Pesticides vary in their effects on bees. Contact pesticides are usually sprayed on plants and can kill bees when they crawl over sprayed surfaces of plants or other areas around it. Systemic pesticides, on the other hand, are usually incorporated into the soil or onto seeds and move up into the stem, leaves, nectar, and pollen of plants. Of contact pesticides, dust and wettable powder pesticides tend to be more hazardous to bees than solutions or emulsifiable concentrates. When a bee comes in contact with pesticides while foraging, the bee may die immediately without returning to the hive. In this case, the queen bee, brood, and nurse bees are not contaminated and the colony survives. Alternatively, the bee may come into contact with an insecticide and transport it back to the colony in contaminated pollen or nectar or on its body, potentially causing widespread colony death. Actual damage to bee populations is a function of toxicity and exposure of the compound, in combination with the mode of application. A systemic pesticide, which is incorporated into the soil or coated on seeds, may kill soil-dwelling insects, such as grubs or mole crickets as well as other insects, including bees, that are exposed to the leaves, fruits, pollen, and nectar of the treated plants. Pesticides, especially neonicotinoids, have been investigated in relation to Colony Collapse Disorder. Potentially toxic effects studied in the laboratory have often been followed by field studies that fail to show effects on actual bee populations. Despite gaps in the scientific evidence, regulators have restricted the use of neonicotinoids in Europe and elsewhere largely on the basis of concerns for bee health. Classification Insecticide toxicity is generally measured using acute contact toxicity values – the exposure level that causes 50% of the population exposed to die. Toxicity thresholds are generally set at highly toxic (acute LD50 < 2μg/bee) moderately toxic (acute LD50 2 - 10.99μg/bee) slightly toxic (acute LD50 11 - 100μg/bee) nontoxic (acute LD50 > 100μg/bee) to adult bees. Pesticide toxicity Acute toxicity The acute toxicity of pesticides on bees, which could be by contact or ingestion, is usually quantified by . Acute toxicity of pesticides causes a range of effects on bees, which can include agitation, vomiting, wing paralysis, arching of the abdomen similar to sting reflex, and uncoordinated movement. Acute toxicity may depend on the mode of exposure, for instance, many pesticides cause toxic effects by