JOINT BROOD GUARDING IN PARENT BUGS - AN EXPERIMENT ON DEFENSE AGAINST PREDATION

Females of Elasmucha grisea defend their eggs and small nymphs against invertebrate predators. Females sometimes guard their clutches side b y side on the same birch leaf. We studied benefits of this joint guard ing both in the field and in the laboratory. We found that adjacent fe males had significantly larger clutches than solitary females. In the laboratory, we studied the effectiveness of joint versus single defenc e against ant (Formica uralensis) predators. We established female pai rs from initially singly guarding females by cutting off pieces of lea ves with egg clutches and pasting them beside another female guarding her clutch. In the control group the females with their clutches were similarly cut off but these clutches were placed on another leaf witho ut any female. The birch twigs where females guarded their clutches we re placed in cages in close proximity to laboratory ant nests. In the experimental treatment, two females guarded their clutches together an d at the same nest there was another birch twig without a female. In t he control treatment two twigs with one female on each were placed clo se to another ant nest. Two females defended their clutches significan tly more successfully, losing fewer eggs than did the single females. This primitive form of female sociality in parent bugs resembles colon ial nesting in birds, where communal defence is also important. Howeve r, to our knowledge this is the first experiment where the benefit of joint guarding has been tested directly by manipulating the size of th e breeding group rather than by measuring the risk of predation in gro ups of different size.