POTATO INSECTS 161 



to end with no interruption in the middle. The caterpillars 

 reach maturity in August and transform to brownish pupae 

 within the burrows. The moths are on the wing in September 

 and October. They have an expanse of about 1 J inches. The 

 ground color of the front wings is light yellow, dusted with 

 brown, the outer margin grayish, leaving a yellow spot at the 

 apex. The outer line runs as in P. nitela and is double and 

 the wing is spotted in much the same way as in the variety 

 nehris of that species. The hind wings are light brown. The 

 eggs are deposited singly in cracks and crevices on the stems 

 of the food plants in the fall but do not hatch until the follow- 

 ing spring. There is only one generation annually. 



The injury inflicted by the burdock borer may be avoided 

 by adopting the measures recommended for the common 

 stalk-borer. 



The Potato Scab Gnat 



Pnyxia scabiei Hopkins 



Potatoes are sometimes injured by a small, white, black- 

 headed maggot, about ^ inch in length, that causes an injury 

 similar to that produced by the scab fungus. Outbreaks of 

 this insect have been recorded in West Virginia and in Ohio 

 and infested potatoes have been shipped fn)m Philadelphia. 

 The adult insects have been found in New York and have been 

 collected under dead leaves in the woods in Missouri. It is 

 probable that the normal food of the scab gnat maggot is de- 

 caying vegetable matter. Under certain conditions, however, 

 it will attack potatoes in the field and in storage. The 

 maggots have also been found injuring peony bulbs in Pennsyl- 

 vania. Potatoes are more subject to injury when grown in 

 low ground where there is an abundance of humus or when 

 stored in warm, damp cellars. 



The female fly is -^ to y^ inch in length, pale in color and 

 without wings. The male is somewhat dusky, smaller than 



