African Journal of Agricultural Research Vol. 7(38), pp. 5317-5331, 2 October, 2012 Available online at http://www.academicjournals.org/AJAR DOI: 10.5897/AJAR12.059 ISSN 1991-637X ©2012 Academic Journals

Full Length Research Paper

The role of and mammalian herbivores on the structure and composition of communities found on canopies of Acacia drepanolobium

S. K. Kuria 1* and M. H. Villet 2

1Department of Biological Sciences, Walter Sisulu University, P/B X1 Mthatha 5117, South Africa. 2Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa.

Accepted 4 June, 2012

Acacia drepanolobium Sjøstedt (Fabaceae) constitutes about 99% of the woody vegetation in the cotton soil ecosystem of Laikipia, Kenya. The tree has symbiotic association with four species that discourage large mammalian herbivores from feeding on it. However, there is no information as to whether these ants affect the community of canopy . Therefore, this study investigated the effect of the four ant species and differential vertebrate grazing and browsing pressures on the insect community inhabiting canopies of A. drepanolobium trees. Insect samples were collected using standard fogging and beating methods and identified to family and morphospecies. At the morphospecies level, the insect communities separated into two distinct groups, one comprised of samples collected from trees occupied by mimosae and Crematogaster nigriceps , and the other of samples obtained from trees inhabited by C. sjostedti and penzigi . However, differential vertebrate grazing and browsing patterns did not show any significant effect on the insect community occupying canopies of A. drepanolobium .

Key words: Community structure, coexistence, diversity, herbivory, Mpala, Kenya.

INTRODUCTION

The interactions of ants and other on plant some effect on the species richness of phytophagous canopies are complex and require proper evaluation to insect community in the Eastern Cape, South Africa. elucidate how ants affect or are affected by other Crematogaster liengmei was also shown to prey on the arthropods (Fagundes et al., 2005). Previous studies eggs of the biological control agent Cactoblastis have shown that ants influence the community cactorum (Lepidoptera: Pyralidae) in the Eastern Cape, that inhabits plant canopies in a wide variety of locations which led to the local failure of this insect to control the and habitats. For instance, the presence of ants on tree invasive prickly pear cactus, Opuntia ficus-indica canopies in northern England resulted in a significant (Robertson, 1988; Robertson and Hoffmann, 1989). increase of Periphyllus testudinaceus (Hemiptera: These examples illustrate how intricate ants’ associations Drepanosiphidae), while their removal resulted in a with other and plants can be. If properly decline (Skinner and Whittaker, 1981). The species understood and documented, interactions between ants richness of canopy ants in a semi-deciduous humid forest and other organisms could be used to predict ecological correlated positively with richness of butterflies and conditions within a given habitat by the presence of a canopy beetles (Lawton et al., 1998). Ross (1994) found particular ant species (Agosti et al., 2000). that the presence of ants on Ficus burtt-davyi Hutch had The black cotton soil ecosystem of Laikipia District, Kenya, is heavily used for ranching of both game and cattle (Young et al., 1997), and supports a wooded grassland technically named ‘Acacia bushed grassland’ * Corresponding author. E-mail: [email protected]. Tel: +27 (Young et al., 1995), in which more than 99% of the 5022170. woody vegetation is Acacia drepanolobium (Harms) 5318 Afr. J. Agric. Res.

Sjøstedt. At Mpala Research Centre, Kenya, four ant MATERIALS AND METHODS species that are usually mutually hostile compete for A. drepanolobium trees in which they nest (Palmer et al., Study area 2000). The smaller trees are generally inhabited by Fieldwork was conducted between September 2003 and June 2005 Crematogaster nigriceps and Tetraponera penzigi at the Kenya Long-term Exclosure Experiment (KLEE) plots (Young (subordinate species), while bigger trees are occupied by et al., 1998) at Mpala Research Centre, Laikipia District, Kenya Crematogaster sjostedti and Crematogaster mimosae (0°17 ′ N, 36° 56 ′ E; 1800 m elevation). The study site had nine (dominant species) (Palmer et al., 2000). C. nigriceps experimental plots, six exclosures in the KLEE and three non- sterilises A. drepanolobium trees by chewing off the floral exclosed plots next to the KLEE plots. The KLEE exclosures were buds (Stanton et al., 1999), while T. penzigi destroys the initiated in September 1995 to investigate the effects of herbivores on the savannah ecosystem (Young et al., 1998). The average foliar nectaries of its tree, in effect lowering the chance of annual rainfall at Mpala Research Centre is 580 mm, while the having more aggressive dominant ants take over the vegetation is generally composed of a woody layer which is trees it inhabits (Young et al., 1997; Palmer et al., 2002). composed mainly of A. drepanolobium (> 99%) interspersed with Apart from these direct effects on the host plant, there several grass species (Young et al., 1997). A detailed description of may be direct and indirect effects on the community of the KLEE exclosures was published by Young et al. (1998). The KLEE exclosure plots were 200 m × 200 m, while the three non- other insects inhabiting these trees, and it is important to exclosed plots measured 100 m × 200 m. Two of the KLEE understand the whole ecosystem to see what kind of exclosure treatments, 0 (no large herbivores including cattle were insect community is associated with these four ant allowed), and C (only cattle were allowed) were sampled along with species. a treatment E (all herbivores including cattle were allowed) that lay Ecological pressure from vertebrate grazing and outside the KLEE exclosures but adjacent to C plots. The browsing sometimes have positive or negative effects on treatments were replicated three times in what is referred to as the “North”, “Central” and “South” blocks (Young et al., 1998: Figure 1). invertebrate communities (O’Neill et al., 2003). Apart from It was felt that these three treatments would give a general the potential effects of the ants, the black cotton soil representation of the various insects found on the canopies of A. ecosystem is measurably affected by the vertebrate drepanolobium and how they interact with ants in the face of herbivores inhabiting it, as shown by the Kenya Long- various degrees of vertebrate herbivory. It was also assumed that term Exclosure Experiment (KLEE) (Young et al., 1998, nine years was sufficient time that if excluding large mammals had 2005; McCauley et al., 2006; Goheen et al., 2007; Odadi any effect on the canopy insects, it should have become measurable. et al., 2007; 2011; Riginos and Young, 2007; Maclean et al., 2011), which is designed to distinguish between the effects of six combinations of cattle (grazers); very large, Sampling indigenous browsing herbivores (giraffes and elephants); and other, primarily grazing, indigenous herbivores Four sampling sessions were carried out with intervals of three (buffalo, smaller ungulates and rodents). The effects of months between consecutive sessions, as follows: first sampling these herbivores was reflected in the community of (27 October – 13 November 2003), second sampling (11 – 28 February 2004), third sampling (26 May – 12 June 2004) and fourth spiders found in the herb layer of the vegetation (Warui et sampling (10 – 27 September 2004). To minimise method-induced al., 2005), but their effects on the community of variation and bias, the insects inhabiting A. drepanolobium invertebrates in the canopy has not been published. canopies were collected using fogging and beating. Five trees Since the four ant species associated with A. occupied by each of the four acacia ant species were sampled drepanolobium behave and modify the tree canopies using each method, making a total of 20 trees during each sampling differently, the current study was undertaken to determine session in each of the three herbivory treatments. The two sampling methods combined sampled a total of 40 trees in each of the if this effect influenced the insect communities inhabiting herbivory treatment during one sampling session. In total 1440 A. drepanolobium . Another possibility was that the trees were sampled during four sampling sessions. herbivores might modify the influence of the ants, so this effect was incorporated by conducting sampling at the KLEE site. The knowledge of the abundance and Beating samples diversity of canopy insects was intended to provide an insight to understanding the interactions that may exist Eighty trees were semi-randomly marked using aluminium tags by between these insect community and the four acacia- following a compass direction on a straight line, and tagging trees ants, taking into account a major land use, the types of within a 20 m range, in each of the herbivory treatments. The trees incorporated twenty trees occupied by each of the four acacia-ants vertebrate herbivores in the ecosystem. The aims of this (C. sjostedti, C. mimosae, C. nigriceps and T. penzigi ). Only trees study were therefore: 1) to establish a checklist of the with heights ranging between 1.0 to 2.5 m were tagged, since these insect species that coexisted with the four obligate have been previously shown to be colonized by all four ant species acacia-dwelling ants; 2) to determine the effect of acacia- (Young et al., 1997; Palmer et al., 2000). A total of 720 trees were ants and various grazing / browsing combinations on tagged in the nine plots. Using random numbers, the trees were allocated to one of the four sampling sessions (Zar, 1974). indices of the diversity and evenness of the community of Sample collection involved beating a tree twenty times with a canopy insects; and 3) to determine the effect of acacia- wooden stick and collecting all falling insect samples using four ants and vertebrate herbivores on the community structure sheets (each 1 m 2) spread under the tree. Samples from the four and composition of canopy insects. sheets were pooled, labelled and placed in a polyethylene bag. In Kuria and Villet 5319

