330 JOURNAL OF ECONOMIC ENTOxMOLOGY [Vol. 1 



11. Aestivation and liibentation. The varying conditions of food 

 supply or species habit, which render a resting period imperative, 

 bring about a factor of extreme variation. With the boll weevil the 

 entrance into hibernation in a given locality may extend over two 

 months in the fall, and the emergence frequently lasts from jMarch to 

 July. Even though a parasite species should carry off all of the 

 developing weevil offspring of the earliest weevils, there would still 

 be many more weevils in hibernation, to continue the species. 



12. Endo- and ecto-parasitism. Although a minor factor, still the 

 location of attack by a parasite must be taken into consideration. 

 Zygoharis xanthoxyli Pierce breeds in the l)erries of Xanthoxylum 

 clavahercidis, and pupates in the ground. It is parasitized internally 

 by Sigalpkus zygobaridis Cwfd., which does not kill it until the 

 earthen pupal cell is formed. Thus the host prepares a safe retreat 

 for its parasite. The weevil is also parasitized externally by Catolac- 

 ciis hunteri Cwfd.. which kills the host larva while it is still in the 

 berry. This instance will suggest how this factor may be important. 



13. Premature death. In considering the numerical ratio between 

 two species, there should be prepared data to show the percentage of 

 each species which die before fulfilling their sexual functions. 



II. Other Biological Factors 



1. Insectivorous vertebrates. Insects have many enemies among 

 the vertebrates, such as batrachians, reptiles, birds, and mammals, 

 which show very little discrimination between hosts and parasites. 

 Parasitized insects, being uneasy or frantic in their movements, are 

 moie open to attack by birds than healthy individuals. 



2. Other plant feeding insects. The existence of other plant-feed- 

 ers on the same host plant is an element which has a direct bearing 

 upon the status of the given insect. For example the cotton squares 

 and ])olls are l)()red by the boll weevil, the boll worm, several square 

 borers, such as Calycopis and Uranotes, and even by the leaf worm. 

 The activity of any of these Lepidoptera in squares necessarily cuts 

 down the food supply of the boll weevil. When the leaf worms defo- 

 liate the cotton, they stop its gi'owth. eliminate the weevil's food 

 supply, and remove the shelter from the sun, with the result that 

 the survivors nuist disperse. An extended region of defoliation may 

 cause the starvation of multitudes of flying weevils in search of 

 fresh fields. Finally, the limiting of the food supply and the off- 

 spring of the weevils limits the parasites, while the work of the sun 

 made possible by defoliation forestalls them, and the dispersion leaves 

 them still less of a chance for great multiplication. Other examples 



