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A Study of the Group-Feeding Behaviour of Larvae of the Jack Pine Sawfly, Neodiprion Pratti Banksianae Roh 1)

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(i) This study was undertaken to determine the causal factors and selective mechanisms responsible for the development of the group-feeding behaviour displayed by Neodiprion pratti banksianae Roh. when feeding on the needles of its host, Pinus banksiana Lamb. (ii) Preliminary studies revealed that hatching orientation is constant, all larvae being directed towards the needle tip. Newly-hatched larvae have creamy white head capsules, and are unable to orient to light until an hour after hatching, by which time head capsule pigmentation is well advanced. Egg-bearing needles receive no preferential selection in the establishment of the first feeding sites. (iii) Larval aggregations were found to be consistently too large and too frequent to be accountable by chance, and were seen to be the product of a strong aggregative stimulus. Aggregations formed at wide extremes of light, temperature, and humidity conditions, and were evidently independent of these physical factors. The stimulus to aggregate appeared to remain equally strong throughout larval life, for although the size of the feeding groups diminished, this appeared to be entirely attributable to increasing head capsule size. Larvae displayed no interest in each other when removed from their pine foliage, dispersing in a direct manner on the surface of a grid until their distribution satisfied the Poisson calculations for a randomly dispersed population. Larvae were found to be markedly responsive to the odour of pine foliage, and to the odour of the saliva of feeding insects, and it was postulated that established feeding sites attract additional larvae through an olfactory response. (iv) A small experiment failed to reveal a difference between the rate of development of larvae reared in groups and the rate of development of those larvae that managed to survive in solitary rearings. This same experiment suggested that, although the first instar mortality rate was high in both group and solitary rearings, it might be substantially higher in solitary rearings. Additional studies showed the strong statistical significance of this difference. Mortality was negligible after the moult from first to second instar in both group and solitary rearings. (v) Feeding aggregations were found invariably to form around a feeding site established by an individual larva. So long as this first larva remains alone, the feeding site is restricted to a small portion of the needle periphery along the needle edge. As additional larvae join the site they are at first in close contact with each other, and it can be seen that they are taking advantage of the existing incision to gain purchase in biting into the remaining uncut needle periphery. Once the feeding site is extended beyond the minimum limits imposed by the head capsules the larvae become freely distributed at the feeding site, and no longer maintain close contact. The provision of artificially cut feeding sites significantly increases the establishment of solitary larvae. (vi) It is postulated that the survival advantage gained in the first instar by exploitation of the feeding incisions made by the few larvae that manage to cut into the tough needle cuticle and epidermis is the selective mechanism that has brought about the evolution of group-feeding. (vii) The concept of division of labour is analysed. By "division of labour" we mean, generally, a specialization in various components of a task. Below this is a level where labour is divided as it is within a caste in a complex insect society: by the additive effect of equipotential contributions. Neodiprion larvae are at the threshold of this lower level of division of labour. Their most difficult task in first instar is to establish openings in the pine needles where they may feed, and each larva attempts to do this alone. As soon as individual larvae succeed in establishing feeding sites, individual ef fort by unsuccessful larvae is abandoned in favour of group exploitation. This primitive level of integration foreshadows the continuum of sociality leading to those levels where we may speak of the "survival of the most cooperative".

Affiliations: 1: Department of Zoology, University of Toronto, and the Forest Insect Laboratory, Sault Ste. Marie, Canada

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