Unequal maternal investment in offspring quality in relation to predation risk

Equal investment within broods does not always maximize parental reproductive value if the reproductive value of some of the young is low. We examined maternal investment in terms of offspring size in relation to the prospects of survival from predation within broods of the shield bug Elasmucha ferrugata Fabr. (Heteroptera; Acanthosomatidae). Shield bug females guard eggs and first instar nymphs against invertebrate predators by covering the clutch with their body and by behaving aggressively towards their enemies. Survival of eggs was not possible without maternal care. When females were allowed to guard their brood, eggs at the periphery were more vulnerable to predators than eggs at the centre. We found that females laid significantly larger eggs in the safest, central part of the clutch. There seems to be an advantage of large nymph size, since when nymphs were reared separately with low food resources, the larger ones were more likely to survive. Larger nymphs were also more likely to push themselves to the safest, central part of the clutch. Females seem to allocate their resources more to the offspring with the highest probability of avoiding predation. Thus our study supports unequal maternal investment within broods of E.ferrugata.