Spray the Pest & Spare the Bees

It depends on three things: how toxic the product is to bees, whether the crop or weeds beneath it are in bloom, and whether bees are foraging at the time you spray. The calculator combines all three. The single most dangerous case is spraying a highly toxic product onto a blooming crop while bees are actively foraging (roughly 06:00–20:00 above about 13 °C / 55 °F). The safest case is a low-toxicity product, or any product applied in the evening after foraging has stopped so residues can weather overnight.

RT25 is the number of hours after a foliar application until weathered residues fall to a level where fewer than 25% of exposed bees are killed on contact. It is the standard residual-hazard measure from PNW 591. A product like spinosad has a short RT25 (~3 hours, so it is safe once dry), while carbaryl has a very long RT25 (~75 hours), meaning residues remain hazardous to foraging bees for days.

Honey bees typically forage from about 06:00 to 20:00, with the heaviest activity from mid-morning to late afternoon. Foraging requires warmth — below roughly 13 °C / 55 °F bees stay in the hive. This is why the recommended pollinator-safe spray window is the evening, after foraging has ended, when a product also has the overnight hours to weather before bees return at dawn.

Bees visit flowers, not foliage. If the crop is in bloom — or there are flowering weeds beneath it — foragers are in direct contact with the treated surface, so a toxic spray reaches them immediately. If nothing is blooming, foragers have little reason to be on the crop, which sharply lowers the hazard. Mowing flowering weeds before you spray is one of the simplest ways to protect bees.

The neonicotinoids (imidacloprid, clothianidin, thiamethoxam), most organophosphates (chlorpyrifos, malathion, dimethoate), the carbamates (carbaryl, methomyl) and pyrethroids (lambda-cyhalothrin, permethrin, bifenthrin) are all rated highly toxic to bees. Carbaryl is among the worst because it combines high toxicity with a very long residual (RT25 ≈ 75 h). The dataset table below lists the toxicity class and RT25 for each.

Spinosad is moderately toxic to bees when the spray is wet, but its residue weathers quickly — RT25 is about 3 hours. That means it is hazardous only briefly: if you apply it in the evening after foraging stops, it dries and becomes essentially safe before bees return the next morning. It is one of the more bee-friendly options when timed correctly.

Use the RT25 value for the product. Residues remain hazardous for roughly that many hours after application. For example, a 24-hour RT25 product applied at 21:00 still has hazardous residue through the next morning's foraging, so it can flag a caution even though you sprayed at night; a 3-hour RT25 product applied at the same time has cleared well before dawn. The calculator draws this residual arc on the 24-hour clock.

Often yes for short-residual products, because bees are not foraging at night and the residue weathers before morning. But for long-residual products (high RT25), even a night application leaves toxic residue on the bloom the next day, so spraying is still hazardous. The tool checks whether the residual window overlaps the next foraging period and flags caution when it does.

Most herbicides, such as glyphosate, are practically non-toxic to adult bees on contact and carry no meaningful residual bee hazard, so the tool rates them safe. The caution with herbicides is indirect — removing flowering weeds reduces forage. The acute spray-kill risk that this calculator addresses is mainly an insecticide and, to a lesser extent, fungicide concern.

Yes. Below about 13 °C / 55 °F bees do not forage, so even a daytime application during bloom can be low-risk if it is cold. The calculator treats foraging as active only inside the daily window and above that temperature threshold. Always confirm local conditions; a warm, sunny midday during bloom is the highest-risk combination.

The recommended window opens at the end of foraging in the evening (around 20:00) and is sized so the product's residue weathers below its RT25 before bees forage again the next morning. Short-residual products can be sprayed any time after dusk; long-residual products need to be applied earlier in the evening — or avoided in bloom altogether — so the residue clears in time.

No. The label's bee-hazard box is the legal authority and may carry application restrictions (for example, no application to blooming crops where bees are foraging). This calculator is a planning aid built on PNW 591 and UC IPM data to help you choose a safer product and time; always read and follow the label and notify nearby beekeepers before spraying.