Pesticides vary in their effect on bees. Contact insecticides, those which kill by touching the organism, affect the worker bee that is directly sprayed. Systemic insecticides, those that are incorporated by treated plants, can contaminate nectar or pollen, and kill bees in the hive.
Dusts and wettable powders tend to be more hazardous to bees than solutions or emulsifiable concentrates.
Actual damage to bee populations is a function of the degree of toxicity of the compound, in combination with the mode of application; a highly potent insecticide applied only to the soil, for instance, would be expected to kill mainly soil-dwelling insects, such as grubs or mole crickets, and not bees.
LD50 is an incomplete measure of toxicity to bees and other social insects because it is a measure of individual toxicity, not colony toxicity. It does not account for the ways in which bee behavior can mitigate or exacerbate the effects of the pesticide on the colony. For example, a moderate to low toxicity pesticide (by LD50 measurement) which is used in granular form and is collected and concentrated along with pollen can be highly lethal to the colony. On the other hand, a pesticide which is so toxic that the exposed bees die in the field can be less dangerous to the colony than a less toxic pesticide which allows the exposed bees to return to the hive and contaminate their fellows. Likewise, a highly toxic pesticide (according to LD50 measures) is "safe" for bees if it is applied on a grass lawn or other location without blooming flowers which would attract the bees. Furthermore, LD50 studies are conducted against adult bees and do not measure the effects on larvae, etc.
Bee kill rate per hive
The kill rate of bees in a single bee hive can be classified as:
Bees poisoned with carbaryl can take 2-3 days to die, appearing inactive as if cold. It allows them time to take contaminated nectar and pollen back to the colony. Some crops treated with Sevin® under the wrong conditions (in bloom, using a dust formulation, with large numbers of bees in the field) have been responsible for disastrous kills. Sevin® is one of the United States' most widely used insecticides for a wide variety of insect pests. It is also one of the most toxic to honey bees, in certain formulations. There are formulations, however, which are determined to be less toxic (see tables). Usually, applicator-beekeeper communication can effectively be used to adequately protect bees from Sevin® poisoning.
By far the most potentially damaging pesticides for honey bees are those packaged in tiny capsules (microencapsulated). Microencapsulated methyl parathion (PennCap M®), for example, is a liquid formulation containing capsules approximately the size of pollen grains which contain the active ingredient. When bees are out in the field, these capsules can become attached electrostatically to the pollen-collecting hairs of the insects, and at times are collected by design. When stored in pollen, the slow-release feature of the capsules allows the methyl parathion to be a potential killer for several months. At the present time, there is no way to detect whether bees are indeed poisoned by micro-encapsulated methyl parathion, so a beekeeper potentially could lose replacement bees for those already poisoned by the pesticide. It is, therefore, strongly recommended by experts that this formulation be used only when honey bee exposure is not a possibility.
safened by repellency under arid conditions. Permethrin is also the active ingredient in insecticides used against the Small hive beetle, which is a parasite of the beehive in the temperate climate regions.
^  Pollinator protection requirements for Section 18 Emergency Exemptions and Section 24(c) special local need registration in Washington State; Registration Services Program Pesticide Management Division Washington State Dept of Agriculture, Dec 2006
^ Hunt, G.J.; Using honey bees in pollination Purdue University, May 2000
Commonly Used Insecticides for Soybeans Kansas State University Extension, Aug 2004
Honey Bees and Pesticides, 1978, Mid-Atlantic Apiculture Research and Extension Consortium
Mayer, D.F., Johansen, C.A. & Baird, C.R.; How to Reduce Bee Poisoning from Pesticides, PNW518, A Pacific Northwest Extension Publication, Washington, Oregon, Idaho, Copyright 1999 Washington State University. Includes an extensive list of toxic chemicals such as pesticides that affect bees.
McBride, Dean k.; Protecting Honeybees From Pesticides, 1997 North Dakota State University
Sanford, Malcolm T.; Protecting Honey Bees From Pesticides, University of Florida Institute of Food and Agricultural Sciences Extension, April 1993