Northeasterners may very well see a spike in tick-borne disease during this spring and summer, but not because of the non-winter we’ve had. The numbers of blood-seeking, disease-infected black-legged nymphal ticks will be due to a series of events that began with the acorn crop of 2010.
You may recall having to rake, and rake, and rake acorns off your lawn in 2010. It was a bumper acorn crop that led to an increased white-footed mouse population in 2011. More food equals more mice. Not only are mice the preferred host for Ixodes scapularis – black-legged ticks – but mice are really good at transmitting Borrelia burgdorferi – Lyme disease – to ticks. Actually, mice are better at infecting tick larva than other tick hosts such as deer, birds, squirrels, chipmunks, fox and opossum. More food equals more mice equals more disease-infected tick larva.
This surge in 2010 was followed by very few acorns in the autumn of 2011. Since acorns are a major forage food of white-footed mice, fewer acorns equals fewer mice. Fewer acorns last autumn also led to particularly heavy deer browsing of ornamental plants, shrubs and trees and extremely persistent squirrel marauding of bird feeders during the autumn of 2011 and non-winter of 2011-2012, but back to mice and ticks.
Ticks need blood meals in each of their three life-cycles. Tick larva, the one-year olds, hatch free of disease but become disease transmitters after feeding on a diseased host. Larva drop off hosts after feeding and grow into nymphs. Tick nymphs, the two-year-olds, feed on hosts, drop off, and grow to adults which feed on hosts to allow them to reproduce what will become tick larva. The larval ticks of 2011 will become the nymphal ticks, which are as small as a poppy seed, of 2012. Nymphs will seek out blood hosts from May through July. With fewer mice available as hosts, nymphs will seek other hosts. To a hungry nymphal tick, people, dogs and cats are just as attractive as any mouse.
I heard Dr. Richard S. Ostfeld, from the Cary Institute of Ecosystem Studies in Millbrook, NY, explain this during his presentation at New England Grows in February. Ostfeld’s and colleagues’ work of more than two decades is also the focus of the article, Lyme Disease Surge Predicted for Northeastern US: Due to Acorns and Mice, Not Mild Winter, in ScienceDaily.
One aspect of Ostfeld’s work I found intriguing was his research on host permissiveness – analysis of where hosts travel, what they eat, how many ticks they support, and how well hosts transmit Lyme disease to black-legged ticks. Ostfeld’s group caught small hosts, kept them until they were free of ticks, re-inoculated them with larva (remember the larva are born disease-free), then tested the number of ticks that survived and were infected with Lyme disease. Ostfeld reported mice as the most efficient at transmitting Lyme disease to ticks. He said 92% of ticks from mice were infected, as were 55% of ticks on chipmunks, and just 4% of ticks on opossum. He listed percentages of infected ticks from other mammals as well, but mice, chipmunks and opossum were the three that stood out to me.
Ostfeld explained that grooming has a lot to do with the number of ticks that survive on hosts. Apparently opossum are very good at killing ticks while grooming. Mice are not. On the other hand, Ostfeld noted, mice are very good at surviving in the fragmented environments humans have created by building homes and communities in what was formerly forest or meadow.
My experience in south-central Connecticut matches what Ostfeld outlines. Tons of acorns in 2010, tons of mice – many in the house – during the 2010-2011 winter. No acorns in 2011, and few mice in the winter months of 2011-2012. Ostfeld also said to expect increased adult tick activity during mild winters. Adult ticks become active and seek hosts when temperatures are a bit above freezing. During a wooded walk on a warm February 2012 day my camera caught this little beauty crawling on my pants. I found two other adult ticks doing the same. Adult ticks can also transmit disease, but are less likely to do so than nymphs. Needless to say, a tick check followed my hike.
We cannot completely avoid ticks while living in the Northeast, and gardeners, by the very nature of what we do, are at high risk of becoming a tick host. Tick numbers are highest in forest interiors, second highest at the edges of wooded or shrubby areas, and lower in cut lawns. While doing any outside activity it’s advisable to wear light colored clothing so you can more easily see and pick off any ticks and the tiny nymphs. While hiking or gardening it’s best to wear long-sleeved shirts and long pants tucked into your socks or boots. How many gardeners actually dress this way in the heat of summer? Certainly not me. Therefore it’s really important to make a full-body tick check part of your daily routine. Finding and removing ticks from your skin early lessens the likelihood they will transmit disease to you.
Garden thoughtfully … and remember those daily tick checks,
Read more about tick habitats:
Lyme-ticks thrive in Japanese Barberry thickets and More on the Japanese Barberry-Lyme Tick Connection