Lettuce growing in greenhouses. Page: 24
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24 FARMERS' BULLETIN 1418, U. S. DEPT. OF AGRICULTURE
is very poisonous, and there is grave danger to the operator and
also to the plants being fumigated unless extreme care is exercised
in handling the material. However, in experienced hands it is a
safe, practical, and economical fumigant. One-eighth to one-fourth
ounce of calcium cyanide per 1,000 cubic feet of air space, depending
on the tightness of the house, should be used during an overnight fumigation
with the gas it produces. As this dosage does not kill immature
forms of whiteflies. repeated treatments will be necessary to kill
the adult flies after they emerge from the pupal stage. More detailed
information on the proper method of fumigating with this material
can be obtained by consulting your State entomologist or the Bureau
of Entomology and Plant Quarantine, Washington 25, D. C.
Nicotine fumigation is used extensively and effectively for the control
of plant lice infesting greenhouse crops. This method, while
convenient, is usually more expensive than hydrocyanic acid gas funligation.
The nicotine is made available by various methods, either by
burning tobacco stems, specially prepared paper or powder treated
with nicotine, or finely ground tobacco dust or by vaporizing liquids
containing nicotine on special burners or hot steam pipes.
Nicotine papers or powders, tobacco dusts, and tobacco or nicotine
liquids prepared for fumigating purposes contain varying proportions
of nicotine, and it is advisable to follow the directions for their
use as given by the manufacturers. The grower is advised to observe
carefully the results obtained and if the fumigation is not effective
to increase the quantity of material.
Nicotine and its compounds are violent poisons, and care should
be exercised in their use.
The cabbage looper,'2 one of the caterpillars commonly found on
cabbage, often gains entrance to greenhouses by being inadvertently
carried there; or the moth may enter the house in late fall and deposit
eggs upon lettuce or other food plants.
When attacked by these caterpillars, the lettuce should be dusted
with derris or pyrethrum dusts.
A dust containing 1 percent of rotenone is effective and may be prepared
by mixing 10 pounds of derris, or cube, powder containing 4
percent of rotenone with 30 pounds of talc or pyrophyllite dust. Obviously,
a powder containing a higher or lower percentage of rotenone
should be mixed with correspondingly more or less of the inert material
to give a 1-percent mixture.
The pyrethrum powder mixtures used should be freshly prepared.
These mixtures standing for a year after preparation lose a high degree
of their effectiveness.
If the insects are successfully controlled during the first outbreak,
there will be little danger of their recurrence during the season.
" Trichoplusia ni (Hbn.).
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Thompson, Ross C. (Ross Calvin), 1896-. Lettuce growing in greenhouses., book, May 1949; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc3455/m1/26/: accessed July 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.