During the entire study, no Oecophylla colony was found that did not attend homopterans, and all major trails of this ant in the canopy lead to aggregation sites with homopterans (AS).
A total of 490 AS was recorded on the 30 trees studied (including repeated censuses on six trees). Most AS involved membracids or coccoids, while associations with aphids, cicadellids, and lycaenids were uncommon (Table 1). The median number of individuals per AS was highest for coccoids (22) and aphids (11), followed by membracids (6), cicadellids (1·5) and lycaenids (1). Mixed aggregations of coccoids and membracids (4 AS), or coccoids and lycaenids (1 AS), were rare. Homopteran species are listed in Table 2 (note that all membracids, aphids and cicadellids represented a single species each).
Table 1. Distribution of aggregation sites (AS) of Oecophylla smaragdina tending different homopterans and lycaenids on trees and climbing plants. Data are considered for a total of 30 trees (including eight trees without lianas). Numbers following the tree species correspond to tree labels of the crane site. Cell entries are numbers of AS with the number of investigated tree canopies in brackets. Total numbers of AS and trees are smaller than pooled numbers where different homopterans or food plants co-occurred on the same AS vs. the same tree, respectively. Species of homopterans and lycaenids are listed in Table 2
|Homopteran host plant species1||Coccoidea2||Membracidae||Aphidae||Cicadellidae||Lycaenidae3||Total|
|Trees||143 (10)|| 3 (2)||6 (4)||2 (2)||3 (3)||156 (15)|
| Acmena graveolens L.S. Smith (MYRT) #3032|| 4 (1) M|| –||–||–||–|| 4 (1)|
| Cardwellia sublimis F. Muell. (PROT) #3028 #4025 #5027|| 11 (3) C|| –||–||2 (2)||–|| 13 (3)|
| Cryptocarya hypospodia F. Muell. (LAUR) #7066|| 13 (1) Co|| –||2 (1)||–||–|| 15 (1)|
| Endiandra microneura C.T. White (LAUR) # 2002|| 68 (1) C M E|| –||1 (1)||–||1 (1) Ar|| 69 (1)|
| Endiandra cf. monothyra B.P.M. Hyland (LAUR) #1004|| 13 (1) C M|| 2 (1)||–||–||1 (1)|| 16 (1)|
| Myristica insipida R.Br. (MYRI) #1059|| 7 (1) M|| 1 (1)||–||–||–|| 8 (1)|
| Synima cordierii Radlk. (SAPI) #7094|| –|| –||–||–||1 (1) An|| 1 (1)|
| Syzygium cormiflorum B.P.M. Hyland (MYRT) #5034|| 19 (1) C M|| –||–||–||–|| 19 (1)|
| Syzygium sayeri B.P.M. Hyland (MYRT) #3023|| 5 (1)|| –||–||–||–|| 5 (1)|
| Unident. #1023|| –|| –||1 (1)||–||–|| 1 (1)|
| Unident. #3034|| –|| – ||2 (1)||–||–|| 2 (1)|
| Unident. #7072|| 2 (1)|| –||–||–||–|| 2 (1)|
| Unident. #7093|| 1 (1)|| –||–||–||–|| 1 (1)|
|Climbing plants|| 49 (15)||289 (17)||–||–||–||334 (22)|
| Caesalpinia traceyi L. Pedley (CAES)|| 3 (3) C||196 (4)||–||–||–||199 (4)|
| Entada phaseoloides Merrill (MIMO)|| 28 (9) C P|| 90 (13)||–||–||–||114 (14)|
| Merremia peltata Merrill (CONV)|| 8 (4) M|| 1 (1)||–||–||–|| 9 (4)|
| Melodinus australis Pierre (APOC)|| 5 (1) M||–||–||–||–|| 5 (1)|
| Stephania japonica Miers (MENI)|| 1 (1)||–||–||–||–|| 1 (1)|
| Ficus pantoniana King (MORA)|| 1 (1) I||–||–||–||–|| 1 (1)|
| unident.|| 2 (1)|| 2 (2)||–||–||–|| 4 (3)|
| Flagellaria indica Linn. (FLAG)|| 1 (1)||–||–||–||–|| 1 (1)|
|Total plants||188 (19)||288 (17)||6 (4)||2 (2)||3 (3)||490 (30)|
Table 2. Trophobiotic partners of Oecophylla smaragdina in this study
|Coccidae||Coccus sp. Milviscutulus sp. |
|Pseudococcidae||Planococcus citri Risso |
|Margarodidae||Icerya sp. |
|Aphidae||Toxoptera aurantii (Boyer de Fonscolombe) |
|Membracidae||Sextius cf. ‘ kurandae ’ 1|
|Cicadellidae||Austrotartessus sp. |
|Lycaenidae||Anthene seltuttus (Röber) |
| ||Arhopala centaurus group |
Homopteran taxa were very unequally distributed among host plant species and life forms (Table 1). Nearly all AS with membracids (99%) were found on lianas, especially Entada phaseoloides and Caesalpinia traceyi. In contrast, most coccoids (74% of AS) were consuming sap from trees. The few AS with aphids, cicadellids and lycaenids were found exclusively on trees. The unequal distribution of AS with coccoids vs. membracids on trees vs. climbing plants was highly significant (χ2 = 296·1; d.f. = 1; P < 0·0001). Many coccoid AS (82 of 188 AS) occurred on trees without suitable lianas in the crown area. Considering only those trees that carried lianas, distribution of coccoid AS across plant growth forms was much more even (61 vs. 45 AS, respectively) and did not significantly deviate from an equal distribution (χ2 = 1·4; d.f. = 1; P= 0·24).
