20 CIRCULAR 45 7, U. S. DEPARTMENT OF AGRICULTURE 



concentration of gas obtained will vary greatly from one fumigation 

 to another. Experiments have shown that the manure fly (Meg- 

 aselia) is killed by concentrations of between one-fourth and one- 

 half of a milligram per liter, which can be obtained under favorable 

 conditions at the recommended dosage (2 ounces per 1,000 cubic 

 feet). The mushroom fly (Sciara) however, requires from three- 

 fourth to 1 milligram per liter, which is too high to be safe for 

 the crop. 



Temperature seems less important than humidity and moisture. 

 The beds should be as dry as possible, and the floor moist, but not 

 wet, before the calcium cyanide is spread. 



By light watering and proper temperature, especially in the early 

 flushes, it is possible to force mushroom growth ahead of the develop- 

 ment of the fly maggots, thus producing a crop in spite of the in- 

 festation. If dusting to control the adult flies is begun early, all the 

 eggs that these flies can lay will have been laid within a short time, 

 and oviposition reduced to a minimum thereafter. When the mag- 

 gots in the beds have pupated, the spawn is free to grow without 

 further interference. 



CECIDOMYIID FLIES 



The adult cecidomyiid flies are extremely small, delicate insects, 

 about 1 millimeter, or one twenty-fifth of an inch in length, brownish 

 and with orange abdomen. The species most common at Arlington 

 Experiment Farm, Va., has been determined as Mycophila fungicola 

 Felt. They are very inconspicuous, as they remain flat against the 

 bedboards and usually fly only when disturbed. Those that have 

 been observed in flight were unable to maintain their initial altitude 

 in the absence of ascending air currents, and were able to fly only 

 a few feet at a time. They live only a few days at most. Each female 

 is capable of laying from three to five or more relatively large eggs. 

 The eggs hatch into small, threadlike, legless larvae, or maggots, which 

 rapidly mature. Mature larvae are about one-tenth of an inch in 

 length, white, and semitransparent. A very peculiar feature in the 

 life history of these flies is that they are "paedogenetic," i. e., able 

 to multiply in the larval condition. Each young larvae feeds for a 

 time until it reaches full size, becomes quiescent, the larval skin splits, 

 and from 2 to 18 young larvae emerge, leaving the remains of the 

 parent behind.. Under favorable conditions this process may take 

 place every 7 or 8 days. At times the larvae give rise to a thread- 

 like form, much less robust than the parent, which is apparently 

 better able to withstand adverse conditions. At other times bright 

 orange forms are produced, some of which may pupate and emerge 

 as adults, the remainder continuing to reproduce paedogenetically. 

 All forms of larvae seem to be mutually interchangeable, but the 

 conditions governing the production of the different forms or those 

 governing the formation of pupae are not known. 



Normally these larvae are minor pests of mushrooms, but at times 

 they may appear in large numbers on the casing soil and mushrooms, 

 and if sufficiently numerous may cause injury to the spawn, and to 

 the mushrooms by eating small holes into the stems and caps. They 

 are probably brought into the houses with the casing soil as a rule, 

 being, in the wild state, feeders on wild fungi and molds and organic 

 material in the soil. 



