Pyrethrum is a name for a group of natural insecticides that are extracted from chrysanthemum flowers (Chrysanthemum spp.), which are hardy to U.S. Department of Agriculture plant hardiness zones 5 through 9. The extract is referred to as pyrethrum, and the insecticides within that extract (the groups pyrethrins I and pyrethrins II) are referred to collectively as pyrethrins. While they are less harmful than many chemical products to humans and pets, pyrethrum-based insecticides can be toxic to other creatures in your landscape, including bees.
1. About Pyrethrins
Pyrethrins exist in chrysanthemum flowers as a combination of six chemicals. These are categorized as pyrethrins I (pyrethrin I, jasmolin I and cinerin I), and pyrethrins II (pyrethrin II, jasmolin II and cinerin II). Pyrethrum is extracted as oil from flowers of the plant, which are harvested soon after blooming as part of the drying and powdering process. Pyrethrins are used in household insect sprays, outdoor and indoor herbicides, lice treatments, and flea and tick treatments. They’re often mixed with other chemicals that help make them effective longer. These other chemicals usually include pyrethroids, which are the synthetic version of pyrethrum. Insecticides labeled as containing pyrethrum and one of these other chemicals are usually more toxic than pyrethrum-based insecticides alone.
2. Toxicity to Bees
Bees are necessary for plants that require pollination to reproduce. Any insecticides that kill bees in your landscape may also affect your plants. Pyrethrum is categorized as level three in terms of its toxicity to bees. This means it is moderately toxic and can kill bees. If you are careful in determining the method of application and timing you can avoid harming bees while treating your plants for pests such as aphids, thrips or mites.
3. Preventing Harm to Bees
To avoid harming the bees in your garden, apply pyrethrum insecticides only in the late evening, night or early morning. During this time bees are not active and are least likely to come in contact with the insecticide. Do not apply pesticides to blooming plants on nights when dew is forecast or when temperatures are expected to be below 60 degrees Fahrenheit. In these conditions, the residual effects of the insecticide are typically twice as hazardous to bees. Liquid formulations of pyrethrum insecticides are usually less hazardous to bees than dusts or granular forms, and they won’t stick to bees’ hairs. Pyrethrum dusts can be carried on the bees’ hairs back to the hive, where the insecticides may affect the queen.
4. Safe Application Tips
While pyrethrins are only mildly toxic to humans, piperonyl butoxide, a chemical enhancer used to increase the residual activity of these insecticides, is considered a human carcinogen. Be careful when applying these insecticides to minimize your exposure to these chemicals. Wear eye protection, long pants and a long-sleeve shirt, and wear a respiratory mask when spraying to minimize your exposure to pyrethrins. Always check the label before you buy or use a pesticide to ensure it is formulated for use on the pest you’re targeting. Heed all label warnings and follow all instructions listed by the manufacturer as well. Pyrethrins rapidly break down after application. To ensure you contact the most target insects possible, apply these insecticides when the pests are present.
- National Pesticide Information Center: Pyrethrins and Pyrethroids
- Scorecard: Good Guide: Chemical Profile for Piperonyl Butoxide
- California Department of Pesticide Regulation: Environmental Fate of Pyrethrins
- University of California Extension: How to Prevent Bee Poisoning
- University of California Extension: Pesticide Information: Active Ingredient: Pyrethrin
- Cornell University Cooperative Extension: Pyrethrins
- University of Vermont: Chrysanthemum (Dendranthema)
- University of Minnesota: Pyrethrum TR
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