Insecticides and Bees

Frank E. Todd, S. E. McGregor.

Bees are so important in agriculture and so important to so many of us that we cannot afford to destroy them along with the harmful insects.

Two-thirds of the 5,600,000 colonies of bees in the United States are east of the Mississippi River. About one-half of them are in the Southern States. Of the 1,946,000 colonies west of the Mississippi, the Plains States have about 30 percent, California 24 percent, and the Intermountain and the Southwestern States 20 percent each. Six percent are in the Pacific Northwest. Although 500,000 persons keep bees in the United States, 8o percent of the colonies belong to about 50,000 beekeepers, about 1,000 of whom depend on bees for their livelihood. The latter group controls about two-fifths of the colonies.

But the extensive use of insecticides has driven beekeeping out of many localities. In apple-growing areas, for example, growers have to pay rental fees to entice beekeepers into the areas during blossom time to insure pollination, and growers of legume seed are beginning to follow this practice. In many cotton-growing areas, spreading arsenical dusts by airplane has nearly wiped out the bee industry.

About three-fourths of the annual honey crop comes from cultivated alfalfa, buckwheat, clovers, cotton, and oranges. The honey crop is the beekeeper's source of livelihood, and unless it covers his expenses he cannot stay in business. A widespread abandonment of beekeeping in turn would reduce the supply of pollinators for agricultural crops. Although bees visit most of the flowering plants to obtain food for colony maintenance, few species contribute enough nectar to make a honey crop. Apple blossoms, for example, contribute pollen and nectar for colony maintenance, but apple honey is unknown on the market.

Dandelion, mustard, goldenrod, and gum weeds are examples of important sources of food for maintenance only; their elimination by weed spraying may limit the amount of beekeeping an area can support. Native bees depend on weeds even more than honey bees do. Their foods must be obtained locally. Often their survival requires a continuity of sources, usually weeds. Spraying grain fields in the Sacramento Valley of California to remove weeds has caused the near disappearance of star thistle honey from the market. Herbicides are also being applied along roadsides for sweetclover and mesquite, both important sources of the honey crop. Bees are not killed by weed sprays, but food sources may be seriously reduced by their widespread use.

Controlling harmful insects on agricultural crops is often beneficial to beekeeping. Almost always can insecticides be applied so as not to harm bees but control programs that disregard bees usually are followed by an acute bee poison problem.

The problem began in the early 1870's. A strange malady appeared then among colonies of honey bees. In the spring dead bees piled up around the hives, colonies failed later to recover strength, and many died outright. The malady and the use of Paris green to control codling moth on apples and pears appeared at the same time; the use of Paris green spread rapidly and so did the sickness among bees. Beekeepers soon learned the source of trouble : The trees were being sprayed while they were in bloom.

C. M. Brose, of the Colorado Agricultural Experiment Station, in 1888 reported finding arsenic in dead bees fed London purple and Paris green in sirup. He found no arsenic in the stored honey. A. J. Cook, of the Michigan Agricultural Experiment Station, reported that bees died soon after they fed on sirup or water containing London purple at the strengths used in spraying. He strongly advocated laws to prohibit the spraying of fruit trees in blossom.

Beekeepers made vigorous complaints. In 1891 the Association of Economic Entomologists appointed a committee to find an answer to the question: "Will arsenical sprays applied to flowering fruit trees kill honey bees?" The chairman was F. M. Webster, of the Ohio Agricultural Experiment Station.

In 1892 Webster made the committee's first report. He sprayed a flowering plum tree with paris green and then caged it, enclosing a colony of bees. An analysis of the dead bees showed arsenic, before and after they were washed to remove external contamination. Experiments on apple trees in the open were less convincing. His second report in 1895 covered more detailed work. He found arsenic in dead bees taken from sprayed apple trees and in bees taken from a colony that had died shortly after the apple orchard in which it was located had been sprayed. That evidence convinced everybody. Research gave proof also that bees do not injure fruit (although they may suck juices from overripe fruit after it is punctured by birds and yellow-jackets) and that honey bees are an economic necessity as pollinators.

About 1920 two new factors arose the development of the cheaper calcium arsenate dust and the use of the airplane to apply insecticides.

THE BOLL WEEVIL had become established in the South, but repeated applications of calcium arsenate reduced its damage. The land of cotton covered more territory than the fruit areas; calcium arsenate was just as toxic as the sprays used in orchards; consequently losses of bees were more extensive. Not uncommonly did beekeepers lose 500 colonies in a season. The choice was to move the bees or go out of business. Moving often meant transporting several truck loads of hives 100 miles or more to strange and less profitable honey locations; besides, the beekeeper often would not know whether insecticides were being applied in the locality until he saw a dusting plane or found an apiary already poisoned. Beekeeping in cotton areas declined. Most beekeepers quit, some moved to distant areas, and a few developed specialties, such as rearing queens away from the cotton areas.

Arsenicals applied on crops elsewhere caused corresponding losses of bees, even though the crops (such as tomatoes, potatoes, or lettuce) were unattractive to bees. The reason was that the dust was drifting onto plants attractive to bees along the borders or outside the treated fields. Analyses disclosed that, regardless of place or method of application, all arsenicals were highly toxic to bees about one-third of a part per million of the bee's body weight was enough to cause death. Furthermore, any arsenical carried into the hive with pollen on the bee's legs and stored for future food remained poisonous for months.

One would expect that some of the enormous quantities of insecticides applied on cultivated crops would show up in honey. That is not the case. Nectar is carried in the honey sac, a specialized part of the alimentary tract. When the nectar contains poison, the carrier is quickly affected. Instead of returning to the hive, the bee attempts to throw off the effect of the poison and becomes lost or dies in the field. Should the bee return with a load of Poisoned nectar there is a second safety factor. Every drop of nectar is rehandled by the hive bees, which are exposed to poison longer than the field bees. Hive bees tend to leave the colony when poisoned, carrying with them the Poisoned nectar. It is therefore unlikely that poisons would ever be stored with honey. Chemical analysis of honey stored in the brood nests of colonies affected with arsenical poisoning has failed to reveal any trace of arsenic.

Calcium arsenate was used in increasing amounts until 1946. Other materials were below it in volume or toxicity. The arsenicals are some 50 times more toxic to bees than cryolite. Large amounts of sulfur were used, but as applied to field crops it is safe for bees. Several plant derivatives nicotine, pyrethrum, sabadilla were applied, but losses were minor as they are safe for bees within a few hours after application.

Since 1946 the synthetic insecticides have brought new problems. They differ in relative killing powers and affect colonies differently. Some, like the arsenicals, cause a large number of bees to die near the hive entrance. Others, like chlordane, cause the bees to die in the field away from the hive. Benzene hexachloride causes bees from affected colonies to be furiously mean. DDT causes slight stupefaction. Lethal effects from exposure to dieldrin may continue for a week and from parathion (less toxic than dieldrin but very dangerous) for 2 to 4 days. Losses from applications of toxaphene may be negligible.

Toxaphene, the least dangerous to honey bees, gives good control of a number of harmful insects. Applied to such crops as alfalfa, the protected crop produces more flowers actually a benefit to the honey bees. That consideration led to the establishment by the Department of Agriculture of a laboratory at Tucson, Ariz., in 1949 to study the effects of insecticides on bees.