N

North Plots

Central Plots

South Plots

400 m

Figure 1. Arrangement of experimental blocks (dotted rectangles) and treatment plots (solid squares) at the KLEE Plots site. Treatment 0 - no herbivores were allowed access; Treatment C - only cattle were allowed access; Treatment E - all herbivores and cattle were allowed access. The treatments were replicated three times (North, Central and South blocks). Diagram after Young et al. (1998).

the laboratory, samples collected using beating were pooled each of the four ant species were sampled, making a total of 20 together with those collected by fogging from a neighbouring tree samples. A hand pumped knap-sack sprayer was used to fog trees and later sorted to order, family and finally to morphospecies. using Alphacypermethrin (100 g/L). The insecticide was diluted with These taxonomic groupings were later confirmed at the National water using the recommended dose rate (5 ml: 10 L). Roughly 300 Museums of Kenya and Plant Protection Research Institute ml of the diluted insecticide was used to sample one tree. To (Pretoria). minimize the effect of wind, fogging was carried out in the mornings (07:30 - 10:30 h) and only in dry conditions. Each tree was fogged for 30 - 40 s, making sure the fog penetrated the canopy. All Fogging samples arthropods falling from the canopy were collected on four sheets (each 1 m 2) placed beneath the tree. After 40 - 50 min, the catch Fogging was carried out in the same plots in which samples were was removed from the sheets and placed in polythene bags. All collected using beating method, with the same number of trees and specimens were later preserved in 70% ethanol. Sampling dates of similar heights. For each sampling session five trees occupied by were the same as those for the beating method earlier mentioned. 5320 Afr. J. Agric. Res.

