POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



351 



to subsequent larval dispersal. Crombie 

 worked with two species of granary insects: 

 the beetle Rhizopertha dominica and the 

 moth Sitotroga cerealella. The larvae of 

 these infest wheat grains. It is possible to 

 introduce a designated number of larvae 

 into the seeds and thus establish larval 

 densities of either species ranging from one 

 to eight larvae per grain. It is also possible 

 to introduce larvae of both species into the 

 same grain so that interspecies relations 

 can be studied. 



Crombie's findings can be summarized 

 briefly in this way. The adult females of 



"When two larvae in the first, second, third 

 or fourth instars were put together into a small 

 hole drilled in a wheat grain and watched 

 under a binocular microscope, they were often 

 seen to attack each other with their mandibles, 

 and eventually either one or both left the hole. 

 When a larva entered such a hole it always 

 went to the bottom and turned round so as to 

 face outwards. Other larvae trying to enter the 

 hole were fiercely attacked. Sometimes such 

 combats resulted in the body wall of one of 

 the antagonists becoming punctured and its 

 bleeding to death. In their tunnels in wheat 

 grains larvae of all instars were always found 

 curled up with the head facing towards the 

 way they had entered. Furthermore, in all 



Table 24. Effect of Density on the Reduction in Numbers of Rhizopertha First Instar Larvae 

 Competing for the Same Grain of Wheat (Crombie, 1944) 



both Rhizopertha and Sitotroga oviposit in 

 wheat grains that contain niches suitable 

 for the development of the larvae. These 

 larvae, however, usually move during their 

 first instar and thus "choose" the exact 

 spot in which to develop. It is the cause 

 of this change of grain and its relation to 

 crowding that we wish to examine. Crom- 

 bie concludes that the larvae disperse not 

 because of limitations of food, oxygen sup- 

 ply, or conditioning within the seed, but 

 rather as the direct result of competition for 

 space leading to fighting. "Larvae will at- 

 tack each other directly after encounters at 

 random within wheat grains, and the super- 

 numerary individuals are either killed or 

 forced to migrate." Thus the probability 

 that a certain larva will live is in inverse 

 proportion to the initial number of larvae 

 in the seed. Crombie's observations about 

 what happens when Rhizopertha larvae 

 meet are informative since this is the specif- 

 ic coaction responsible for the observed 

 density efiFect. 



grains dissected during the experiments to be 

 described, whenever two larvae were found 

 in the same tunnel at least one of them was 

 always dead. It thus seems probable that when- 

 ever two larvae meet within a grain they will 

 attack each other, wdth the result that either or 

 both wall migrate or be killed" (p. 138). 



An actual illustration of the relation of 

 crowding within grains to survival and 

 movement in Rhizopertha appears in Table 

 24. 



This table clearly shows how population 

 pressure, as measured in terms of increased 

 larval death and dispersion, is accentuated 

 by larval crowding. Columns 6 and 7 are 

 particularly informative. 



An ecological principle of considerable 

 general significance emerges from Crom- 

 bie's investigation. This can be formulated 

 some\\'hat as follows: When the total en- 

 vironment is essentially unlimited or at 

 least unsaturated— that is, when there are 

 many wheat grains available for larval oc- 

 cupation—the increased emigration induced 



