Gardening has changed a lot over the last 30 years. We use more perennials and shrubs than annuals. We practice grasscycling and leaf cycling, leaving lawn clippings and shredded leaves on the ground to nourish our lawns, which we mow less regularly to let the plants that feed our pollinators flourish. Many gardeners install meadows to radically reduce mowing or use native plants to create wildlife-friendly environments. But a major challenge is looming in Canada, crossing the border from the United States, and is threatening to roll back this fine progress: ticks.
The black-legged tick or deer tick (Ixodes scapularis), found in eastern and central North America, and the western tick (I. pacificus), found in North America west of the Rocky Mountains, are the main sources of concern in Canada and the United States. Their European cousin, the sheep tick (I. ricinus), plays a similar role in Europe. Although there are several species of tick, any mention of them in this text will refer to Ixodes, for simplicity’s sake, since they are the main vectors of Lyme disease.
The Life Cycle of Ticks
Ticks have four stages of development: egg, larva, nymph and adult. They feed only three times during their lives, once per stage after hatching.
- In spring, the female lays eggs on the ground.
- In summer, the egg hatches into a six-legged larva. It feeds on the blood of small mammals and birds, mainly the blood of the white-footed mouse (Peromyscus leucopus), the main vector of Lyme disease. At birth, the tick larva is not a priori infected with the Borrelia burgdorferi bacterium, but it may acquire it from its first host..
- In its second year of life, the larva transforms into a nymph. It is most active from May to July. It is at this stage that a nymph, infected the previous year in the larval stage, could transmit a pathogen to a human or pet. Nymphs again feed on the blood of small mammals and birds, including the white-footed mouse, and once again risk being infected by this second host. Because of their small size (2 mm), which makes them difficult to detect, nymphs may transmit pathogens to humans more frequently than adult ticks.
- In autumn, the nymphs molt into adults that seek a third, larger host to feed on and mate on, mainly cervids, including white-tailed deer (Odocoileus virginianus) or European roe deer (Capreolus capreolus). Although white-tailed deer are important tick carriers, they cannot be infected with the bacterium that causes Lyme disease, and are therefore not vectors for transmission to ticks.
Ticks in Our Gardens
Government bodies have issued a variety of recommendations to reduce the presence of ticks on our properties. The main ones are
- Eliminate tall grass and undergrowth around houses and lawn edges.
- Place a wood chip or gravel barrier 1 meter (3 feet) or wider between wooded areas and recreational areas, patios and playground equipment.
- Mow the lawn regularly and rake up dead leaves.
- Pile firewood carefully in a dry place to prevent tick-feeding animals from taking refuge in it.
- Keep play equipment, decks and patios away from wooded areas, and place them in a sunny spot, if possible.
- Keep your yard clean by picking up any garbage that may be lying around, to avoid creating habitats for small rodents.
In recent years, gardeners and the general public have become increasingly interested in biodiversity, pollinators and the health of our ecosystems in general. We recommend letting lawns grow taller to allow flowers to feed our pollinators, leaving wilted leaves and plants in our garden to feed the soil and microorganisms as they decompose, and inviting a diversity of flora and fauna into our green spaces. Government recommendations seem to be in direct conflict with a more ecological vision of our gardens.
Ticks or Biodiversity?
As with many issues, reality is far more nuanced and, in my opinion, this clash of ideas is far smaller than one might think. First and foremost, while I’m firmly on the side of biodiversity, I fully share concerns about Lyme disease, a serious condition with no cure for its chronic form, the seriousness of which cannot be ignored.
Lyme disease is caused by bacteria of the Borrelia burgdorferi sensu lato complex, and can cause skin, neurological, joint, heart and eye problems. Manifestations vary from patient to patient. Lack of awareness of the disease, even within the medical community, makes detection complex and diagnosis difficult (see Get Ticked Off: Gardening With Lyme Disease in Mind by Paul Hetzler). While an infection is usually successfully treated with antibiotics if detected early, no treatment exists for its chronic form.
If you are bitten, try to keep the tick once removed and contact a health professional. Ticks usually need to remain attached to the skin for at least 24 hours to develop the disease. For more information, visit Centers for Disease Control and Prevention, Canada.ca or European Centre for Disease Prevention and Control.
In fact, it would seem that an increase in biodiversity could reduce the presence of Lyme disease in ticks, without necessarily reducing their population. When these mites feed on several species, rather than just the white-footed mouse, which is the main reservoir of Lyme disease, there is a dissolution effect and fewer infected ticks. Certain animals are more difficult to transmit the bacterium that causes the disease. The presence of predators of the mouse, such as foxes, snakes and birds of prey, would also help reduce its occurrence.
For the time being, studies in Europe do not demonstrate as pronounced an effect between biodiversity and the presence of Lyme disease as in North America. More research remains to be done.
So, what can we do in our garden to encourage the presence of diverse flora and fauna, while protecting ourselves from the presence of ticks? Next week, we’ll look at the subject of forest fragmentation and its effect on the prevalence of Lyme disease, as well as some details on garden maintenance in at-risk areas. In the meantime, you can read Larry Hodgson’s very detailed article, A Gardener’s Guide to Avoiding Ticks.