Data analysis F = 2.87, d.f. = 3, P = 0.002). Pair-wise comparisons revealed that the total number of orders (S) was The data for two neighbouring trees, one sampled by beating and significantly higher on trees occupied by C. sjostedti (S = the other by fogging but occupied by the same ant species, were pooled and regarded as one sampling unit. Specimens were 3.69) compared to those colonized by C. mimosae (S = identified to order, family and morphospecies, and three descriptors 3.01: t = 2.72, P = 0.010) and T. penzigi (S = 3.31: t = of community diversity were calculated at each taxonomic level: the 2.02, P = 0.010). Total numbers of orders was again total number of taxa (S), the Shannon-Wiener diversity index (H ′) significantly different ( t = 1.79, P = 0.030) on trees and Pielou’s evenness index (J ′). These descriptors were subjected occupied by C. mimosae (S = 3.01) as compared to those to permutation analysis of variation as implemented in the software program PERMANOVA (Anderson, 2005). For all analyses, 999 inhabited by C. nigriceps (S = 3.46). The diversity of the permutations were used to generate the p -value. Each time a insect community was significantly different for insect significant difference (P<0.05) was recorded, further pair-wise samples collected from trees inhabited by C. sjostedti (H ′ comparisons were carried out using 99 permutations. = 1.02) compared to those found on trees colonized by C. Ordination by non-metric multidimensional scaling (nMDS) was mimosae (H ′ = 0.83; t = 2.34, P = 0.010) and T. penzigi performed in the PRIMER software (Warwick, 1988) using a Bray- (H ′ = 0.89; t = 1.98, P = 0.010); those collected on trees Curtis similarity matrix derived from the log transformed (x’ = log(x ′ ′ +1)) data and ten iterations. Points on the maps that represented occupied by C. mimosae (H = 0.83) and C. nigriceps (H similar treatments or ant species were later joined using convex = 0.97; t = 1.71, P = 0.030); and finally those insect hulls. samples sampled from trees occupied by C. nigriceps (H ′ = 0.97) and T. penzigi (H ′ = 0.89; t = 1.49, P = 0.050). There was no significant effect due to differential RESULTS grazing and browsing pressure on the community descriptors at the level of taxonomic order (S: F = 1.08, Abundance of canopy insects associating with the d.f. = 2, P = 0.399; Pielou’s J ′: F = 2.22, d.f. = 2, P = four acacia ant species 0.088; Shannon-Wiener H ′: F = 1.58, d.f. = 2, P = 0.219). The location of the three experimental blocks did not A total of 115 morphospecies belonging to 24 families show any effect on the three community descriptors (S: F and seven orders were identified. Out of a combined total = 1.50, d.f. = 2, P = 0.246; Pielou’s J ′: F = 1.07, d.f. = 2, P of 10,134 individuals collected, 31.9% were associated = 0.411; Shannon-Wiener H ′: F = 1.98, d.f. = 2, P = with C. sjostedti , 24.6% with C. nigriceps and 24.8% with 0.109). There was also no interaction effect between T. penzigi (Appendix 1). 84 morphospecies were sampled location × treatment ( F = 0.89, d.f. = 4, P = 0.596), from trees inhabited by C. sjostedti , while 66 morpho- location × ants ( F = 1.03, d.f. = 6, P = 0.426), treatment × species were collected from trees colonized by T. penzigi ants ( F = 0.73, d.f. = 6, P = 0.897) and between location (Appendix 1). A. drepanolobium trees occupied by C. × treatment × ants ( F = 1.29, d.f. = 12, P = 0.104) for the nigriceps had the highest percentages of individuals in total number of orders. PERMANOVA results showed the orders Blattodea (cockroaches: 49.8%) and that there was no interaction effect for Pielou’s evenness Hemiptera (bugs: 47.5%), respectively. From the pooled index between location × treatment ( F = 0.62, d.f. = 4, P samples for the four ant species, 57.7% of = 0.890), location × ants ( F = 1.14, d.f. = 6, P = 0.306), (other ants apart from the four obligate ant species) and treatment × ants ( F = 0.72, d.f. = 6, P = 0.865) and 30.3% of Coleoptera (beetles) came from trees colonized location × treatment × ants ( F = 1.29, d.f. = 12, P = by C. sjostedti (Table 1). 0.074). However, there was an interaction effect between Moreover, out of the 24 insect families recorded, 18 location × treatment × ants ( F = 1.46, d.f. = 12, P = 0.029) associated with all four ant species. However, members for the Shannon-Wiener diversity index. Further analysis of the family Polyphagidae (Blattodea) were not found on did not reveal any significant difference between the trees inhabited by C. mimosae and C. nigriceps , while a treatments in the three experimental blocks for the four singleton of the family Meenoplidae (Hemiptera) was ant species, except that in the central block there was a collected from a tree occupied by C. sjostedti (Appendix significant effect on trees inhabited by C. nigriceps 1). Members of the families Coccinellidae and between two plots, one in which both wildlife and cattle Bostrichidae were not found to occur on trees colonized were allowed and the other in which only cattle were by C. nigriceps and T. penzigi , respectively. allowed to graze ( t = 1.34, P = 0.030).

Effect of ants and differential grazing and browsing Familial level pressure on insect community structure Analysis was also carried out at the family level in case Ordinal level the community response at this taxonomic level differed from that at the order level. Results indicated that there Two of the three community descriptors varied was an effect on the total number of families and the significantly between the ant species (total number of Shannon-Wiener index for the factor ants (Table 2). taxa: F = 3.32, d.f. = 3, P = 0.001; Shannon-Wiener: Nonetheless, there was no significant effect between Kuria and Villet 5321

Table 1. Relative frequencies (expressed as percentages) of individuals belonging to seven insect orders occurring on canopies of A. drepanolobium trees inhabited by four acacia ants collected using two sampling techniques (fogging and beating).

Insect order C. sjostedti C. mimosae C. nigriceps T. penzigi Blattodea 28.64 (407) 14.99 (213) 49.82 (708) 6.54 (93) Coleoptera 30.28 (1308) 19.91 (860) 20.16 (871) 29.65 (1281) Hemiptera 9.00 (74) 17.64 (145) 47.45 (390) 25.90 (213) Hymenoptera 57.65 (708) 22.07 (271) 6.60 (81) 13.68 (168) Mantodea 32.21 (67) 15.87 (33) 13.46 (28) 38.46 (80) 33.09 (631) 17.09 (326) 17.88 (341) 31.93 (609) Phasmatodea 7.02 (16) 21.93 (50) 42.11 (96) 28.95 (66)

The expected value was 25% in all cases if the distributions were random.

