I was recently trying to fix some problems in my lawn. Unfortunately, my local gardening store didn’t know which (if any) of the chemicals they sold were safe to use around honeybees. Here are some notes on the active ingredients in some of the more common treatments sold in our area. If you find something that’s missing, email me with the active ingredients and we’ll see if we can get it added to the list.
Note: Regardless of the listing here, you must read the label carefully and follow all the instructions, especially the rules on dosage and allowable times of use. Some of these chemicals are highly toxic to humans, pets or other organisms even if they are relatively safe for the bees. And remember, underdosing can be even worse than overdosing if it leads to resistance in the target pest.
Note 2: This list is currently biased toward a listing of conventional or "synthetic" pesticides. Organic pesticides (which consist of a vast array of mineral and botanical pesticides) are largely unregulated and rarely evaluated to the same scientific standards as the more conventional pesticides. A recent study found that, contrary to the conventional wisdom that "organic is better", choosing organic pesticides over synthetic pesticides may not effectively mitigate environmental risk. If anyone has hard data on common organic pesticides, please add them to the list.
2,4-D (also called 2,4-Dichlorophenoxyacetic acid)
Sold as or in: Weed-B-Gon Max, Scotts Lawn Pro, Triplex, Triplet, Bayer Advanced
Sold as or in: Scotts Lawn Pro, Bug-B-Gon, Talstar
A contact and stomach poison which affects the nervous system and causes paralysis in insects. It is not absorbed by plant foliage. Bifenthrin is registered as a “restricted use pesticide” to only be sold to and used by Certified Pesticide Applicators because of its toxicity to fish and other aquatic organisms.
Carbaryl (also called 1-Naphthyl-N-methylcarbaramate)
Sold as or in: Sevin, Sevimol, Latox, Bug-Geta
Carbaryl is one of the most widely used broad-spectrum insecticides.
Do not apply while the plants (or any nearby weeds) are in bloom. Do not allow it to drift to areas that the bees are visiting. Used primarily as a powder, it is collected by the forager bees and taken back to the hive where it will kill the rest of the colony.  
Dicamba (also called 2-Methoxy-3,6-dichlorobenzoic acid)
Sold as or in: Weed-B-Gon Max, Trimec, Tripower, Scotts Lawn Pro, Triplet, Bayer Advanced
An herbicide used to control broadleaf weeds. 
Glyphosate (also called N-(phosphonomethyl) glycine)
Sold as or in: Roundup, Touchdown, Rodeo, Accord
A non-selective systemic herbicide absorbed through the leaves. 
Repeated studies have shown no direct effects from glyphosate on honeybees. Furthermore, some studies have concluded that the use of glyphosate on crops genetically-engineered for resistance has led to a reduction in the other, more toxic herbicides.
Other researchers, however, have noted that as glyphosate is used in commercial settings, it kills not only the weeds in direct competition with the crop but also the blooming plants in field borders and irrigation ditches, dramatically decreasing the available pollen diversity and potentially leading to poor nutrition which can increase the bees' susceptibility to stress diseases such as nosema.
Sold as or in: __
Imidacloprid is rapidly absorbed by the plant tissues and is primarily toxic only to those insects which then attempt to eat the plant. It is a remarkably safe chemical to everything else. Unfortunately, imidacloprid is preferentially expressed in the pollen. While it is not toxic to the worker bee who collects and stores it, it is thought to be harmful when it is eventually fed to the larvae. Imidacloprid was temporarily banned in France and has been implicated as a possible cause of Colony Collapse Disorder (though the reported patterns of CCD do not appear to correlate well with the known patterns of usage of Imidacloprid).   
Malathion (also called Carbophos, Maldison or Mercaptothion)
Sold as or in: Cekumal, Cythion, Fosfothion, Fyfafon, Malixol, Maltox, Sadophos, Zithiol
Mecoprop (also called MCPP or 2-(4-chloro-2-methyl phenoxy) propionic acid)
Sold as or in: Weed-B-Gon Max, Scotts Lawn Pro, Triplet, Tripower, Bayer Advanced
Mecoprop is a slow-acting herbicide absorbed by plant leaves and translocated to the roots. 
