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PLNA e-News: Plants, Pests & Diseases

AmericanHort Warns: Milkweeds and Systemic Insecticides? Please Don’t!

Monday, May 15, 2017   (0 Comments)
Posted by: Craig Regelbrugge, AmericanHort
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MilkweedThere’s a chain email making its way around the country, forwarded by garden enthusiasts up in arms over the allegation that Home Depot is selling Asclepias spp., milkweeds, bearing a label indicating that the plants have been treated with a neonicotinoid.  Yikes!  Leaf-feeding caterpillars and systemic insecticides probably aren’t going to get along very well. 

Of course, many milkweed species attract an array of pollinators and other beneficials, and some are obligate hosts for the iconic monarch butterfly caterpillar.  The migratory monarch faces habitat loss where it overwinters in Mexico, and is the focus of a massive restoration effort across North America. In this instance, our industry can be part of the problem or part of the solution.

What the Experts Say

I asked our pollinator task force members, and our own in-house scientist Dr. Jill Calabro, for perspective. They suspect one of two things is going on. 

On one hand, the plants perhaps were treated with a neonicotinoid, as labeled. This could be a problem for leaf-eating caterpillars and perhaps other pollinators and beneficials, depending on treatment details and possible insecticide residues. It’s also a public relations black eye for the industry.

Or, maybe the plants were NOT treated at all, but an overly cautious grower inserted the standard Home Depot label anyway. If this is the case, pollinators won’t be harmed, but our industry still gets the black eye, and activist groups pushing for outright pesticide bans will reload their grenade launchers... 

Aphids and Thrips

Regardless, we know that many Asclepias varieties are susceptible to aphid infestations. Though effective, neonicotinoids should not be used here. Alternatives include horticultural oils, pyrethroids, pymetrozine (Endeavor), and several biological controls. As always, check labels before making applications. Thrips, on the other hand, have fewer alternatives, including a predatory mite, and planting Asclepias varieties that are resistant to thrips. Dr. Dan Potter, University of Kentucky, is evaluating thrips resistance in Asclepias varieties, research partially funded by our Horticultural Research Institute. One final thrips control strategy is to simply move the plants outside as soon as the frost-free date has arrived; thrips typically become a non-issue in the field.

Know the Best Management Practices

It’s important for the industry to have tools in the toolkit to effectively manage invasive pests. Equally important that we use them wisely. Under Jill’s guidance, our Horticultural Research Institute recently published a set of best management practices for protecting bee pollinators.  The BMPs will be refined and expanded as research results come in. 

But please be aware that the BMPs focus on flowering plants and bee pollinators. Leaf-feeding larvae merit special consideration, too, especially for plants we are explicitly growing to attract and sustain butterflies. Let’s be part of the solution!  


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