block locations, treatments and ant species on Pielou’s J ′ significant difference in distribution between insect (Table 2). Further analysis on the total number of families samples collected from trees inhabited by the different revealed that there was a significant difference in the ant species except those collected from trees colonised insect community between trees occupied by C. sjostedti by C. sjostedti and T. penzigi , and those collected from (mean ± SE, S = 5.48 ± 0.16, n = 180) and those C. mimosae and C. nigriceps (Table 3). However, there colonized by C. mimosae (S = 4.13 ± 0.16) , C. nigriceps was no interaction effect between location × treatment, (S = 4.60 ± 0.15) and T. penzigi (S = 4.59 ± 0.15). A location × ants, treatment × ants, and between location × significant difference was also found between C. treatment × ants for the Pielou’s evenness index (Table mimosae and C. nigriceps (Table 2). The Shannon- 3). Pair-wise comparisons for the Shannon-Wiener Wiener diversity index was higher on insect samples diversity index showed that insect samples collected from collected from trees inhabited by C. sjostedti (H ′ = 1.36 ± trees inhabited by C. sjostedti (1.45 ± 0.03) had a higher 0.03) compared to those collected from trees occupied by diversity than those collected from trees occupied by C. C. mimosae (H ′ = 1.09 ± 0.04), C. nigriceps (H ′ = 1.19 ± mimosae (1.14 ± 0.04) and C. nigriceps (1.22 ± 0.04) 0.03) and T. penzigi (H ′ = 1.20 ± 0.04) (Table 2). There (Table 3). Trees inhabited by T. penzigi (1.34 ± 0.04) had was, however, no interaction effect between location × insect community that had a significantly higher diversity treatment, location × ants, and location × treatment × compared to that occupying trees colonized by C. ants for the three community descriptors (Table 2). mimosae (1.14 ± 0.04), while insect community inhabiting trees colonized by T. penzigi had a higher diversity (1.34 ± 0.04) compared to that found on trees colonised by C. Morphospecies level nigriceps (1.22 ± 0.04). There was no interaction effect for Shannon-Weiner diversity index (H’) between location Some effects on community descriptors are more × treatment, location × ants, treatment × ants, and pronounced at the lower taxonomic units as opposed to location × treatment × ants (Table 3). There was also no higher taxonomic units (Biaggini et al., 2007). Therefore, significant effect on the insect community due to analysis was also carried out at the morphospecies level. herbivory treatment (total number of morphotaxa S, Results revealed that the insect community was Pielou’s J’ and Shannon-Wiener H ′) and location (total significantly different for the three community descriptors number of morphotaxa S, Pielou’s J ′ and Shannon- (total number of morphotaxa S, Pielou’s J ′ and Shannon- Wiener H ′) (Table 3). Wiener H ′) for the factor ants (Table 3). Pair-wise comparisons for the total number of morphotaxa showed that insect samples collected from trees colonized by the Effects of experimental block location, herbivory four ant species were significantly different except those treatment and acacia-ants on insect community collected from C. mimosae and C. nigriceps (Table 3). structure The total number of morphotaxa of the insect samples collected from trees occupied by the different ant species The nMDS ordination did not reveal any specific pattern were as follows C. sjostedti (means ± SE, 6.23 ± 0.20), C. on insect communities based on the block locations. mimosae (4.39 ± 0.18), C. nigriceps (4.78 ± 0.16) and T. However, each sampling session seemed to be unique penzigi (5.38± 0.20). and all of the sampling sessions separated from each Results obtained did not reveal any interaction effect other (Figure 2). Ordination indicated that there was a between location × treatment, location × ants, treatment × tendency of insect communities that were collected ants, and location × treatment × ants for the total number during the same sampling session to segregate in one of morphotaxa (Table 3). Nonetheless, analysis on direction. The convex hulls of the samples collected from Pielou’s evenness index showed that there was a the different locations overlapped at the order, family and 5322 Afr. J. Agric. Res.

Table 2. Results of PERMANOVA to test the effect of location (North, Central and South) treatments 0 (no herbivores and cattle allowed), C (only cattle allowed) and E (all herbivores and cattle allowed), and ants ( C. sjostedti , C. mimosae , C. nigriceps and T. penzigi ) on total number of families (S), Pielou’s evenness index (J’) and the Shannon-Wiener diversity index (H’) at family level.

Variable Source df MS F P perm Location 2 15.132 0.763 0.590 Treatment 2 21.257 1.641 0.174 Ants 3 80.743 4.011 0.001 Location*Treatment 4 16.069 0.849 0.619 S Location*Ants 6 19.826 0.985 0.475 Treatment*Ants 6 12.951 0.643 0.910 Location*Treatment*Ants 12 18.931 0.940 0.568 Residual 108 20.132

Ant species t P perm C. sjostedti vs. C. mimosae 2.997 0.010 C. sjostedti vs. C. nigriceps 2.118 0.010 C. sjostedti vs. T. penzigi 2.249 0.010 C. mimosae vs. C. nigriceps 1.567 0.040 C. mimosae vs. T. penzigi 1.392 0.120 C. nigriceps vs. T. penzigi 1.079 0.290

Location 2 3.101 1.399 0.249 Treatment 2 1.701 1.103 0.386 Ants 3 2.728 1.387 0.174 Location*Treatment 4 1.441 0.623 0.881 J’ Location*Ants 6 2.217 1.127 0.305 Treatment*Ants 6 1.542 0.784 0.789 Location*Treatment*Ants 12 2.313 1.176 0.196 Residual 108 1.968

Location 2 1.548 1.475 0.251 Treatment 2 0.880 1.168 0.328 Ants 3 3.220 3.025 0.001 Location*Treatment 4 0.662 0.669 0.822 H’ Location*Ants 6 1.050 0.986 0.486 Treatment*Ants 6 0.754 0.708 0.873 Location*Treatment*Ants 12 0.990 0.930 0.619 Residual 108 1.064

Ant species t P perm C. sjostedti vs. C. mimosae 2.606 0.010 C. sjostedti vs. C. nigriceps 1.863 0.010 C. sjostedti vs. T. penzigi 1.944 0.010 C. mimosae vs. C. nigriceps 1.245 0.200 C. mimosae vs. T. penzigi 1.323 0.190 C. nigriceps vs. T. penzigi 1.232 0.200

*Significant at α = 0.05.

morphospecies levels (Figure 2). In most cases, samples Furthermore, the two-dimensional configurations collected from the North block were positioned between generated using insect abundances at the three those collected from Central and South blocks in the taxonomic levels (order, family and morphospecies) ordinations (Figure 2). showed that there was a tendency of communities to Kuria and Villet 5323

Table 3. Results of PERMANOVA to test the effect of location (North, Central and South) treatments 0 (no herbivores and cattle allowed), C (only cattle allowed) and E (all herbivores and cattle allowed), and ants ( C. sjostedti , C. mimosae , C. nigriceps and T. penzigi ) on total number of morphospecies (S), Pielou’s evenness index (J’) and the Shannon-Wiener diversity index (H’) at morphospecies level.

variable Source df MS F P perm Location 2 31.396 1.030 0.423 Treatment 2 24.458 1.259 0.303 Ants 3 149.407 5.063 0.001 Location*Treatment 4 23.833 0.950 0.502 S Location*Ants 6 30.470 1.033 0.419 Treatment*Ants 6 19.421 0.658 0.891 Location*Treatment*Ants 12 25.088 0.850 0.731 Residual 108 29.509

Ant species t P perm C. sjostedti vs. C. mimosae 3.297 0.010 C. sjostedti vs. C. nigriceps 2.767 0.010 C. sjostedti vs. T. penzigi 1.773 0.030 C. mimosae vs. C. nigriceps 1.601 0.070 C. mimosae vs. T. penzigi 1.957 0.010 C. nigriceps vs. T. penzigi 1.679 0.010

Location 2 5.797 1.680 0.172 Treatment 2 2.258 1.183 0.337 Ants 3 8.567 3.096 0.001 Location*Treatment 4 2.299 0.872 0.607 J’ Location*Ants 6 3.450 1.247 0.205 Treatment*Ants 6 1.909 0.690 0.877 Location*Treatment*Ants 12 2.636 0.953 0.581 Residual 108 2.767

