PARASITISM 137 



circumstances, there may be even quartenary parasites. This relation- 

 ship is well illustrated by the parasites of the larvae of the the White- 

 marked Tussock Moth which is very common on shade trees in our cities 

 and towns. The caterpillar may be parasitized by Amorphota orgyiae 

 which is one of the Ichneumons as is shown by its club-shaped abdomen 

 and its sting-like ovipositor. The larva of this parasite lives within 

 the caterpillar until the entire body has been devoured. Tlie larva then 

 leaves the skin of the caterpillar and spins an oval cocoon within which 

 it enters the pupal stage. At this time one of the Chalcids (Spilochalcis 

 debilis) which, it will be remembered, have elbowed antennae and heavy 

 hind legs, comes along and "stings" the cocoon laying an egg in the 

 body of the Ichneumon pupa. When the food supply is exhausted, the 

 larva spins its cocoon within the other cocoon. Dibrachys boucheanus, 

 another one of the Chalcids now lays an egg within these cocoons. This 

 insect pupates without spinning a cocoon. The pupa of this parasite 

 fall a prey to a little Chalcid called Asecodes albitarsis. Hence the 

 leaves, used by the caterpillar to build up its own body, were successive- 

 ly transformed into four other insects, for none of these parasites partook 

 of any food save that which was in the original caterpiller. 



The importance of the ipart played by parasites in controlling the in- 

 crease of injurious insects may be made more clear by a discussion of 

 some of our common pests. In 1895 the White-marked Tussock Moth, 

 already mentioned, threatened to become a plague in Washington, D, C, 

 during the latter part of August and the early part of September almost 

 every poplar, soft maple, box elder, elm, elder, birch, and willow in the 

 city was completely defoliated, while other trees were badly damaged. 

 Somewhat later in the fall Dr. Howard, U. S. Entomologist, found that 

 about 90% of the caterpillars were parasitized. Nevertheless, the 10% 

 which survived produced enough so that next spring there were large 

 numbers of the caterpillars. At this time Dr. Howard gathered 624 

 cocoons. From these only 12 moths emerged while there were 916 para- 

 sites of which only 64 were primary. In other words parasites destrayed 

 98% of the larvae. A month later it was almost impossible to find any 

 larvae when the year before at the same time there had been thousands. 



In handling the Cotton-boll Weevil of the South parasites are made 

 to play a very important part. There are a number of weevils closely 

 related to the Cotton-boll Weevil but which never bother the cotton. 

 They live in the grass and weeds about the cotton fields, and are para- 

 sitized to a great extent. At the time when these parasites have emerged 

 and before they have again laid their eggs, the weeds and grass contain- 

 ing the w6evils are destroyed. The female parasites, as a second choice, 

 then hunt up the Cotton-boll Weevils and parasitize them, greatly lessen- 

 ing the damage done by that insect. 



In many localities the main factor which keeps the Hessian fly 

 check is the presence of parasites. When climaic conditions are favor- 

 able the Hessian fly may become so numerous as to do much damage. 



