July 20, 2019

Plant Buyer, Be Wary

by Amy Campbell, Master Gardener

Plant Buyer, Be WaryThere is nothing more fun for me as a gardener than to poke around nurseries in the spring looking for plants! It was gardening that got me into beekeeping because I thought bees would be perfect companion animals in the flowery landscape. Several years later, it is my interest in bees and my learning about some of the problems associated not only with honeybees but also with native bees (Maine has more than 270 species) that is taking me back to gardening. The article “Nectar pointers” in Financial Times   that I read this winter suggested that backyards can offer varied habitats for pollinators, complete with a plentiful selection of nectar- and pollen producing plants, and could even provide places for them to survive, safer havens than intensely managed agricultural areas, for example. This idea adds a greater purpose to the pleasurable aesthetic pursuit that ornamental gardening truly is. To foster that idea, our bee club started a new project working with nurseries to help them identify for shoppers good foraging plants for bees. But a worm of a thought brought a concern – what pesticides do nurseries use and could they possibly present a problem for bees once the plants came home?

One pesticide that is used in nurseries and greenhouses as well as an active ingredient in a myriad of home and garden products is imidacloprid. This and other compounds in this class of insecticides with the tonguetwisting name of neonicotinamides, such as chlothianidin and thiamethoxam, are rated “ highly toxic” to bees where the lethal dose that will kill 50 percent of a population is less than 2 micrograms per bee. An acute lethal dose for these insecticides is less than 50 nanograms. There are some people that suggest that imidacloprid is a main causative agent of colony collapse disorder or CCD. “Marathon” and “Benefit” are formulations that are specifically geared toward nursery/greenhouse use. As are all of the neonicotinamides, it is a systemic pesticide that is taken up by the plant either through a root or leaf application. From these areas it spreads throughout the plant and can be found in (as opposed to on) leaf and flower, as well as in the plant’s pollen and nectar, even long after (up to 540 days) the application is made. There are a few studies that show that this indirect exposure through foraging can be harmful to bees and other beneficial insects, an unfortunate unintended consequence.

Information about how plants are grown is often difficult or impossible to obtainInformation about how plants are grown is often difficult or impossible to obtain. Many of the nursery/greenhouse plants in Maine are grown out of state so even if a local retailer doesn’t use systemic pesticides, that does not mean they haven’t been treated. If plants are started as small plugs and shipped to be grown to a bigger size, they might come with a burden of chemicals that persist throughout the growing season and beyond, for imidacloprid is known to last up to two years in nursery pots. Mail-order plants might be treated due to quarantine issues. (I recently opened a box of plants from a nursery from a state that has Japanese beetles and gypsy moths, both federally quarantined pests. Despite the fact that Maine already has both of these pests, the plants had been treated in this case with a non-systemic pyrethroid insecticide.) Furthermore, even though it might not migrate through the soil once the plant is put in the garden, if other plants are nearby and they are vigorous growers, their roots might take up the pesticide adding to possible exposure for foraging insects. Add in the research that indicates that chronic exposure of bees to minute doses of imidacloprid can be actually more toxic than one larger dose. Little is known about the effect on solitary native bees, some of which are important crop pollinators besides their role as a keystone organism in the ecosystem.

I recently received an email from Mace Vaughan, Pollinator Program Director of The Xerces Society, an organization dedicated to invertebrate conservation, that echoes my concern. “Indeed you highlight a problem that causes us grave concern at The Xerces Society,” he replied to my query whether systemic pesticides in nursery plants could be harmful. He continues, “Unfortunately for beneficial insects, ornamental plants may be treated with significantly higher doses of these chemicals than plants grown as food crops. The result is that home gardeners may be growing poisonous plants without even knowing it.”

My hunt for scientific studies continues and I plan to follow up this brief article as more information is available. One lab in Minnesota that has research underway with bees is currently working on a website. In the meantime, there are several things a gardener concerned about pesticide use can do.

  1. Grow plants from seed. There are many great bee fodder plants that are annuals and easy to start at home. And it’s not too late!
  2. Try to find smaller local nurseries that grow without the use of chemicals. Even organic pesticides can be dangerous.
  3.  If you do purchase plants via mail-order, place the order by phone so you can ask what chemicals have been used.
  4. If you already have plants that attract bees and other pollinators such as butterflies and flower flies, keep a list and when you can, try to divide them to increase their number. Bees especially like large swaths of the same kind of plant, not just onesies and twosies.
  5. If you do shop for plants at typical garden centers, be sure to ask what they use. If they don’t know, ask if they can find out. The more people that ask, the more the issue will be looked into. Ask specifically about systemic insecticide use—Marathon and Benefit in particular. Often imidacloprid, the active ingredient, will not be recognized, but the product name will be. And tell them why you want to know.
  6. According to state horticulturist Ann Gibbs, woody plants are not as likely to be treated with pesticides. There are many shrubs and trees that bees find very enticing. Native selections would be great choices since they attract a much larger number of species than nonnatives!
  7. Perhaps, and this is just an idea on my part, you can find older potted plants that are hiding in the back of a nursery for a couple of years and haven’t had any treatment in that time. They might not look good for a year but have patience.

I don’t want to be an “alarmista” but rather a “cautionista” when it comes to this information. There are other perhaps more distressing stories about bees coming in direct contact with systemic neonicotinamides, especially near farmlands. And unfortunately, homeowners have a huge array of products containing imidacloprid available to purchase for use in the garden. Often the tendency is to think that “more is better” despite the label instructions. We can’t keep the bees at home, but if we are already avoiding insecticide use on our gardens and are planting specifically to attract a good variety of pollinators and to help increase their numbers, it just seems sensible to be wary when adding nursery plants to the collection.

Many thanks to Gary Fish, Maine Board of Pesticides Control, for all his help answering pesticide questions and also Lois Berg Stack, Extension Specialist, Ornamental Horticulture, for our email correspondence which has inspired a lot of my thinking about gardening.

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