Wednesday, July 10, 2013

Lessons Learned from the Wilsonville Bee Kill

Before I saw it with my own eyes, I wouldn’t have believed that 50,000 bumble bees forage in the trees of a suburban big box store parking lot. They do. We know that now, because at least that many died in the Wilsonville, Oregon Target parking lot in a single week in late June. 

This picture was taken on June 17th, two days after the trees were sprayed for aphids with a product called Safari. Each black dot on the pavement is a dead bumble bee, almost all of them Bombus vosnesenkii.
Picking up dead bumble bees.

The pattern was the same under each of 55 European linden trees (Tilia cordata), and the bees kept dying for a week until the City of Wilsonville led a heroic effort to cover each tree with shade cloth. Though this isn’t a traditional invasive species story, it contains lessons for invasive species warriors. 

Lesson 1: Oregon’s wild bee populations are healthy. There are a couple of known exceptions, but in general, we can be thankful that our native bees are thriving. This is important because wild bees pollinate many of our native plants and agricultural crops. Honeybees get all the credit, but native bees do much of the work. On a larger scale, healthy pollinators indicate healthy habitats; we’re lucky in Oregon to have an environment that is in pretty good shape – and worth protecting.
Lesson 2: Pesticides are powerful tools. Used correctly, they are often our most effective tools for early detection and rapid response programs (EDRR) programs.  Most eradication programs depend on them. However, they can occasionally cause problems. We need to learn from these instances to prevent them from happening again.

Lesson 3: Pesticide experts commonly refer to, “The label is the law.” That is true, but labels and laws are sometimes flawed and often open to interpretation. It could be months before the official investigation of this incident is complete and the Oregon Department of Agriculture (ODA) determines whether there was a violation. Given this parking lot site, how would you interpret the Safari label language? “Do not apply this product or allow it to drift to blooming crops or weeds if bees are visiting the treatment area.” A supplemental label adds: “For trees in forests that are pollinated by bees or other invertebrates, make applications post-bloom.”

Lesson 4: Linden nectar by itself can be toxic to bees!* Dead bumble bees are not uncommon under unsprayed linden trees. The publicity generated by the Wilsonville bee kill resulted in many more reports of dead bees. Most of them turned out to be relatively small numbers probably not related to pesticides. So far, we’ve only found one other tree in Hillsboro that was associated with dead bee numbers similar to those in Wilsonville. That tree had been sprayed with the same pesticide last March. Other linden trees along the same street receiving the same treatment didn’t have any dead bees. That will be a puzzle for the investigators!

Interestingly, honey bees weren’t affected to the same extent as bumble bees.  The honey bees were foraging on the linden flowers, but few were dying. One reason might be that honey bees are from Europe; they evolved with European linden nectar.  We’ll have to wait for the investigators and scientists to sort out whether toxic nectar played a part in this incident.

Placing netting over blooming linden trees.

Lesson 5: When people work together, they can do amazing things. On Thursday, June 20, ODA hosted a conference call bringing together officials from the City of Wilsonville, entomologists from ODA, Xerces Society, and Oregon State University, and other interested parties. We discussed how to stop the bee kill as well as a long list of less palatable options, from cutting the trees down to spraying the trees with garlic-based insect repellent. The only option that seemed to have promise was to cover the trees with bee-proof netting, and it wasn’t at all clear how to do that. City of Wilsonville staff figured it out by the end of the day. The very next day, 60 people and six bucket trucks representing eight agencies/organizations went to work. Crews stapled panels of shade cloth together, passed 40’ X 40’ sheets to bucket truck crews, and with much tugging and adjusting, the netting was draped over the 25’ tall trees. At 5:10 PM, the last net was tied off at the trunk, and the Wilsonville bee kill was over.

It was a day I’ll never forget, and none of us should ever forget the lessons we learned. 

Dan Hilburn 

*Pawlikowski,T. 2010.  Pollination activity of bees (Apoidea: Apiformes) visiting flowers of Tilia cordata Mill. And Tilia tomentosa Moench in an urban environment. J. Apicultural Sci. 54(2): 73-79.

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