123 
becomes abundant, a sooty fungous growth appears on the leaves 
covered with it, which hastens the death of the plants. 
Parasites. — In the university greenhouse, at Urbana, many of 
the plants, especially Salvia and pansy-geranium, were badly in- 
fested in the midwinter of 1908 by the little white-fly, but two 
months later these insects were found to have been almost exter- 
minated by a minute hymenopterous parasite, which proved to be 
a new species, and has since been described by Mr. A. A. Girault 
as Encarsia versicolor.^ It attacks the immature forms of the 
white-fly, and the infested young may be recognized by their black 
color, those not infested being pale greenish. 
Remedies. — Fumigation with hydrocyanic acid gas is by far the 
most efiicient means known for controlling the white-fly in green- 
houses. In ordinary greenhouses it may be used at a strength of 
one ounce of cyanide of potassium' to 3,000 cubic feet of space. In 
new tightly built houses a smaller quantity may be used; but in 
very loosely built or weather-shrunken houses more must be used. 
The mode of using this gas is described on pages 156-158. It is 
commonly necessary to fumigate three or four times, at intervals 
of about two weeks. 
Fumigation with tobacco stems, or with any of the commercial 
tobacco extracts, is partially effective against this insect, and where 
the use of hydrocyanic acid gas is impracticable this treatment is 
recommended (pages 159 arid t6o). More fumigation will be re- 
quired, but, if persistently followed up, the white-fly can be kept in 
check by this means. 
The Onion Tiirips 
Thrips tabaci Lind. 
This insect, which is very injurious to cucumber plants in the 
greenhouse, is discussed in connection with its injury to the onion 
(page 137). In the greenhouse its life history is the same as out-of- 
doors, excepting that the life cycle varies greatly in length under 
the various artificial conditions which prevail in greenhouses. As 
a rule, cucumbers are started under glass in fall, and it is at this 
time that the thrips migrates from outdoor vegetation into the 
houses. It breeds successfully on almost any kind of greenhouse 
vegetation, and may soon become well established ; but it is not 
usually until midwinter that it becomes abundant enough to alarm 
the gardener. 
Remedial Measures. — Clean culture is one of the most im- 
portant means of holding this pest in check. As a rule the grasses 
*Encarsia versicolor species novum, a Eulophid Parasite of the Greenhouse 
White-fly, Aleyrodcs vaf^orayionim Westwood. Psyche, Vol. XV, No. 3 (June, 
1908), pp. 53-57. Cambridge, Mass. 