Ant species t P perm C. sjostedti vs. C. mimosae 2.078 0.010 C. sjostedti vs. C. nigriceps 2.053 0.020 C. sjostedti vs. T. penzigi 1.276 0.240 C. mimosae vs. C. nigriceps 0.888 0.570 C. mimosae vs. T. penzigi 1.845 0.010 C. nigriceps vs. T. penzigi 2.043 0.010

Location 2 2.012 1.597 0.222 Treatment 2 1.067 1.397 0.253 Ants 3 4.765 3.842 0.001 Location*Treatment 4 0.889 0.825 0.636 H’ Location*Ants 6 1.260 1.016 0.432 Treatment*Ants 6 0.764 0.616 0.932 Location*Treatment*Ants 12 1.076 0.868 0.725 Residual 108 1.240

Ant species t P perm C. sjostedti vs. C. mimosae 2.872 0.010 C. sjostedti vs. C. nigriceps 2.332 0.010 C. sjostedti vs. T. penzigi 1.548 0.080 C. mimosae vs. C. nigriceps 1.242 0.230 C. mimosae vs. T. penzigi 1.844 0.010 C. nigriceps vs. T. penzigi 1.697 0.020

*Significant at α = 0.05. 5324 Afr. J. Agric. Res.

(a) (b)

(c)

Figure 2. Ordinations of log-transformed abundances of insects (pooled) collected using two sampling methods (fogging and beating) to test the effect of block location (N = north, C = central and S = south) on insect communities; (a) Two- dimensional nMDS of orders; (b) two-dimensional nMDS of families and (c) two-dimensional nMDS of abundances of morphospecies. Digits represent the sampling sessions.

separate reflecting the four ant species (Figure 3). At the based on the different herbivory treatments. The order level, the convex hulls of samples collected from configurations were similar at all levels (order, family and trees inhabited by C. sjostedti did not overlap at all with morphospecies). The convex hulls from all three treat- those samples collected from trees colonized by C. ments overlapped. This could imply that the insect nigriceps and C. mimosae (Figure 3). The ordination also community was not directly affected by grazing or revealed that samples collected from trees occupied by browsing pressure. T. penzigi overlapped with those collected from trees inhabited by C. sjostedti and C. mimosae but not C. nigriceps, while insect samples collected from trees DISCUSSION colonized by C. mimosae overlapped with those obtained from trees inhabited by T. penzigi and C. nigriceps but The lignified stipular spines of A. drepanolobium provide not C. sjostedti (Figure 3). domatia for four species of ants that differ in their aggres- A similar pattern was observed at family level (Figure siveness, body size and colony size (Hocking, 1970; 3). However, at the morphospecies level, samples Goheen and Palmer, 2010; Martins, 2010). Superficially, collected from trees which were colonized by T. penzigi the relation appears to be mutualistic because the plant is and C. sjostedti separated into one group and those apparently protected from invertebrate herbivores by the collected from trees inhabited by C. mimosae and C. ants, but this assumption needs verification especially in nigriceps (Figure 3) separated into a second distinct relation to phytophagous insect herbivory which has not group. Therefore, the two dimensional nMDS confi- been well-studied in this system. This study provides guration generated using abundances at order, family strong proof that the ants do affect the composition of the and morphospecies level did not reflect any pattern insect community. Kuria and Villet 5325

(a) (b)

(c)

Figure 3. Ordinations of log-transformed abundances of insects (pooled) collected using two sampling methods (beating and fogging) to test the effect of ants on insect communities: (a) two-dimensional nMDS of orders; (b) two-dimensional nMDS of families; (c) two-dimensional nMDS of abundances of morphospecies. Digits represent the sampling sessions.

Insect abundance and composition A. drepanolobium is defended by symbiotic ants (Young et al., 1997; Stanton et al., 2002), stipular thorns The variation in insect community composition occurring (Young, 1987; Milewski et al., 1991) and tannins (Ward in A. drepanolobium trees inhabited by different species and Young, 2002); therefore a small number of of ants may be due to the ants deterring insects from taxonomic units were expected in the canopies. visiting these plants or through their modification of the Nonetheless, 115 morphospecies of the class Insecta canopy. Ants have been shown to deter insect herbivores were collected from the canopies. The number was large from visiting plants (Skinner and Whittaker, 1981; Del- considering that the tree has invested heavily in Claro et al., 1996; Oliveira et al., 1999; Izzo and defensive mechanisms against herbivory. Even this Vasconcelos, 2005), but the deterring capacity may differ number was low for this ecosystem relative to the true since some herbivores possess mechanisms allowing diversity because scale insects are known to inhabit them to feed on the plant despite the presence of ants these trees and were actually seen during sampling, (Vasconcelos and Casimiro, 1997; Ruhren, 2003). The none of them were collected in any sample. Butterflies size, abundance and aggressiveness of the ants can also were also sighted perching on the trees and again these affect their protective abilities (Itioka et al., 2000; Bruna et two methods failed to collect any of them. This was a al., 2004). For instance, chrysomelids may possess clear indication that even though both fogging and adaptations that allowed them to coexist with ants beating methods were used to improve the thoroughness (Selman, 1988; Jolivet, 1991). Therefore, ant diversity of sampling of the community, the two techniques still should be considered as one factor that enhances missed some insects. The study has shown that the most biodiversity of arthropod community on ant-inhabited speciose invertebrate taxa in the A. drepanolobium plants. For instance, each of the four ant species found in canopy, apart from the four acacia ants, are Myllocerus this ecosystem associated with more than 60 sp. A (Curculionidae), Periplaneta sp. 1 (Blattidae), morphospecies (Appendix 1). Camponotus sp. (Formicidae), Ectatoderus sp. A 5326 Afr. J. Agric. Res.

