I am trying to create a native plant garden that attracts bees and butterflies. What chemicals should I be staying away from? What questions should I ask a nursery to ensure I don't plant things that end up being harmful to bees and butterflies.
Frederick County Maryland
In general, any pesticide is intended to kill pests (insects, usually) and should be avoided. That said, the more low-impact options include horticultural oils and insecticidal soaps (because their persistence is brief) and those products more targeted to the pests, such as those derived from organisms naturally harmful to just that specific kind of pest. For example, Bacillus thuringiensis (B.t.) is a soil bacteria which makes proteins that can be toxic to some insects but harmless to birds and mammals. Strains of this bacteria are even more specific, like kurstaki (B.t.k.) that affects only caterpillars and israelensis (B.t.i.) that affects only fly larvae, like mosquitoes and fungus gnats. Spinosad is derived from another soil bacterium that produces compounds toxic to insects but safe for other wildlife. By first identifying the pest in question and judging how severe its damage is, you can decide if it is worth treating and by what means.
More study is still needed to assess the impacts of systemic (plant-absorbed) insecticides on pollinators. The degree to which such compounds are moved into flower pollen, nectar, and other secretions isn't fully understood, and may vary from species to species. Avoiding systemically-treated plants would be best, though in time such treatments will be metabolized by the plant and wear off. How long this takes will depend on many factors, like chemical used and plant growth rate. Wholesale (grower) and retail nurseries vary widely in their choices regarding pesticide use, so you can ask a retailer if they use suppliers who avoid pesticides (or systemic pesticides) and if they themselves spray their plants.
Some growers opt to use beneficials (predatory or parasitic insects, mites, nematodes, or fungi) to combat pest outbreaks, but this also means that some pests or their damage may persist in low levels on the plants for sale. This is not problematic, but since the gardening public at large prefer spotless, healthy-looking plants, this threshold for tolerable damage is a hurdle to overcome. In addition, use of beneficials would be more common at the wholesale level (and likely native-only nurseries) simply because the plants spend more time at that facility than at a retailer, making such slower-acting treatments impractical.
The rising popularity of native plant use and gardening for pollinators and butterflies means that more wholesalers are responding with plants suited to these uses - either with no pesticides or low-impact treatments only. Some area wholesalers proudly declare such practices on their websites, and while they do not (or infrequently) sell to individual consumers, they probably list or can tell you which area garden centers stock their plants.
The Maryland Native Plant Society also has a page listing area native plant vendors (not all listed are native-only), and many of these also are likely to minimize pesticide use.
Thank you for the response and the links.