoppers was high. This shortfall of the impact of spray applications in the long-term. Cer- grasshopper cadavers was far less apparent in the feni- tainly this latter point would help explain why hori- trothion plot, where grasshopper natural enemies pre- zontal transmission following spray applications is sumably suffered a similar fate as their prey. generally far less apparent than predicted by theoretical The circumstantial evidence for enhanced suscepti- models (Thomas et al., 1995). It is clear that such in- bility of live but Metarhizium-infected grasshoppers to traguild interactions may be complex and warrant local field predation is supported by small-scale studies in level investigation. other environments. Both the rice grasshopper Hierogl- In conclusion, biopesticides offer an environmentally yphus daganensis (Krauss) in northern Benin and the safe alternative for locust and grasshopper control. Al- brown locust Locustana pardalina (Walker) in the South though selectivity of biopesticides is normally consid- African Karoo that were infected with M. anisopliae var. ered from the perspective of environmental safety, the acridum and tethered in the field, suffered higher rates of implications for efficacy are poorly understood. These predation during the disease incubation period com- data support growing evidence that the effect of grass- pared with controls (Arthurs and Thomas, 1999; Tho- hopper and locust control measures (both chemical and mas et al., 1998). During field trials with the same fungal biological) on indigenous natural enemies should be isolate in the desert, signs of fungal infection viewed more holistically and not considered simply in could not be observed on Schistocerca gregaria (Forskaal) terms of direct short-term mortality rate, especially hopper bands treated in the field because of severe can- where treatments are conducted over a large area. The nibalism and heavy predation by birds, which resulted in testing of microbial control products should be en- three out of four treated bands being completely elimi- couraged at the appropriate scales across time and space nated within 4–5 days despite apparently little mortality to infer useful generalities associated with operational of locusts within untreated bands (Kooyman and use. Godonou, 1997). During similar trials in the Buzi floodplain in Mozambique, the cohesive break down and disappearance of bands of red locust, Nomadacris Acknowledgments septemfasciata (Serville), was attributed to the fungus, although few cadavers were found (Price et al., 1997). The authors are grateful to members of the LUBIL- The present studies and those of Peveling et al. (1999) OSA team from IITA (Cotonou) and DFPV (Niamey) provided no evidence that M. anisopliae var. acridum for field assistance. LUBILOSA is funded by the de- directly infected and killed (i.e., acted as an intraguild velopment agencies of Canada (CIDA), Netherlands predator) indigenous non-target arthropods under field (DGIS), Switzerland (SDC), and UK (DfID). Com- conditions. Rather, by selectively consuming live but ments from John Lawton, Carlos Bogram and two infected prey, natural enemies of grasshoppers may have anonymous reviewers improved the manuscript. functioned proximately as intraguild predators of the fungus, although this did not appear to be the case postmortem once the fungus had started saprophytic References development (Table 1). The type of interspecific inter- actions described above may influence the efficacy of a Arthurs, S., Thomas, M.B., 1999. Factors affecting horizontal trans- spray campaign. Given the slow action of entomopath- mission of entomopathogenic fungi in locusts and grasshoppers. 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