(Gryllidae) and Anthicidae sp. A (Anthicidae). These five The different grazing/browsing patterns morphospecies contributed 67.25% of all insect abundance collected from the A. drepanolobium canopy. A previous study on the same site had shown that There are several possible explanations as to why a large removing large herbivores led to reduction in the length of number of morphospecies coexist with the four acacia- lignified stipular spines of A. drepanolobium (Young and ants. First, the ants may have little or no effect on other Okello, 1998) as the plant invested less in defence. invertebrates, and therefore the insect diversity may be Huntzinger et al. (2004) showed that production of controlled by the tree canopy. Secondly, the fact that the extrafloral nectaries at the KLEE site declined by 25% in symbiotic ant species reduces herbivory by large plots where all herbivores were excluded for seven years. mammalian herbivores (Madden and Young, 1992; The reduction of extrafloral nectaries was expected to Martins, 2010) may have resulted in an increase in translate into fewer rewards for ants, forcing them to look microhabitats for the different insect morphospecies. for alternative food sources to supplement their diet. This Thirdly, different insect herbivores could have attracted was further hypothesised to result in the ants defending predatory insects to these tree canopies thereby their plants at a reduced intensity, and hence result in increasing the diversity. many insect species gaining access to the canopies to feed and live there. In contrast to these hypothesized scenarios, this study’s results of PERMANOVA have Effect of ant species on insect community structure shown that the different grazing / browsing patterns had no significant effect on total number of taxa, Pielou’s Elsewhere, mutualistic ants defend trees against evenness index and the Shannon-Wiener diversity index vertebrate browsers (McKey, 1974; Agosti et al., 2000; at order, family and morphospecies levels. However, Madden and Young, 1992) and insect herbivores (Koptur, other studies have shown different findings; for example 1984; Itioka et al., 2000; Offenberg et al., 2004). Ant-plant arthropods were more abundant and diverse in grazed mutualisms are imperative in structuring the community than in ungrazed plots (González-Megías et al., 2004). of canopy arthropods in cerrado habitats in Brazil Species richnesses of nectar-seeking butterflies and (Oliveira and Freitas, 2004). The acacia ants colonizing bumble bees in south-central Sweden were shown to be A. drepanolobium at Mpala Research Centre modify their negatively correlated with grazing intensity as reflected tree’s canopies differently (Young et al., 1997) and they by grass height (Söderström et al., 2001). also differ in aggressiveness, with T. penzigi being the This study did not reveal any significant effect on the least aggressive and C. mimosae being the most community structure due to vertebrate grazing / browsing aggressive (Palmer et al., 2000; Martins, 2010). The patterns. The likely explanation could be that, since these canopy insect communities were therefore expected to insect communities were collected from the canopies, the be ant-specific, and the pattern to reflect the effect of the effect of either grazing or browsing was minimal. The four ant species. The study revealed that the ant species livestock mainly feeds on grass and herbaceous in fact played a key role in determining the structure of vegetation but not on A. drepanolobium on this eco- the canopy insect community. However, the nMDS system, therefore, the presence of livestock might have ordination did not reveal a specific pattern at either the little effect on the insect community. However, some of order or the family levels since the convex hulls the largest herbivores in this ecosystem, mainly the overlapped, but there was a tendency of insect samples elephants and giraffes, browse on A. drepanolobium , but collected from canopies of trees colonized by C. their effect were minimal in this ecosystem since the mimosae and C. nigriceps aggregating distinctly from symbiotic ants deter their feeding (Madden and Young, those collected from trees inhabited by T. penzigi and C. 1992). sjostedt i. Moreover, at the morphospecies level, the nMDS ordination revealed two separate communities, one consisting of insect samples collected from trees Conclusion occupied by C. mimosae and C. nigriceps and the other one from trees colonized by T. penzigi and C. sjostedti . The study has shown that ant species defending A. This shows the importance of analysing data at different drepanolobium against herbivory still allow a large taxonomic units because clear separation of the number of insect species to inhabit the canopy. This communities became apparent only at the morpho- study recorded more than 100 insect species that coexist species level. with the four acacia-ants on canopies of A. This finding is unique in the sense that these two drepanolobium at Mpala Research Centre and these communities had one dominant ant species and one included herbivores, omnivores and predators. From the subordinate ant species. The likely explanation could be foregoing discussion, it is clear that the ant species play a that different ant species behave differently to other key role in the structure and composition of the insect insects relative to the way they respond to other ant community on A. drepanolobium canopies. However, it species. These findings show that ant species play a role was not clear how the ant species influence the in structuring the insect community in this ecosystem. communities apart from modifying the canopies Kuria and Villet 5327

