296 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



contain two kinds of blossoms; one is all female and the other is 

 all male. Here again pollen must be carried from the male flower to 

 the female. Since pollen from these plants is also sticky and heavy, 

 the indispensable services of pollinating insects is apparent. 



There are still other types of plants in which a single plant pro- 

 duces only male flowers, while another plant of the same species bears 

 only female flowers. The wild persimmon and holly are good illus- 

 trations of this type. Since the wind cannot be depended upon to 

 disperse the pollen adequately, insects again come into the picture. 



There are approximately 50 cultivated crops grown in the United 

 States that require insect pollination. In addition to those already 

 mentioned, there are many other important plants — for example, 

 alfalfa, sweetclover, red clover, alsike clover, white Dutch clover — 

 which must have each tiny flower visited by an insect in order to 

 produce seed and thus perpetuate themselves. Pollinating insects are 

 either essential or highly desirable in the production of seeds of many 

 vegetables, such as carrot, onion, cabbage, cauliflower, and Brussels 

 sprouts, to name only a few. 



Insect pollination is a "must" in our present-day agriculture, but 

 why is it more important today than it was 30 or 40 years ago ? 



There was a time in the history of agriculture, and not so many 

 years ago either, when it was not uncommon to hull 6 to 10 bushels of 

 red clover or alfalfa seed per acre. It is only rarely that such produc- 

 tion is experienced today — in fact, the average yield of both these 

 crops for the United States is slightly under 1 bushel per acre. Yet 

 alfalfa and red clover grow just as well and blossom as profusely as 

 in bygone days. But why has seed production fallen so low ? Could 

 inadequate pollination be one of the contributing factors? 



There was at one time in this country an adequate population of 

 native insects to take care of all pollination needs. These are insects 

 that maintain themselves and raise their young largely on pollen and 

 nectar. Their whole livelihood depends upon their flower-visiting 

 habits. As farming operations expanded, the nesting sites of many 

 of these insects were destroyed. Where the population of native in- 

 sects could adequately pollinate a 10-acre field of clover or alfalfa, 

 the same number of insects fell woefully short when the acreage 

 jumped to several hundred acres. Since most plants have blooming 

 periods of short duration, it is only logical that the numbers of pol- 

 linating insects be stepped up in proportion to the increase in acreage 

 if seed yields are to be adequate and profitable. We have gone ahead 

 increasing acreages manyfold but have made no effort to provide a 

 proportionate increase in the number of pollinators. 



Many factors have contributed to the decline of native pollinating 

 insects. The plowing and clean cultivation of large tracts of land 

 deprive these insects, many of which build their nests in the ground, 



