Experiments on the poisoning of honeybees by insecticidal and fungicidal sprays used in orchards



Losses of bees by poisoning have been greatly increased in recent years by the growing practice of applying insecticidal and fungicidal sprays to fruit trees. Preliminary laboratory tests showed that, of the common constituents of spray mixtures, only lead arsenate and flowers of sulphur were likely to cause serious honeybee poisoning, though Derris emulsion may cause slight poisoning. Syrup containing lime sulphur, nicotine sulphate, or copper sulphate was strongly repellent to the bees.

It seemed possible that spray mixtures might be made repellent to the honeybee by the addition of suitable substances. Further trials showed that lead arsenate solution, at least in the concentrations normally used, was no more attractive to the bee than distilled water. Concentrations of 1/500 lime sulphur or 1/2000 nicotine sulphate were sufficient to reduce the uptake of M/1 sucrose to less than 10% of that of unadulterated sucrose solution, and very much lower concentrations appreciably affected the uptake. The presence of lead arsenate in these solutions seemed to make them even more repellent. The repellent effect was reduced, though not entirely destroyed, when the solutions were evaporated to dryness and taken up in distilled water again.

Creosote (0.13%), which has been suggested as a possible repellent, gave erratic results under laboratory conditions, and, in view of the danger of phytocidal action it is probably unwise to attempt to use its repellent property.

Open flowers of apple trees were sprayed with various spray mixtures in an orchard experiment, and counts of bees showed both lime sulphur (1%) and nicotine sulphate (0.05 % nicotine) to have retained their repellent value for at least 7 days, in spite of heavy rain. If bees can thus be deterred from visiting the open blossom, it should be possible to deter them even more thoroughly from collection of water contaminated by sprays applied at the correct time.

Data collected throughout the country during the last 3 years show calcium or lead arsenate to be the constituent of orchard sprays causing honeybee poisoning. Bees may collect arsenic when visiting fruit trees, or plants growing beneath or near the trees, for pollen or water, contaminated water apparently being the chief cause of poisoning; about 0.00005 mg. internal arsenic per bee has been found to be a lethal dose. When arsenical sprays alone are applied to trees, particularly if to the open blossom as is sometimes the case with gooseberries and cherries, severe poisoning from the pollen may be expected. On the other hand, much of the danger from contaminated water should be removed by the incorporation of lime sulphur (1% or more) or nicotine sulphate (0.05% nicotine) into sprays intended for application either before flowering or after petal fall, provided that alternative sources of water are available for foraging bees. Provision of water within the hive is suggested as a further safeguard.