differently and exhibiting different aggressive behaviours. intensity of ant defence among three species of Macaranga For instance, were the predators (e.g. praying mantises) myrmecophytes in a Southeast Asian dipterocarp forest. Biotropica 32(2):318-326. occupying A. drepanolobium canopies attracted by the Izzo TJ, Vasconcelos HL (2005). Ants and plants size shape the presence of prey on these canopies, or by the ant structure of the arthropod community of Hirtella myrmecophila , an species, or by the tree itself? If by A. drepanolobium or its Amazonian ant-plant. Ecol. Entomol. 30:650-656. ants, are they therefore acting as a defensive Jolivet P (1991). Ants, plants, and beetles: A triangular relationship. In: Huxley, CR, Cutler, DF (eds) Ant-plant-interactions. Oxford University mechanism? Press, New York. Pp. 390-396. This study has clearly demonstrated that is Koptur S (1984). Experimental evidence for defense of Inga not a straight case of two or a few species benefiting from (Mimosoideae) saplings by ants. Ecology 65(6):1787-1793. each other, but a complex system involving many Lawton JH, Bignell DE, Bolton B, Bloemers GF, Eggleton P, Hammond PM, Hodda M, Holt RD, Larsen TB, Mawdsley NA, Stork NE, organisms interacting at various levels and intensities. Srivastava DS, Watt AD (1998). Biodiversity inventories, indicator Insect herbivores in this case may affect the performance taxa and effects of habitat modification in tropical forest. Nature of A. drepanolobium and therefore indirectly reduce the 391:72-76. habitat for ants. Maclean JE, Goheen JR, Doak DF, Palmer TM, Young TP (2011). Cryptic herbivores mediate the strength and form of ungulate impacts on a long-lived savanna tree. Ecology 92:1626–1636. Madden D, Young TP (1992). Ants as alternative defences in ACKNOWLEDGEMENTS spinescent Acacia drepanolobium . Oecologia 91:235-238. Martins DJ (2010). Not all ants are equal: Obligate Acacia ants provide different levels of protection against mega-herbivores. Afr. J. Ecol. We are grateful to N. Georgiadis and the entire Mpala 48:1115–1122. Research Centre community for their support during the McCauley DJ, Keesing F, Young TP, Allan BF, Pringle RM (2006). fieldwork; to J. Lemboi and S. Ekwam for their dedication Indirect effects of large herbivores on snakes in an African savanna. during sample collection, sorting and identification and to Ecology 87:2657–2663 McKey D (1974). Ant-plants: Selective eating of an unoccupied Barteria C.D. McQuaid for help with the PRIMER analyses. The by a colobus monkey. Biotropica 6(4):269-270. project was financed by National Science Foundation Milewski AV, Young TP, Madden D (1991). Thorns as induced (NSF DEB-0444741) through a grant to T. M. Palmer, M. defences: experimental evidence. Oecologia 86:70-75. L. Stanton and T. P. Young. Odadi WO, Young TP, Okeyo-Owuor JB (2007). 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vol. 42 (Ed. By P Jolivet, E Petitpierre, TH Hsiao), pp. 463-473. Warwick RM (1988). The level of taxonomic discrimination required to Kluwer Academic, Dordrecht. detect pollution effects on marine benthic communities. Mar. Pollut. Söderström B, Svensson B, Vessby K, Glimskar A (2001). Plants, Bull. 19:259-268. insects and birds in a semi-natural pasture in relation to local habitat Young TP (1987). Increased thorn length in Acacia drepanolobium an landscape factors. Biodiversity Conserv . 10:1839-1863. induced response to browsing. Oecologia 71:436-438. Stanton ML, Palmer TM, Young TP, Evans A, Turner ML (1999). Young TP, Patridge N, Macrae A (1995). Long-term glades in acacia Sterilization and canopy modification of a swollen thorn acacia tree bushland and their edge effects in Laikipia, Kenya. Ecol. Appl. 5:97– by a plant-ant. Nature 401:578-581. 108. Stanton ML, Palmer TM, Young TP (2002). Competition-colonization Young TP, Stubblefield CH, Isbell LA (1997). Ants on swollen-thorn trade-offs in a guild of African acacia-ants. Ecol. Monogr . 72(3):347- acacias: species coexistence in a simple system. Oecologia. 109:98- 363. 107. Skinner GJ, Whittaker JB (1981). An experimental investigation of inter- Young TP, Okello BN (1998). Relaxation of an induced defense after relationships between the wood-ant ( Formica rufa ) and some tree- exclusion of herbivores: Spine length in Acacia drepanolobium. canopy herbivores. J. Anim. Ecol . 50:313-326. Oecologia 115:508-513. Vasconcelos HL, Casimiro AB (1997). Influence of Azteca alfari ants on Young, TP, Okello, BD, Kinyua, D, Palmer, TM (1998). KLEE: the the exploitation of Cecropia trees by leaf-cutting ant. Biotropica Kenya long-term exclosure experiment. Afr. J. Range Forage Sci. 29(1):84-92. 14:94-102. Ward D, Young TP (2002). Effects of large mammalian herbivores and Young TP, Palmer TM, Gadd ME (2005). Competition and ant symbionts on condensed tannins of Acacia drepanolobium in compensation among cattle, zebras, and elephants in a semi-arid Kenya. J. Chem. Ecol. 28(50):921-937. savanna in Laikipia, Kenya. Biol. Conserv. 122:351-359. Warui CM, Villet MH, Young TP, Jocqué R (2005). Influence of grazing Zar JH (1974). Biostatistical Analysis . Prentice-Hall, New Jersey. Pp. by large mammals on the spider community of a Kenyan savanna 662. biome. J. Arachnol. 33: 269-279.

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Appendix 1. The checklist of the various morphospecies found associating with the four acacia-ants on canopies of A. drepanolobium . Showing orders, families and accumulative number of individuals collected for each insect species. They were collected using beating and fogging.

Species Order Family C. sjostedti C. mimosae C. nigriceps T. penzigi Acrididae sp. 1 Orthoptera Acrididae 53 6 5 37 Acrididae sp. 2 Orthoptera Acrididae 135 23 16 118 Acrididae sp. 3 Orthoptera Acrididae 27 6 2 28 Acrididae sp. 4 Orthoptera Acrididae 14 31 17 17 Acrididae sp. 5 Orthoptera Acrididae 42 5 2 47 Acrididae sp. 8 Orthoptera Acrididae 0 1 0 0 Acrididae sp. 9 Orthoptera Acrididae 1 0 0 0 Acrididae sp. 10 Orthoptera Acrididae 1 0 0 0 Acrididae sp. 11 Orthoptera Acrididae 5 1 0 11 Acrididae sp. 12 Orthoptera Acrididae 5 0 0 8 Acrididae sp. 14 Orthoptera Acrididae 11 1 1 22 Acrididae sp. 15 Orthoptera Acrididae 2 2 1 1 Acrididae sp. 16 Orthoptera Acrididae 12 0 0 11 Acrididae sp. 17 Orthoptera Acrididae 1 0 0 0 Acrididae sp. 19 Orthoptera Acrididae 0 2 1 0 Acrididae sp. 20 Orthoptera Acrididae 1 1 0 0 Acrididae sp. 21 Orthoptera Acrididae 0 0 0 1 Acrididae sp. 23 Orthoptera Acrididae 0 0 1 0 Acrididae sp. 24 Orthoptera Acrididae 1 0 0 0 Acrididae sp. 26 Orthoptera Acrididae 0 0 1 0 Acrididae sp. 27 Orthoptera Acrididae 1 0 0 0 Acrididae sp. 28 Orthoptera Acrididae 2 0 0 2 Acrididae sp. 30 Orthoptera Acrididae 0 1 0 0 Acrididae sp. 31 Orthoptera Acrididae 0 1 0 0 Ectatoderus sp. A Orthoptera Gryllidae 309 239 286 303 Gryllodes sp. A Orthoptera Gryllidae 4 2 2 0 Gryllacris sp. A Orthoptera 1 1 1 4 Pamphagidae sp. 1 Orthoptera Pamphagidae 0 0 5 1 Pamphagidae sp. 2 Orthoptera Pamphagidae 0 0 1 0 Pamphagidae sp. 3 Orthoptera Pamphagidae 3 2 2 2 Pamphagidae sp. 4 Orthoptera Pamphagidae 1 0 0 2 Agrilus sp. A Coleoptera Buprestidae 3 3 0 0 Agrilus sp. B Coleoptera Buprestidae 0 6 3 1 Agrilus sp. D Coleoptera Buprestidae 1 0 0 0 Agrilus sp. G Coleoptera Buprestidae 1 0 0 0 Buprestid sp. 1 Coleoptera Buprestidae 18 6 3 16 Buprestid sp. 2 Coleoptera Buprestidae 1 5 0 0 Hoplistura sp. A Coleoptera Buprestidae 21 45 18 34 Chrysobothris sp. A Coleoptera Buprestidae 1 2 1 4 Sjoestedtius sp. A Coleoptera Buprestidae 2 0 0 0 Sjoestedtius sp. B Coleoptera Buprestidae 0 1 0 0 Sjoestedtius sp. C Coleoptera Buprestidae 3 20 3 0 Chrysomelidae sp. 1 Coleoptera Chrysomelidae 0 0 0 1 Chrysomelidae sp. 3 Coleoptera Chrysomelidae 0 0 1 1 Chrysomelidae sp. 4 Coleoptera Chrysomelidae 1 13 19 2 Chrysomelidae sp. 5 Coleoptera Chrysomelidae 1 0 1 0 Chrysomelidae sp. 6 Coleoptera Chrysomelidae 1 2 1 1 Cryptocephalus sp. A Coleoptera Chrysomelidae 1 0 0 0 Cryptocephalus sp. B Coleoptera Chrysomelidae 1 0 0 0 5330 Afr. J. Agric. Res.

