To survive the attacks from their many natural enemies, plants have evolved some complex defense strategies, including the attention of ants to help combat potential herbivores. Extrafloral nectaries promote nectar secretion that attract ants that locate and capture intruders, hence reducing the scale of damage on the plant. In return, ants get a nutrient-rich meal produced by the extrafloral nectaries. We report on a previously undetected phenomenon in which the large leaf galls induced by an Apion weevil are patrolled by dozens of ants on the herbaceous plant Croton antisyphiliticus (Euphorbiaceae). Unlike any other galling insect, the weevil caused the host plant to produce hundreds of extrafloral nectaries on the gall walls. We report on the interactions of the weevil with the host plant and the network of interactions of the galls, ants and gall natural enemies.
Results/Conclusions
Extrafloral nectaries on leaves and galls have similar morphology but differed in height (281.14µm leaves; 446.13µm galls), and probably in functionality because the diameter of secretory head was larger in extrafloral nectaries on galls (1080.34µm) than on leaves (505.32µm). In average gall area was 228.2 mm2 (N=56 galls). Each gall had on average 59.96 extrafloral nectaries (N=30). In contrast, we found an average of 11.34 (N=35 leaves) extrafloral nectaries on the edge of a leaf. One Croton antisyphiliticus plant supports up to 4 galls and has an average of 5.55 leaves per plant (N=90 plants). Therefore, on a single plant, the expected number extrafloral on the leaves is 11.34 X 5.55 = 63, while the number of nectaries associated with galls can be as high as 59.96 X 4 = 240. Therefore, gall nectaries offer a significantly higher quantity of resources for ants compared to ungalled leaves.
More than 25 species of ants have been recorded on this plant, avidly patrolling the plant day and night and many of which exhibit strong defense behavior against insects that otherwise would drink from the extrafloral nectaries or feed on the gall. Distinct ant communities defend the gall extra floral nectaries. Curiously, ants simply ignore adults on the gall inducing Apion weevils. The quantity of the Apion individuals per gall varied between 17 to 65 (N=30), and each gall had on average 19.28 adult weevils (N=30). We present the diurnal and nocturnal ant interaction networks and reported on the behavior of the entire insect community centered on the galls.