An Update on the Activity of the Beetle in Manitoba
By Ian Wise
Whenever an invasive insect spreads to a new habitat, climatic and environmental parameters will determine whether the insect is able to establish. The scarlet lily leaf beetle, Lilioceris lilii, has now been in Manitoba for over 20 years. Initially found in Portage la Prairie, this pest is now present throughout Winnipeg and recently was located in Brandon. The beetle was first found in North America in Montreal in 1943, and began to invade other lily growing areas of Canada and United States in the 1970’s and 1980’s with the growing popularity of potted lilies. This leaf beetle is not suited for long distance travel because of its poor flying ability, but readily has taken advantage of human’s propensity to unknowingly distribute all forms of life. Manitoba experienced the effects of this previously with the arrival of the European elm bark beetle, and is witnessing a further repeat of this tendency with the recent arrival of the emerald ash borer. Unfortunately, expect this problem to continue.
Dispersal of the lily beetle over long distances is almost exclusively done through the movement of contaminated plants, or poorly cleaned bulbs. The lily beetle is a threat to our native lilies, particularly the prairie lily. The only saving grace is the beetle is a poor disperser, and has a difficult time finding isolated populations of prairie lilies that are not located near populated areas. Bans on the movement of lilies from infested areas will effectively protect prairie lilies in remote areas. The lily beetle is not a threat to fritillaria. Adults will feed on this plant, but do not cause sufficient damage to kill the plant. Larvae are unable to develop on fritillaria, so lily beetle populations will not survive if fritillaria is the only food source for adults.
Since their arrival in Winnipeg, the lily beetle mostly has gone through major increases in its population as it methodically attacked lily plants throughout the city. At many sites, plants were almost completely eaten or damage was so severe plants either did not flower or produced only small, short-lived flowers. In the past few years, however, changes in the development of beetle populations have been observed. This all lends itself to environmental variabilities that may arise and impact the establishment of an invasive pest like the lily beetle that was alluded to earlier. In 2016 unusually warm temperatures early in the spring caused adult beetles to emerge before the lilies, greatly reducing food availability. Beetles that overwintered in more protected environments and emerged later were less affected and, because of the beetle’s high reproductive rate, produced enough offspring to enable the population to remain stable. The past two years, though, have seen the population of beetles drop noticeably. Damage to lilies has been sporadic at best, with many lily growers reporting few or little injury to their plants. What is causing this decline in beetle numbers is not known. It could be the relative absence of lilies in many parts of Winnipeg because of their removal by gardeners, coupled with better management practices by dedicated lily growers that has reduced the ability of beetle populations to flourish. Also, the prairie provinces are not an optimal environment for this beetle. Its origins in eastern Europe experience temperatures more suitable for the beetle. While sufficient snowfall usually will provide sufficient protection for adults in the prairies, hot weather during larval development in the summer can either slow larval growth or kill the larvae.
Whether the trend in the past few years are any indication of future lily beetle population levels is not possible to predict. Ecologically, the lily beetle has been in Manitoba for a very short period. Leaf beetle species typically are very genetically diverse. That means they contain individuals with a wide range of tolerances to environmental conditions. Over time individuals with the ability to tolerate suboptimal conditions if they arise will be selected through their ability to produce more offspring and become the dominant genotype in the population. The result will be lily beetles more capable of withstanding changes to their environment, and damage to lilies will be more consistent each year. The one wild card in this scenario is climate change; specifically, how fast changes occur in Manitoba relative to the ability of the lily beetle to adapt.
The establishment of the beetle has had a severe effect on the growing of lilies even though this insect is not any more damaging than other leaf beetles found in Manitoba such as the Colorado potato beetle, the sunflower beetle, and crucifer flea beetle. The major difference is all these beetles attack commercial crops where farmers apply pest management practices to minimize their impact. Outside of dedicated lily growers, most gardeners are apt to discard their lilies in favour of less labour-intensive flowers. This, despite that methods to control the lily beetle are relatively easy to apply.
An early examination of the plants soon after they emerge, will enable growers to remove adults before females have begun laying eggs. Continuous inspection for adult beetles, eggs and larva, the latter which may be found on the undersurface allows early manual removal before damage to the plant occurs. A final observation in early August should be done to remove any adults that developed on neighbours’ plants and had migrated to feed on other lilies. Once the beetles have pupated, adults feed voraciously to build up fat reserves for overwintering, and generally overwinter at or near the base of lily plants. One cultural practice that might provide some value is the removal of leaf litter from areas around lilies either in the fall before snowfall or after the snow has melted in spring. The fall removal would expose overwintering adult beetles to colder temperatures that may increase their mortality. It may also increase the mortality of the lilies, so it is a bit of a gamble. The early removal of leaves in the spring will expose overwintering adults to warmer temperatures and induce them to emerge early relative to the emergence of the lilies. This will result in beetles being present before lilies have emerged and asynchronize beetle development with the lilies. Lilies will be subjected to less feeding damage and beetles will find fewer food and egg laying sites.
I still grow dozens of lilies and have never used insecticides to control the lily beetle in my garden. However, formulations that contain pyrethrin can be applied if the larvae are found on the undersurface of leaves. Excellent coverage that directly contacts the larvae is needed.
One final note is a parasitic wasp, Tetrastichus setifer, of the lily beetle has been found to be able to overwinter in Manitoba. This opens the possibility of establishing this wasp in Manitoba and permanently reducing lily beetle populations to levels conducive for most gardeners, as has been the case in other areas of North America where the parasitic wasp was introduced.
Ian Wise is the chair of “The Prairie Garden” and Lecturer of the Assiniboine Community College Master Gardener Program. He has a BSc in Biology and MSc in Pest Management from Simon Fraser University. From 1980-1988 he worked as a Research Coordinator for Hoechst Canada and from 1988-2013 with Agriculture Canada at the Cereal Research Centre. With Agriculture Canada he mostly worked with plant breeders on developing spring wheat cultivars with resistance to the wheat midge