MCPA (also called 2-Methyl-4-chlorophenoxyacetic acid)
Sold as or in: Tripower, Triamine II
Neonicotinoids are a group of chemicals used as insecticides, primarily in commercial/agricultural settings though also in suburban settings to a lesser extent. Agricultural use is commonly in the form of a seed coating which is absorbed by the seedling and which makes the plant matter toxic to herbivaceous insects. Neonicotinoids are far less dangerous to mammals (including the farmers applying them) than predecessor insecticides.
Studies looking at the effects of neonicotinoids on bees have been widely reported. Neonicotinoid bee kills have been confirmed in, for example, Germany in the situation where improperly coated seeds are then applied in a manner which aerosolizes the coating as dust around the planter. Studies assessing the impact of the pesticide on bees (once properly planted) have been more ambiguous.
In March 2013, UK DEFRA (Department for Environment, Food & Rural Affairs) published an assessment of key evidence about Neonicotinoids and bees which concluded that:
- “Evidence suggests that populations of bees in free-ranging situations do not normally experience the levels of neonicotinoids that result in sub-lethal toxic effects. This, together with the dilution effect of bees not always feeding upon treated crops, is the most likely reason why field studies do not demonstrate the same effects as studies where bees are given artificial doses of pesticide.”
- “Insects are significant pollinators of crops like oilseed rape where yields can collapse in the absence of pollinators [12-18]. In the UK, neonicotinoids have been used as seed treatments on OSR for 10 years. This suggests that if pesticide use was reducing pollinator effectiveness then this would also be detrimental to crop productivity. Consequently, the claim that treatment of OSR with neonicotinoids kills pollinators is partly countered by the success of the crops themselves.”
- “While this assessment cannot exclude rare effects of neonicotinoids on bees in the field, it suggests that effects on bees do not occur under normal circumstances. This assessment also suggests that laboratory based studies demonstrating sub-lethal effects on bees from neonicotinoids did not replicate realistic conditions, but extreme scenarios. Consequently, it supports the view that the risk to bee populations from neonicotinoids, as they are currently used, is low.”
Sold as or in: Scotts Crabgrass Preventer, Prowl, Soverign, Stomp
Sold as or in: Bug-B-Gon, Spectracide, Ambush, Torbedo, Dragnet
Permethrin is a widely used insecticide and acaricide and an insect repellent. It is a pyrethroid and functions as a neurotoxin, killing on contact. Severe losses may be expected if bees are present at treatment time or within a day thereafter. It should not be applied, or allowed to drift, to crops or weeds in which active foraging takes place.  
Sold as or in: Anvil 10+10
Piperonyl Butoxide is a synergist - that is, a chemical that makes other insecticides more effective. It does not have insecticidal properties on its own. It works by interfering with the insect's ability to metabolize the other insecticide. It is often applied with pyrethroids such as Sumithrin to reduce the incidence of resistance.
Alone, it has little toxicity to bees (and even less to mammals). 
Sold as or in: Anvil 10+10
Sumithrin is a contact neurotoxin in the pyrethroid family. It is frequently sprayed as a fog to control mosquitos. It is highly toxic to bees exposed during fogging and may not be applied to blooming crops or weeds when bees are actively visiting the treatment area (though local mosquito control activities are often granted a waiver to this restriction).
Sumithrin is an oily substance with low solubility in water. It decomposes by exposure to light and air with a half-life of less than one day in the air or on plants and other surfaces subject to sunlight.   
Sold as or in:
Trichlorfon is an organophosphate with is toxic both through ingestion and contact. Low toxicity to bees when used according to the label instructions. 
Triclopyr (also called 3,5,6-trichloro-2-pyridinyloxyacetic acid)
Sold as or in: Garlon, Access, Weed-B-Gon
Triclopyr is a systemic herbicide used to control broadleaf weeds. [http://pmep.cce.cornell.edu/profiles/extoxnet/pyrethrins-ziram/triclopyr-ext.html 
|high to extremely high toxicity to honeybees||moderate toxicity to honeybees||low to non-toxic to honeybees||toxicity is still under investigation|