121 



It will be noticed by referring" to the table that the greater percent- 

 age of parasites come from the smaller larvae; or, in other words, that 

 the larvae are destroyed before they are half grown and consequently 

 before they have done the greater part of their injury to the plants. 



HYMENOPTEKOUS PARASITES. 



Practically all the parasites bred from the young larvae belong to 

 the same species, JficropUtis nigripennis Ashm., of the famil}^ Bra- 

 conid*. This species was first bred in the laboratory from a larva 

 collected on tobacco at Paris July 5, 1904, but it was not observed in 

 any numbers until the middle of August, when a large percentage of 

 the boUworms feeding on alfalfa were found to be parasitized b}^ it. 

 Its frequent occurrence on cotton after that time has been referred to. 



The eggs of the parasite are deposited in larvae which are about 

 10 or 12 mm. in length, or much more rarely in larger specimens. 

 After it has been stung by the Braconid, the larva continues to feed, 

 but at a much diminished 

 rate, for two or three days. 

 It now becomes very slug- 

 gish and eats th-e involucre 

 of the square or boll in a 

 rather desultory manner, 

 chewing it into small bits, 

 mam^ of which it fails to 

 swailow. These small pieces 

 usually remain webbed to- 

 gether loosely by delicate 

 strands of silk spun by the 

 boll worm, and present quite 

 a characteristic appearance. By the time the parasitic grub is full- 

 grown, the larva lias ceased feeding entirely and is scarcely larger 

 than when first attacked. The Microplitis now bores its way out 

 through the skin of the quiescent bollworm, leaving a large black scar 

 (see PI. XIX, fig. 1) to mark the point of its emergence, which is gen- 

 erally on one side of the body, near the first pair of prolegs. In one 

 case the grub was watched during the act of emerging from the boll- 

 worm. It had protruded the pointed head and was gradually working- 

 its fleshy body through the small hole in the larva's skin. In doing this 

 the body had to be constricted dumb-bell shape in order to pass through 

 the small orifice. Once free from the larva, it spins its pupal cocoon 

 (see PI. XIX, ^g. 1, and text fig. 23) on some surrounding object, gen- 

 erally a stem or a leaf, spending several hours in this work. The cocoon 

 is about 5 or 6 mm. in length, oval in shape, pale dingy yellow in color, 

 and furnished with a few coarse longitudinal ribs. When first spun it 

 is almost white and rather delicate. After a few hours, however, it 

 acquires its mature color and its consistency becomes ver}^ tough. 



Fig. 23. — Microplitis nigripennis: adult, larva, and 

 cocoon (original). 



