ABSTRACT

Over the past two decades, monarch butterfly populations have been declining. This decline has been partly attributed to the extensive loss of breeding habitat through the reduction of common milkweed (Asclepias syriaca, the larval host plant) through herbicides. While the decline of milkweed has been well documented in the US, less is known about its decline in Canada. To take a first step in quantifying its potential decline, we compared roadside milkweed abundance at sites around Ottawa, Ontario, Canada in 2018, to a county-wide estimate of milkweed abundance in 1943 and 1944. We also evaluated the effect of roadside characteristics on milkweed abundance at these sites. Current milkweed density in the Ottawa region is 33–86% lower than it was 75 years ago. We found milkweed at 67% of our 100 sites and found less milkweed at sites with a higher number of lanes in the road adjacent to the roadside. Interestingly, mowing indices were not significant predictors of milkweed abundance. Here we document the first quantitative evidence for milkweed decline over the past 75 years in Canada, which has likely contributed to the decline of breeding monarchs in Canada.

摘要

在过去的二十年里，帝王蝶的数量一直在下降。这种下降的部分原因是由于常见马利筋（ Asclepias syriaca）的减少导致繁殖栖息地的广泛丧失，幼虫寄主植物）通过除草剂。虽然马利筋的衰退在美国已有充分的记录，但对其在加拿大的衰退知之甚少。为了迈出量化其潜在下降的第一步，我们将 2018 年加拿大安大略省渥太华附近地点的路边乳草丰度与 1943 年和 1944 年全县范围内的马利筋丰度估计值进行了比较。我们还评估了路边特征对这些地点的乳草丰度。目前渥太华地区的乳草密度比 75 年前低 33-86%。我们在 100 个地点中发现了 67% 的马利筋，而在靠近路边的道路上车道数量较多的地点发现马利筋较少。有趣的是，割草指数并不是乳草丰度的重要预测指标。