Between 10 and 2115 homopterans (median: 455) were counted on each of 11 tree crowns where we were able to examine most parts of the crown (n = 26 surveys). These homopterans were tended by a similar number of Oecophylla workers (median: 449, range: 20–1218). Numbers of homopterans and attended ants per tree were significantly correlated (Pearson's r = 0·79; P < 0·001). Additional coccoids were usually tended inside the nests, but were not counted here as this would have required nest destruction. A closer inspection of a single nest that had been previously abandoned by Oecophylla revealed 895 scale insects. Numbers of homopterans and ants were highly variable between trees and different surveys (months), although no clear seasonal pattern could be found and ranking of trees was not stable (Fig. 1a). It is unclear how much these results were affected by post-cyclone succession. Ants attended homopterans more continuously on lianas throughout the year, while homopteran attendance occasionally dropped to zero on trees (Fig. 1b,c).
Figure 1. (a–c). Total number of Oecophylla smaragdina workers attending homopterans outside their nests during five surveys between October 1999 and January 2001: (a) on six trees incl. lianas (2–5 surveys per tree). Tree numbers (see Table 1) are: #1004 (open squares), #2002 (filled squares), #3023 (open circles), #5027 (open triangles), #5034 (filled circles) and #7066 (filled triangles); (b) on liana vs. tree as homopteran host plant on #1004 and (c) on tree #5027. Smooth curves between the data are plotted for the purpose of readability.
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Homopterans varied strongly in aggregation size. There was a significantly higher number of coccoids (mean ± SEM: 59·1 ± 9·2; n = 182) than membracids per AS (10·5 ± 0·9; n = 286) (Mann–Whitney U = 11007, P < 0·0001).
Ant visitation rate (VR) was negatively correlated with the number of homopterans (H) present at an AS. On a log-log plot (Fig. 2), the relationship was linear. The negative regression was significant for both AS with coccoids (Hc) (log VR = −0·22 log Hc + 1·4; r2 = 0·39; n = 174; P < 0·0001) and membracids (Hm) (log VR = −0·25 log Hm + 1·5; r2 = 0·23; n = 277; P < 0·0001). There was no significant difference between the regression slopes for these two homopteran taxa (t = 0·84; d.f. = 447; P = 0·40). These findings lead us to incorporate H as covariate in the following analysis of the effect of plant life form on VR, with data from coccoids and membracids combined.
Figure 2. Relationship between ant visitation rate ( VR ) and total number of homopterans per aggregation site, plotted on log-log scale. Coccoids are represented by open circles, membracids by closed squares.
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Ant recruitment to trophobionts was significantly affected by host plant life form. Considering only trees that carried lianas, VR was 64% higher to homopterans that were feeding sap from lianas compared to trees (Fig. 3). This effect was highly significant (ancova: F1;367 = 10·6, SSeffect = 0·97, SSerror = 0·05; P = 0·0016), using H as covariate. The effect on VR was slightly weaker, but still highly significant when only coccoids were considered (52% higher on lianas), when all trees were included irrespective of liana presence (45% higher on lianas), or when the covariate was not included.
Figure 3. Visitation rate ( VR = number of Oecophylla smaragdina workers per individual homopteran), compared between lianas vs. trees (where lianas available) as homopteran hosts (mean ± SEM). Mean values vary significantly between plant life-forms ( ancova , see text).
