LONCHOCARPUS, DERRIS, AND PYRETHRUM CULTIVATION 31 
The stage of blossom development at time of picking is important, - 
particularly with respect to the keeping quality of the flowers during 
subsequent storage. The pyrethrin content of flowers increases up to 
the point where the majority of the disk florets open. After pollina- 
tion, the pyrethrin content does not diminish, but the development of 
the ovaries causes a considerable increase in the weight of the floral 
head without a comparable increase in pyrethrins so that insecticidal 
quality gradually declines. More important than the small differences 
in pyrethrin content at various stages of floral development is the 
degree of its stability under storage conditions. Gnadinger et al. (7) 
have shown that immature and nearly mature flowers lose little 
pyrethrin during the first 60 days of storage but that mature flowers 
may lose as much as 13 percent. 
Figure 16.—Mechanical pyrethrum picker developed by U. 8. Department of 
Agriculture. 
After harvesting, the flowers should be thoroughly dried either in 
the sun, in ventilated shelters, or in heated driers before they are baled 
for storage and shipment. A common belief is that pressure baling is 
necessary to prevent loss of pyrethrins in storage, but it has been shown 
that flowers packed under pressure of 16,000 pounds per square inch 
for a period of 263 days lost only slightly less pyrethrins than a 
comparable lot of loosely baled flowers (7). 
PRODUCTION IN THE WESTERN HEMISPHERE 
Brazil 
Pyrethrin production in the Western Hemisphere has increased 
significantly in the past 5 years, with Brazil accounting for almost the 
entire amount. In 1946 total United States imports from all world 
sources reached 20,476,000 pounds, which was more than for any pre- 
