129
It is a general principle that insecticides should never be applied to plants while they are flowering. However, there are occasions in which a crop must be treated for a serious pest even while the crop is in bloom. Fortunately, there are ways to reduce bee kill even in these situations.
First, it is important to underscore that not all insecticides are equally hazardous to bees, and bee species vary in their susceptibilities (Mayer et al., 1994b). Appendix 3 rates some pesticides according to their relative hazard to bees based on the chemical’s toxicity and its persistence after it is applied.
Secondly, the formulation of an insecticide affects its toxicity to bees. Granules and solutions are generally less hazardous than wettable powders and dusts. Non-sugary bait formulations are also relatively safe. For example, carbaryl-laced wheat bran flakes used to control rangeland grasshoppers are relatively safe to alfalfa leafcutting bees (Peach et al., 1994, 1995).
Third, many insecticides are deadly to bees when they are first applied, but they degrade within hours to relatively non-hazardous levels. This means that certain acutely toxic insecticides can be applied in the late evening, and by morning the pest will be controlled and the residues will be sufficiently degraded so that day- flying bees can forage with relative safety (see Appendix 3). One exception to this scenario will be of concern to growers who rely on bumble bees or squash bees for pollination. These bees frequently spend the night in cucurbit flowers, and evening insecticide applications are no guarantee for their safety.
Finally, an emphasis on pest scouting, treatment thresholds, precision sprays, and other practices of integrated pest management (IPM) can be expected to reduce the number of sprays needed per season. IPM practices since the 1960s and 1970s in the western US have reduced the number of lucerne sprays per season from 6–8 to 1–3 with a noticeable benefit to pollinators (Peterson et al., 1992).
Insecticidal bee kills are sometimes unacceptably high even in crops that bees rarely visit. This has been the case for bee-keepers in the southwestern US who report large bee kills following insecticide applications to blooming Bermuda grass grown for seed. In this case, bees apparently are exposed to the insecticide when they are forced to forage on Bermuda grass for lack of better forages (Erickson and Atmowidjojo, 1997). Moving bee colonies away from the area seems to be the only practical solution to a problem like this. In other cases, the bee exposure results when bees visit blooming weeds that are within the drift zone of a spray applied to a crop; Erickson et al. (1994) suggest clean cultivating to reduce this problem in snap bean.
Research in biological and botanical insecticides is yielding products that are safer for bees. Appendix 3 lists some of these
130 Chapter 14
products, usually ones with the active ingredients Bacillus thuringiensis and diflubenzuron. Many of these products are specific to particular pest species and are relatively innocuous to non-target organisms. However, not all B. thuringiensis insecticides are safe for bees. For example, the label for XenTari®(Abbott Laboratories), with active ingredient B. thuringiensis aizawai, reads ‘This product is highly toxic to honey bees exposed to direct treatment. Do not apply this product while bees are actively visiting the treatment area.’
Nevertheless, many biological insecticides are relatively safe for honey bees and may, in some cases, allow growers to spray crops during bloom, something inadvisable with ‘harder’ chemicals. For example, treating blooming canola with azadirachtin (an extract of the neem tree) at concentrations of up to 150 p.p.m. did not repel honey bees or other pollinators, and no residues of azadirachtin were found on the bodies of bees foraging on treated plants (Naumann et al., 1994a). Certain doses of the insecticidal fungus Metarhizium flavoviride control pest locusts but kill relatively few honey bees (Ball et al., 1994). These kinds of pesticides may be promising alternatives when growers are faced with emergency pest outbreaks during crop bloom.
One should always avoid untested mixes of two or more pesticides because the combined activity can be unpredictable.
Sometimes a blend of two or more pesticides can be more toxic than any one of the ingredients by itself. This interactivity is called synergism. As an example, the insecticide Karate® is already highly toxic to honey bees (see Appendix 3), but when blended with the fungicides Impact® or Sportak® its toxicity increases up to 16 times (Pilling and Jepson, 1993).
Herbicides, plant growth regulators, and fungicides are relatively safe for bees but there are exceptions such as the herbicides 2,4-D and MSMA, the growth regulator carbaryl, and the fungicide binapacryl (Johansen and Mayer, 1990; Drexel Chemical Co., personal communication). Appendix 3 has bee toxicity information on certain herbicides and fungicides.
Sometimes insecticide-damaged honey bee colonies can be rehabilitated. The first step is to move hives away from the hazard.
The colony may recover on its own if only the older adult population was affected and if it has plenty of honey and pollen. However, if brood and nurse bees continue dying then this means that the pollen is contaminated. In this case all combs with pollen must be removed.
It may be possible to salvage these combs by soaking them in water for several hours, washing the pollen out of the cells, and air-drying, but it is safer simply to discard them. Weakened colonies can be fed to stimulate brood production. They can be strengthened with the addition of queenless packages of bees or combined together to make
Bees and Pesticides 131
up stronger colonies. It is important to quickly ascertain if the queen is still functioning normally and replace her if there are signs of reduced egg-laying capacity.
Obviously, pesticide applications must be clearly worked out between the grower and the bee-keeper. It is simple common sense to protect oneself with a pollination contract. Most contracts require the grower to give the bee-keeper a 24- to 48-h notice of a pesticide application (see Appendix 2).
Choosing the correct pesticide for any pest situation is a weighty matter. First, a pesticide should be used only if it is approved by governing authorities for the particular pest and site in question.
Secondly, it is important to consider the best ways to protect groundwater and to conserve beneficial species such as pollinators and pest predators. Thirdly, there are sometimes other pest control practices available that can reduce or eliminate the need for pesticides.
It is a blessing of the developed world that there is an abundance of information resources available to help farmers produce food and fibre in an environmentally responsible manner. Agricultural universities, extension services, crop consultants, and government departments of agriculture are good sources of research-based, environmentally sound pest control recommendations.
132 Chapter 14