Insecticides and Honey Bees

FRANK E. TODD AND S. E. MCGREGOR

PEOPLE who grow seed crops have a double problem with insects: Many of the crops must be pollinated by bees, which are highly sensitive to insecticides. Harmful insects, though, must be controlled, usually by the use of the pest killers.

An experiment with alfalfa, which is typical of such crops, was conducted by Frank E. Todd, Frank V. Lieberman, and .John W. Carlson in 1949 at the U.S. Department of Agriculture Legume Seed Research Laboratory in cooperation with the Utah State Agricultural College at Logan, Utah. There were 24 plots, each 12 by 24 feet, in the experiment. Each plot was covered by a 6-foot-high plastic screen cage to control pollinators.

Harmful insects were controlled on a series of plots of growing plants, but not on another comparable series. Bees were excluded from some of the plots and were provided on others. The seed was harvested in due time.

The plots that had neither bees nor insect control produced only 5 pounds to the acre. Those on which insects were controlled and had no bees yielded 15 pounds. Those with bees but no insect control produced 31 pounds. Plots where both bees and insect control were provided yielded 321 pounds of seed an acre.

To protect the pollinators while controlling the harmful insects requires considerable knowledge of both groups as well as of the flowering habits of plants and the toxicity of the insecticides to bees.

The harmful insects are, relatively permanent residents in a field. They remain, feed, and multiply in it as long as conditions are favorable for them. They damage the plant in obtaining food for themselves. They attack various parts of it and at different stages of its growth.

Bees visit the plant only when it is in flower, and even then they seek only its nectar and pollen. The plant usually is benefited by their visits. The bees spend the night in the hive and visit the field only during daylight hours to collect food for the colony.

Bees usually are not harmed when insecticides are applied to plants not in flower. Even when the plants are flowering, materials having short residual action can be applied during the hours bees do not visit them.

THE BEEHIVE is like a miniature, isolated airport on which many tiny planes converge from all directions. Its prosperity is determined by its traffic, as all food and supplies must be flown in. Its radius of operation is only a mile or so. The effect of an application of insecticide within this area of operation could be compared to the effect of a hurricane. All planes in flight may be downed or seriously damaged, and even the airport itself may be destroyed. But just as airports operate under proper control in regions where hurricanes occur, so may colonies of honey bees survive in areas where insecticides are applied if proper precautions are taken.

Rearing a field bee, like training a pilot, requires time about 6 weeks. Half of this time is spent as a larva or immature bee. The remainder is spent as a young adult bee devoted to hive maintenance or "ground crew" duties.

The best pollination service is derived from a strong colony. It should have enough bees to form a cluster over about 20 brood combs. It should also have sufficient brood of all stages and stores of pollen and honey to maintain this strength during the period its services are needed for pollination.

Just as the licensed pilots represent only a small part of the airport personnel, so do the field bees represent only a small part of this entire unit. They are the ones upon which both grower and beekeeper depend. They are also the first to be exposed to insecticides and often the only ones killed. The loss of this group of bees can make the difference between a strong colony and an inadequate one.

The bees that are seen flying or visiting flowers are part of the colony's field force of perhaps 20 thousand fliers that gather food for the colony. They are at least 3 weeks old and have a normal life expectancy beyond that of 10 to 15 days, but they are continually being replaced if egg laying by the queen is uninterrupted.

A considerable number of them have regular flight schedules as water carriers. A few others scout the range for new food sources. When they find new foraging areas, they return to the hive and communicate to unemployed food gatherers the scent of the source plant, its direction, and distance from the hive These recruits fly directly to the area. Each of them confines its foraging activity to the indicated plant species on only about to square yards. This tiny theater of operation is visited repeatedly, sometimes for days, until the food becomes exhausted. The bee then awaits recruitment in the hive by another scout.

When insecticides destroy these field bees, the flow of food to the colony is cut off. Colony routine is disorganized. Productiveness is reduced. The area where the insecticide was applied is neglected until rediscovery by scout bees and the recruitment of a new field force. This may take only a few hours (if an insecticide with low residual effect was used) or several days (if it was an insecticide having long residual action, because the latter continues to kill searching scout bees).

The greatest concentration of fliers is near the hive. Direct applications of insecticides over the hives, therefore, can cause the greatest destruction.

Even applications of materials to plants unattractive to bees and not over hives may cause heavy mortality if the flight pattern of the bees is through the dust or spray cloud.

An application of insecticide anytime the bees are working on the field usually kills the bees it touches.

If the material is slow acting, the flier limps back to crash land at the hive, and a litter of other dead bees accumulates around it.

If the material is fast acting, the flier crashes in the field and is heard from no more. No external evidence of damage may be seen at the hive except that flight dwindles and ceases.

Some materials kill the "ground crew" also. When that happens, the ventilating and cooling systems are disrupted, the brood is neglected, and it dies. If the colony survives this onslaught, a week or months may be required to restore normal conditions. During this time the colony may superficially appear normal, but it is a useless unit to the seedgrower and the beekeeper.

KNOWLEDGE of the flowering habits of crop plants is useful in timing applications of insecticide to avoid the destruction of pollinators.

Some plants produce only pollen. Others produce both pollen and nectar. The grasses (corn, sorghum, bermudagrass, and johnsongrass) are of the first type. They usually yield pollen until about 10 a.m., after which they are unattractive to bees. Insecticides applied to these crops at other times of the day should destroy no pollinators.

The flower of the cantaloup or muskmelon exemplifies one that produces both pollen and nectar and is open only a single day. It opens early in the morning and is attractive to bees through the forenoon. The safest time to apply insecticides to this crop would be after the flowers begin to wilt and close during late afternoon or evening. Other plants in this group are watermelons, cucumbers, squash, and pumpkin.

The cotton flower opens a few hours after sunup and may be visited by bees throughout the day. Although it is open only 1 day, the extra-floral nectaries on the plant are functional for several days and may attract bees early in the morning. Often the highest bee population may be found in cotton-fields then. Usually there are only a few bee visitors in late afternoon, but the best time to treat cotton would be at night.

Another type of flowering is exemplified by alfalfa on which a single floret may be open for a week unless the stigma and anthers are released from the keel of the flower, or "tripped," by bees. Bees do not usually visit alfalfa until 2 or 3 hours after sunup. Pollinators therefore would not be exposed to direct contact with any insecticide that is applied early in the morning.

The flowering habits of other plants may be similarly utilized to reduce insecticide losses.

When agricultural chemicals are to be used around bees, the choice of the least harmful material becomes very important.