Incredible Creatures: Northern Migration - The Good and the Bad of Insect Range Expansion

Common Buckeye. (Photo by Kelly Fiegle)

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Species of animals all have their preferred habitat to live in, and weather conditions in which they can survive. But what happens when things change over time? Habitat loss is a concern in some areas, but another big change for some animals is temperature. This is something that is easy to measure, and scientists have temperature records going back many decades. There is a definite trend to overall warmer temperatures, with the trend being strongest in more northern areas. In Canada, average annual temperatures have increased 1.3ºC since 1948, which is about twice the global average. So how does this affect the species that can be found in an area? Some may argue that more species could be a good thing, but it may depend on perspective. In this month’s Incredible Creatures we will explore the range expansions of some rather attractive butterflies, a tree feeding beetle, and some crop feeding insects.

Buckeyes where no buckeye has gone before
Most people like seeing butterflies. And seeing a butterfly that rarely occurs in an area can be a real treat for many. Some species that normally occur only as far as the south or central parts of the United States are starting to be seen further north, but it varies greatly with the species. One species of butterfly that is significantly increasing the range where it can regularly be found is the buckeye, Junonia coenia. This brown butterfly with superb orange highlights and interesting “eye” markings on each wing is unable to overwinter in Manitoba and was historically rarely seen in Manitoba. But recent sightings have become more common. Research in both Manitoba and eastern Canada has looked at the expansion of range where this migratory species is being sighted. Biologists from Quebec and the University of Ottawa researched the range expansion of 12 species of butterflies that can occur in Eastern Canada, including the buckeye, which they found to be undergoing a large range expansion. From 1990 to 2012 the shift in the northern edge (the average of the 10 northern most records of the species) was averaging 18.6 km per year. In Manitoba the buckeye and another butterfly called the Baltimore increased their northerly ranges by about 150 and 70 kilometers, respectively, from 1972-2004. For butterfly enthusiasts it’s also worth noting that the Gorgone checkerspot and the Delaware skipper have recently expanded their northern ranges significantly.
The period when you may be able to see butterflies flying around may be extending as well. A study here in Manitoba found that 13 of 19 butterfly species showed a significant increase in flight period (31.5 days), extending longer into the autumn over the study period (1972-2004). In a somewhat similar study in Central California, 70% of 23 butterfly species had advanced their first flight date over 31 years, by an average of 24 days. So we may have more opportunities to see more of these cool butterflies, and for longer. Great for butterfly watchers. But there can be a down side to insect range expansion, which has many that work in agriculture and forestry quite concerned.

Mountain Pine Beetles on the Move
As climates change, temperatures may no longer constrain some native plant feeding insects within their historical ranges. Such is the case with the mountain pine beetle, Dendroctonus ponderosae. Mountain pine beetle is a species of bark beetle native to the forests of western North America from Mexico to central British Columbia. It is only about the size of a grain of rice, and has a role in forest ecology. Normally they play an important role in the life of a forest, attacking old or weakened pine trees, and speeding development of a younger forest. However, unusually hot, dry summers and mild winters in their northern range, along with monocrop replantings of lodgepole pine, have led to this insect dramatically increasing its range, and unprecedented damage to pine forests. The beetles introduce a blue stain fungus into the sapwood of the tree. This prevents the tree from repelling and killing the attacking beetles with tree pitch flow. The fungus also blocks water and nutrient transport within the tree. The joint action of larval feeding and fungal colonization kills the host tree within a few weeks of successful attack.
Winter temperatures have historically limited the northern range of this insect to southern British Columbia. Outside this rather restricted area winter temperatures would at some point get cold enough to kill any beetles that attempted to overwinter. Climatic conditions have however become more favourable to this insect over large portions of Western Canada during the past 40 years. In recent years, mountain pine beetle has been spreading north and east, and has spread into western and central Alberta. Mountain pine beetle has now been detected over large areas of north-central Alberta, resulting in widespread infestations extending as far east as Slave Lake. Those dead pine trees you see as you drive into Banff National Park, that’s the impact of mountain pine beetle.
Scientists are trying to determine how far this beetle will spread; are we safe in Manitoba? The temperature at which beetles start to die is not fixed, but varies because of the larvae’s response to daily temperature fluctuations. For example, an under-bark temperature of –37°C will kill 50 per cent of a mountain pine beetle population, even in mid-winter; however, a low temperature of –20°C in the fall, before the beetles are prepared for winter, or in the spring, when beetles are starting to become more active, will also kill beetles if it is preceded by temperatures above 0°C. The relatively warmer temperature causes the larvae to start to lose their natural antifreeze, making them more vulnerable when colder temperatures return. So as long as our winters consistently get some temperatures in the -40°C range, even for brief periods of time, we should be safe from this beetle. See, there is some good to having some bitterly cold temperatures over the winter.

Watching for Migrants to Farms
As an entomologist working on behalf of prairie farmer, myself and entomologists in the other Prairie Provinces try to keep current on the overwintering habits of potential crop pests, and for some that do not overwinter here we monitor when they arrive. We collectively have a group called the Prairie Pest Monitoring Network that does this work. Expanding overwintering ranges would obviously be a concern. One of the insects that we monitor annually is a moth called diamondback moth, the larvae of which can potentially be a big concern for canola growers. Fortunately they do not overwinter well in northern climates. Concern has been expressed regarding the possibility of expanding overwintering populations of this insect towards northerly latitudes, with the potential to increase the frequency of new invasions from the south.
Aphids are another insect where we need to monitor movement patterns. Most of our major grain feeding aphids do not overwinter in the Canadian Prairies, so our Prairie Pest Monitoring Network also tries to keep tabs on when they first arrive, as earlier arrival could be a concern for farmers. In Britain a 1 °C increase in average winter temperature advanced the migration of different aphid species by 4–19 days. So we need to monitor to determine if a similar phenomenon may be happening here.
Expansion of some species to new areas may be exciting in some circumstances, particularly for those who enjoy watching wildlife such as birds and butterflies. But on the down side there are also new potential pest insects showing up or familiar ones arriving earlier. This poses tough questions and choices at many levels. So if you are still looking for a New Years resolution, why not include enjoying the many gifts nature provides and taking small (or big) steps to ensure the health of our ecosystems.
Incredible Creatures is a monthly contribution to provide information on some of the common yet often not well known creatures that we share space with in Manitoba and abroad. John Gavloski is an entomologist living in Carman.