IOWA ACADEMY OP SCIENCE 
209 
A STUDY IN INSECT PARASITISM. 
BY E. L. WEBSTER. 
Introduction. For a number of years the writer has been interested in insect 
parasitism and its relation to the control of injurious insects. Occasional notes 
have been made on different species of parasites and considerable interesting 
data has been obtained concerning the parasites of one of our common Iowa 
insects, the “tomato-worm,” Phlegethontius sexta (Johanssen), sometimes called 
the “tobacco worm” or the “southern tobacco worm.” It is with certain para- 
sites of this particular insect with which this paper has to deal. 
The most of this data, but not all of it, represents a part of a study on 
potato insects, being carried on at the Iowa Agricultural Experiment Station 
at Ames. The host insect mentioned, not content with tobacco and tomato as 
food plants, is also quite fond of potato foliage, hence its consideration as a 
potato insect. 
The Host Insect. I think that most of us have seen these long, fat, green 
“worms,” with diagonal white bars on each side, and with a prominent horn 
at the caudal end. This rather dangerous looking horn has been popularly 
thought of as being poisonous, but such is not the case. When it is picked 
up the tomato-worm is likely to bite one’s fingers with its mandibles, but there 
is no danger to be feared from the caudal horn. The “worms” are common 
insects in Iowa and are found frequently on tomato and potato leaves during 
the summer. 
There are two common species of these insects in Iowa; the “northern tobacco 
worm,” Phlegethontius quinquemaculata (Haworth) and the “southern tobacco 
worm” or “tomato-worm” as it will be called in this paper. The observations 
herein given refer to the tomato-worm, Phlegethontius sexta (Joh.). 
A brief account of the seasonal history of the tomato-worm is given here- 
with. The winter is spent in the soil in the pupa state. Emerging in June, 
the moths deposit their eggs on the leaves of tomato and potato plants and 
the larvae appear in July, maturing in late July^and early August. A second 
generation, which is probably only a partial one, occurs in Iowa, since the 
larvae are found again in September and October. These mature and spend- 
the winter as pupae in the soil. 
The Parasites. The most common primary parasite of the tomato-worm is 
a small, black, hymenopterous insect, a braconid, Apanteles congregatus (Say). 
The small, white, cylindrical cocoons of this parasite are often seen on the 
tomato-worms, although they are usually mistaken by most people for eggs 
of some kind. The parasite, however, deposits its eggs inside the host insect, 
puncturing the skin of the tomato-worm in order to do so. These eggs hatch 
out and the parasite larvae feed on the inside of ‘the host until they become 
mature. Then they cut through the skin of the host to the outside and spin 
their small, white cocoons there, attaching them to the back of the tomato-worm. 
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