Impacts of Pesticides on Honey Bees

This chapter focuses on the detrimental effects that pesticides have on managed honey bee colonies and their productivity. We examine first the routes of exposure of bees to agrochemicals used for crop protection and their application to crops, fate and contamination of water and plants around the fields. Most of the time, the exposure of bees to pesticides is through ingestion of residues found in the pollen and nectar of plants and in water. Honey bees are also exposed to pesticides used for the treatment of Varroa and other parasites. The basic concepts about the toxicity of the different kinds of pesticides are explained next. Various degrees of toxicity are found among agrochemicals, and emphasis is given to the classic tenet of toxicology, “the dose makes the poison,” and its modern version “the dose and the time of exposure makes the poison.” These two factors, dose and time, help us understand the severity of the impacts that pesticides may have on bees and their risk, which are analysed in the third section. Sublethal effects are also considered. The final section is devoted to some practical advice for avoiding adverse impacts of pesticides in beekeeping

Pesticide residues and bees--a risk assessment.

Bees are essential pollinators of many plants in natural ecosystems and agricultural crops alike. In recent years the decline and disappearance of bee species in the wild and the collapse of honey bee colonies have concerned ecologists and apiculturalists, who search for causes and solutions to this problem. Whilst biological factors such as viral diseases, mite and parasite infections are undoubtedly involved, it is also evident that pesticides applied to agricultural crops have a negative impact on bees. Most risk assessments have focused on direct acute exposure of bees to agrochemicals from spray drift. However, the large number of pesticide residues found in pollen and honey demand a thorough evaluation of all residual compounds so as to identify those of highest risk to bees. Using data from recent residue surveys and toxicity of pesticides to honey and bumble bees, a comprehensive evaluation of risks under current exposure conditions is presented here. Standard risk assessments are complemented with new approaches that take into account time-cumulative effects over time, especially with dietary exposures. Whilst overall risks appear to be low, our analysis indicates that residues of pyrethroid and neonicotinoid insecticides pose the highest risk by contact exposure of bees with contaminated pollen. However, the synergism of ergosterol inhibiting fungicides with those two classes of insecticides results in much higher risks in spite of the low prevalence of their combined residues. Risks by ingestion of contaminated pollen and honey are of some concern for systemic insecticides, particularly imidacloprid and thiamethoxam, chlorpyrifos and the mixtures of cyhalothrin and ergosterol inhibiting fungicides. More attention should be paid to specific residue mixtures that may result in synergistic toxicity to bees

Residue loads of the most common pesticides plotted against their prevalence (frequency) in: A) pollen; B) honey or nectar.

<p>Key: 1 coumaphos (total); 2 tau-fluvalinate; 3 thymol; 4 chlorothalonil; 5 acetamiprid; 6 amitraz (total); 7 dithiopyr; 8 thiacloprid; 9 carbaryl; 10 imidacloprid (total); 11 pendimethalin; 12 chlorpyrifos; 13 phosmet; 14 carbendazim; 15 atrazine; 16 thiamethoxam; 17 chlorfenvinphos; 18 fenpyroximate; 19 clothianidin; 20 endosulfan (total); 21 thiophanate-methyl; 22 metolachlor; 23 fenpropathrin; 24 methoxyfenozide; 25 esfenvalerate; 26 tebufenozide; 27 captan (total); 28 bifenthrin; 29 azoxystrobin; 30 lambda-cyhalothrin; 31 diphenylamine; 32 penconazole; 33 trifloxystrobin; 34 fenthion; 35 norflurazon; 36 metribuzin; 37 hexachlorobenzene; 38 HCH (alpha and beta); 39 phorate; 40 gamma-HCH (lindane); 41 heptenofos; 42 methiocarb; 43 DDT (total); 44 vinclozolin; 45 methidathion; 46 malathion; 47 cypermethrin; 48 dimethoate; 49 carbofuran (total).</p

Risk (% probability) and time to reach topical LD50 (T50 in days) for worker bees, estimated as contact exposure with 1 g of contaminated pollen, at average or maximum levels, during 2 days.

1<p>A = acaricide; F = fungicide; I = insecticide; IGR = insect growth regulator; IS = insecticide synergist.</p>2<p>Mixture LD50 estimated in accordance with known synergistic ratios <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-Iwasa1" target="_blank">[67]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-Pilling1" target="_blank">[68]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-Biddinger1" target="_blank">[69]</a>.</p>3<p>Chronic LD50 for 77 days exposure.</p

Life-span of larvae and worker bees and their consumption rates of pollen and honey (After [76]).

1<p>Assuming 5 times the consumption of <i>Apis mellifera</i> in the same proportion.</p

Risk (% probability) and time to reach oral LD50 (T50 in days) for larvae and workers of honey bees feeding on contaminated pollen and/or nectar at average or maximum residue levels (see Table S3).

1<p>Exposure period of 5 days.</p>2<p>Exposure period of 10 days.</p>3<p>Exposure period of 30 days.</p>4<p>Mixture LD50 estimated in accordance with known synergistic ratios <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-Iwasa1" target="_blank">[67]</a>.</p

Comparison of estimated times to LD50 (T50 range in days) for dietary exposure of honey bees to two neonicotinoid insecticides, using standard and cumulative risk approaches.

1<p>Sources: <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-Suchail1" target="_blank">[62]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-DechaumeMoncharmont1" target="_blank">[63]</a>.</p>2<p>Source : <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0094482#pone.0094482-Oliveira1" target="_blank">[73]</a>.</p