I have hives that are close to a field that gets sprayed each year, any help to protect my bees would be nice.
It looks like there isn't much you can do because most of it depends on the farmers. With that said,
The farther colonies are away from fields or orchards that are treated with pesticides, the better chance the bees have against pesticide poisoning.
Establish apiaries at least 4 miles from crops being treated with toxic materials and subjected to drift. However, if your apiary is already located in an agricultural area where pesticide use is high, moving your bees may be the best insurance against future pesticide kills since preventing honey bees from foraging on pesticide-contaminated flowers is almost impossible.
If moving is impossible, covering colonies with a well-ventilated screen to restrict honey bee flight during peak foraging hours may be your only course of action. However, this method has dangerous side effects and could lead to higher mortality than would have occurred from pesticide exposure. Colonies may have difficulty controlling their hive temperature when confined and can easily overheat, so care must be taken. Providing water inside the screen will allow the bees to reduce temperatures. Do not keep the colony covered for more than two days. Covering the colonies with large wet burlap sacks and providing them with shade is another method for preventing overheating.
Do not leave unmarked colonies of bees next to orchards or fields. Post your name, address, and phone number in printing large enough to be read at some distance in all apiaries so you can be contacted readily to move the colonies when hazardous sprays are to be applied.
Cover honey bee colonies with wet burlap for two or three days to protect them from the initial hazards of an insecticide. Such covers should be put over the hives during the night before the crop is treated and should be kept wet during use. This method works; however, most beekeepers find it impractical.
If you can't move the bees to a safe location, it looks like your best bet is to contact the farmers and ask that they notify you before they apply their pesticides so you can try and keep the bees at home.
To tag onto Charlie's answer - I will say it depends on the pesticides in use. I will give a specific example of the application of Neem Oil. (Source: I was raising honeybees to pollinate my large pumpkin patch.)
Neem oil is sometimes utilized along with sodium bicarbonate to both treat various blights, as well as treat pests.
Neem oil works in a couple ways:
- It can suffocate pests that become coated in it.
- It will repel 'munchers' that would otherwise dine on the treated plants.
Organic neem oil would be hazardous to my bees if I applied it during the day while they were active as I could suffocate them inadvertently. By applying it late in the evening after my blooms had closed and the bees were less active, I successfully treated my pumpkin plants (within a hundred yards off my hives) and caused no ill effects to my bees.
Long story short, I would attempt to contact local farmers and coordinate with them as best as possible. This may involve you learning with pesticides they intend to use and researching their effects on your colonies - and presenting an agreeable solution to the local farmers.