On Intraspecific and Interspecific Competition in Larvae of Graminivorous Insects

Abstract

The females of both Rhizopertha and Sitotroga oviposit in environments containing places suitable for larval development, but the larvae themselves, usually during a period of migration in the first instar, choose the actual developmental site. The rate of oviposition of neither species of female bears much relation to the amount of food present for the larvae, and the latter do not refrain from multiple or superinfestation of wheat grains. The competition which ensues is apparently wholly a struggle for space, the limitation of food or oxygen, and the ‘conditioning’ of the medium, being unimportant. Larvae (of any instar) will attack each other directly after encounters at random within wheat grains, and the supernumerary individuals are either killed or forced to migrate. The probability that any particular larva will survive is thus inversely proportional to the initial number present (equation (1)). Except that it tended to favour the survival of Sitotroga with atypical rates of development, overcrowding had no other effect upon the larvae of either of these species. In an unlimited or unsaturated environment migration may lead to survival; but when the environment is limited in capacity a point will be reached when migration from one grain to another merely leads to death in another place. Then the only effect of overcrowding will be to increase mortality. Migration from the grains tends to decrease with later instars. Because of competition for space the number of larvae of the same age which survive in one grain is less than that which the food present in the latter could support. This number (approx. 1.2 per grain) does not vary with density, so that the relationship between survival and larval density is given by equation (2). When the two species are competing the average ratio of the survivors is Rhizopertha to Sitotroga as 1.3r : s, where r and s, respectively, are the initial numbers of larvae of each species. These represent the proportions of the 1.2 larvae surviving per grain which belong to each species. This ratio remains constant at all densities when the larvae enter the grains at the same time in the same instar. Each species thus decreases the probability of survival of the other in direct proportion to its own numbers. When first instar larvae of the two species enter the grains at different times the above relationship changes in favour of the first comer. The most unfavourable time for the second species to enter the grain is apparently when the first is in the second or early third instar. With greater differences between times of entry (i.e. of age) the severity of competition for space decreases, so that more larvae are able to survive and take advantage of the food reserves of the grain. (The survival of the larvae of the second species is apparently then reduced to some extent by the accumulation of the excretory products of the first.) Sitotroga, but not Rhizopertha, was able to take advantage of this decreased competition because of the occurrence of larvae with atypical rates of development in this species. The latter were able to survive the competition of normal larvae of either species where other normal larvae would have succumbed. Crowding to a certain degree tended to increase the proportion of atypical larvae among the survivors of this species. Overcrowding in the immature stages had no effect upon the average develomental period of the larvae, or upon the sex-ratio, weight or fecundity of the adults or either species.