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Mean VR (± SEM) to coccoids on lianas was 1·3 (± 0·2). There was no difference between the two legume species (E. phaseoloides and C. traceyi) and non-legume lianas (ancova: F1;35 = 0·33, SSeffect = 0·01, SSerror = 0·03; P = 0·57). On two coccoid host trees without lianas (which were excluded from the preceding analysis), VR was higher on one tree (Endiandra microneura; mean VR ± SEM = 2·0 ± 0·3; n= 68) and lower on the other (Cryptocarya hypospodia; 0·6 ± 0·1; n = 13). Among trees with lianas, mean VR at coccoids was 1·2 (± 0·4; n = 19) on one Syzygium cormiflorum and lower on the remaining six trees (mean VR ranging between 0·5 and 0·8). Per capita ant recruitment to membracids varied significantly between the legumes E. phaseoloides (3·2 ± 0·4; n = 79) and C. traceyi (2·2 ± 0·1; n = 191) (ancova: F1;267 = 5·3, SSeffect = 39·6, SSerror = 7·4; P = 0·02), but was generally higher compared to coccoids.
Locations of Oecophylla–homopteran aggregations were highly dynamic. The mean (± SEM) establishment rate of new AS per tree was one AS per 6·8 (± 1·7) days vs. 2·6 (± 1·2) days (coccoids vs. membracids, respectively), while one AS per 15·9 (± 5·3) days vs. 3·6 (± 1·2) days was abandoned. Aggregations with coccoids were less dynamic than those with membracids. Mean longevity of AS with coccoids was 130 (± 14) days and only 54 (± 10) days with membracids. The difference in ‘mortality’ (M) of AS with coccids and membracids was significant (t = 2·6; d.f. = 18; P < 0·05; M log-square-root transformed).
Oecophylla constructed pavilions with leaves that were woven together with larval silk in a similar way as nests. Number of pavilions per tree varied (mean: 4·6; range 0–18; n = 26 surveys on 11 trees). Pavilions were usually less than 10 cm in diameter, and therefore considerably smaller than nests of mature colonies. Between one and 15 leaves (median: 3; counted for n = 59 AS) were incorporated in each pavilion. They were found on young foliage of trees and lianas where many homopterans were tended by ants, often several meters away from the nearest ant nest. Plants with large leaves or leaflets (> 5 × 5 cm) were used more frequently for pavilion construction ( E. phaseoloides : 34% of AS; other lianas: 33%, trees: 36%) than C. traceyi (15%), a vine with relatively small leaflets (< 2 × 3 cm). Forty-one per cent of AS with coccoids and 19% of AS with membracids were sheltered within pavilions. Both factors were interrelated, as most membracids occurred on C. traceyi (see Table 1 ). On one occasion some ant larvae were found inside the pavilion, which are necessary for the production of silk. Some pavilions were repaired by ants after damage during the census. Pavilions were also abandoned by ants during maturation of the plant tissue in the same way as nonsheltered AS, and mean (± SEM) longevity was 108 (± 15) days (longer than mean longevity of AS with membracids, similar to AS with coccoids).
Oecophylla nests were found on 39 trees from over 18 species and eight families of plants (six trees unidentified). Nests were woven from tree or liana leaves (tree leaves only: 24 trees, tree and liana leaves: 11 trees, liana leaves only: four trees). A mixture of tree and liana leaves was used either in separate nests or incorporated in the same nest. The most commonly used liana was Merremia peltata (Convolvulaceae). Most common host trees were Acmena graveolens and Syzygium sayeri (Myrtaceae) (each species represented by four trees inhabited by Oecophylla ), Endiandra microneura (Lauraceae), Cardwellia sublimis (Proteaceae), Argyrodendron peralatum (Sterculiaceae), and Myristica insipida (Myristicaceae) (three trees each). All these common hosts were among the 18 most common and largest trees in the crane plot (each with at least 10 trees of d.b.h. ≥ 10 cm). Sizes of leaves or leaflets utilized by Oecophylla were ‘normal’, ranging from c . 5 × 8 cm to 20 × 20 cm (upper end: M. peltata ). The mean height of the 39 Oecophylla host trees was 23·2 m (± 1·1 m SEM), and significantly higher than the mean for the remaining trees (15·2 ± 0·2 m; 667 trees of d.b.h. ≥ 10 cm; Mann–Whitney U = 3429, P < 0·0001). Many examples of common trees that were not recorded as hosts for Oecophylla nests were either bearing very large or tough foliage that was obviously unsuitable for nests, e.g. Alstonia scholaris R. Br. (Apocynaceae) and palms ( Normanbya normanbyi L.H. Bailey, Licuala ramsayi Domin), or they were relatively small understorey trees (height < 15 m), e.g. Cleistanthus myrianthus Kurz (Euphorbiaceae) and Antirhea tenuifolia B.D. Jacks (Rubiaceae).
Between one and five nests were found on each tree (median: 3), which were 10–50 cm (median: 30 cm, n= 34) in diameter. Nests were frequently abandoned by Oecophylla and replaced by new nests. New nests appeared at a rate of one nest per 56 (± 11) days (mean ± SEM), and nests were abandoned at a similar rate (52 ± 11 days). Mean nest longevity was 131 (± 21) days.