Appendix 1. Cont’d.

Dorcathispa sp. A Coleoptera Chrysomelidae 0 0 0 1 Hispa sp. A Coleoptera Chrysomelidae 2 0 1 5 Lema sp. A Coleoptera Chrysomelidae 2 0 2 1 Megalognatha sp. A Coleoptera Chrysomelidae 0 1 1 2 Monolepta sp. A Coleoptera Chrysomelidae 5 11 41 8 Monolepta sp. B Coleoptera Chrysomelidae 18 0 3 13 Monolepta sp. C Coleoptera Chrysomelidae 0 0 1 2 Monolepta sp. D Coleoptera Chrysomelidae 2 0 0 0 Systates sp. A Coleoptera Curculionidae 11 13 1 3 Curculionidae sp. 1 Coleoptera Curculionidae 2 1 7 8 Curculionidae sp. 2 Coleoptera Curculionidae 3 1 2 0 Curculionidae sp. 4 Coleoptera Curculionidae 2 1 1 0 Curculionidae sp. 5 Coleoptera Curculionidae 1 3 0 0 Curculionidae sp. 6 Coleoptera Curculionidae 0 0 0 2 Curculionidae sp. 7 Coleoptera Curculionidae 1 0 0 0 Curculionidae sp. 8 Coleoptera Curculionidae 0 0 0 1 Curculionidae sp. 9 Coleoptera Curculionidae 0 0 1 0 Myllocerus sp. A Coleoptera Curculionidae 612 457 321 851 Neosphrigodes sp. A Coleoptera Curculionidae 7 0 0 12 Philonthus sp. A Coleoptera Staphylinidae 0 0 1 0 Bruchid sp. 1 Coleoptera Bruchidae 0 5 9 0 Bruchid sp. 2 Coleoptera Bruchidae 1 0 0 0 Bruchid sp. 3 Coleoptera Bruchidae 0 0 0 2 Bruchid sp. 4 Coleoptera Bruchidae 0 0 0 2 Anthicidae sp. A Coleoptera Anthicidae 410 114 266 69 Anthicidae sp. D Coleoptera Anthicidae 1 0 0 0 Aphodius sp. A Coleoptera Scarabaeidae 0 0 1 0 Arsinoe sp. A Coleoptera Carabidae 0 0 1 0 Carabidae sp. 1 Coleoptera Carabidae 43 20 9 9 Carabidae sp. 2 Coleoptera Carabidae 0 0 5 0 Carabidae sp. 3 Coleoptera Carabidae 2 0 0 1 Scymnus sp. A Coleoptera Coccinellidae 2 1 0 0 Micraspis sp. A Coleoptera Coccinellidae 1 2 0 1 Bostrichidae sp. 1 Coleoptera Bostrichidae 3 10 1 0 Cleridae sp. 1 Coleoptera Cleridae 98 97 120 199 Enaretta sp. A Coleoptera Cerambycidae 0 17 8 0 Lagria sp. A Coleoptera Tenebrionidae 26 11 16 20 Cilnia sp. A Mantodea Mantidae 14 0 3 9 Galepsus sp. A Mantodea Mantidae 1 8 7 5 Miomantis sp. A Mantodea Mantidae 9 5 1 15 Parasphendale sp. A Mantodea Mantidae 31 13 13 45 Popa sp. A Mantodea Mantidae 5 4 2 1 Mantidae sp. F Mantodea Mantidae 1 0 1 0 Mantidae sp. G Mantodea Mantidae 1 0 0 0 Mantidae sp. H Mantodea Mantidae 1 0 0 0 Mantidae sp. J Mantodea Mantidae 2 2 0 2 Mantidae sp. K Mantodea Mantidae 1 0 0 0 Mantidae sp. L Mantodea Mantidae 0 0 0 1 Mantidae sp. P Mantodea Mantidae 0 1 0 0 Miridae sp. 1 Hemiptera Miridae 17 119 351 107 Miridae sp. 2 Hemiptera Miridae 56 15 3 73 Miridae sp. 3 Hemiptera Miridae 2 5 13 25 Kuria and Villet 5331

Appendix 1. Cont’d.

Hemiptera sp. 5 Hemiptera 4 0 0 2 Hemiptera sp. 11 Hemiptera 3 0 0 3 Pentatomidae sp. 1 Hemiptera Pentatomidae 1 7 8 0 Aeliomorpha ? simulans Hemiptera Pentatomidae 1 1 2 1 Aeliomorpha senegalensis Hemiptera Pentatomidae 1 1 1 2 Anygrus ochreatus Hemiptera Meenoplidae 1 0 0 0 Clonaria sp. Phasmatodea Diapheromeridae 17 50 97 66 Derocalymma sp. A Blattodea Polyphagidae 7 0 0 3 Cyrtotria sp. A Blattodea Blattidae 32 0 3 2 Periplaneta sp. 1 Blattodea Blattidae 366 211 704 91 Camponotus sp. Hymenoptera Formicidae 713 257 72 164 Pheidole crassinoda Hymenoptera Formicidae 0 0 2 0 Polyrhachis viscosa Hymenoptera Formicidae 4 6 2 0 Technomyrmex sp. A Hymenoptera Formicidae 0 0 0 9