Wriggly, crawly, buzzy little friends
The actions of beneficial insects are responsible for more than $1 billion in crop production in Canada every year. Predatory and parasitic insects reduce pest populations.
November 6, 2008 By Carolyn King
Small but mighty:
beneficial insects have big benefits for crop production.
|An Aleochara bilineata larva parasitizing a pupa of the root maggot (the pupal case has been cut away to show the root maggot pupa and the little parasitic beetle feeding on it).
Photo courtesy of
L. Dosdall, University of Alberta.
The actions of beneficial insects are responsible for more than $1 billion in crop production in Canada every year. Predatory and parasitic insects reduce pest populations. Insects like bees and butterflies pollinate plants. And some soil-dwelling insects help decompose organic material, turning it into a form that crops can use. Although there is a lot to learn about how to encourage beneficial insect species, there are various options to help these little friends be part of a grower’s integrated crop management system.
Enemies of crop pests
“It’s hard to over-estimate the role of natural enemies of crop pests, but it is pretty profound,” says Dr. Lloyd Dosdall, an entomologist at the University of Alberta in Edmonton. He studies predatory and parasitic insects, or parasitoids, that help control crop pests.
Dosdall points out that it is almost impossible to artificially replace the beneficial effects of natural predators and parasitoids, with their many ingenious ways of controlling crop pests. For instance, an amazing little beetle, called Aleochara, is a mighty foe of the root maggot, a canola pest. Each voracious beetle eats, on average, 23 maggot eggs per day, and when root maggot eggs hatch into larvae, each beetle eats about three larvae per day. Then, when the beetle’s eggs hatch, the tiny beetle larvae feed as parasitoids on the root maggot pupae. Dosdall says, “If you add up the number of days that the beetles are active and the number of beetles there are, they have an enormous impact. They are marvellous natural enemies of root maggots, completely adapted to our system.”
Other natural enemies also have adapted to regional cropping systems. For example, Dosdall and his students have found 14 different species of native Alberta parasitic wasps that have switched from their native weevil hosts to attack the millions of larvae of cabbage seedpod weevil in canola pods in southern Alberta. This weevil was introduced to North America from Europe in 1931, and was in southern Alberta by 1995. Most parasitoids are wasps or flies. The adult parasitoid does not feed on insects; it travels around laying eggs on or near host insects so the parasitoid’s larval stage can live off the host. Dosdall’s research shows parasitoids are extremely important in bringing bertha armyworm outbreaks to an end. The outbreaks tend to last about three years and then the armyworm populations decline. He explains, “Early on in the outbreak phase there were hardly any natural enemies. Then gradually the parasitoids built up, and finally they built up to such a level that they were quite effective in decimating bertha populations. That really speaks to the importance of limiting insecticide use, because the insecticides that kill bertha armyworms will also kill the parasitoids inside them.”
|A parasitoid wasp (Banchus flavescens) with the larva of a bertha armyworm.
Photo courtesy of Peter Mason, AAFC.
Along with specialist predators and parasitoids that excel at attacking particular insect pest species, there are also generalists who prey on various pests. Examples of these generalists include lady beetles, green lacewings, big-eyed bugs, ambush bugs and beetles in the Carabidae family, or carabid beetles. Dosdall’s students are investigating carabid beetles in studies at Edmonton and Lacombe, Alta. He says, “We’re finding very large numbers of carabid beetles and many species of them. And the more we learn about them, the more fascinating they become. Some of them crawl right up on weeds and feed on weed seeds. Some of them scoop up crop seed, like canola seed that gets spewed out on the ground by combine action, so that there are fewer volunteer weeds growing in the springtime. But the vast majority of the beetles feed on other insects, like diamondback moth larvae or bertha armyworm larvae, and a number of species consume huge numbers of root maggot eggs and larvae.”
Dosdall notes, “One thing that strikes me so dramatically is that we know the species of crop pests, but we know very little about beneficial insects. For example, one of my students documented 56 different species of ground beetles at one site at Lacombe. When you go through the textbooks to find out how these beetles complete their life cycles, where they overwinter, what they prefer to feed on, how they function in the system, all we know is their name.” Similarly, all that is known about the 14 parasitoid wasps of cabbage seedpod weevils are their names.
So it is no surprise that much of the natural enemy research conducted by Dosdall and his students aims at discovering the life cycles and behaviours of these predators and parasitoids, and to ultimately determine how to encourage these insects. For instance, one of his students is evaluating canola production practices, like seeding dates and rates, and conventional versus zero till, to determine what combination of conditions favour Aleochara so farmers can make the most of this beetle’s ability to control root maggots.
|Leafcutter bees are important pollinators for hybrid canola production.
Photo courtesy of Bruce Barker.
While some plants rely on wind, water or gravity for pollination, out-crossing plants, like alfalfa, clovers, trefoil and many fruits and vegetables, are very highly dependent on pollinators. Canola is a special case, explains Dr. Ken Richards of Agriculture and Agri-Food Canada in Saskatoon. “Pollinators visit canola and get a lot of nectar from it, but pollinators aren’t a necessity for seed production. However pollinators are very much required for things like creating hybrid canola, to cross different canola lines.”
Various types of insects act as pollinators; the type of pollinator depends on the plant species. For many of the out-crossing crops grown in western Canada, bees are the most important pollinators. The best known bee types are the ones used commercially: honeybees and alfalfa leafcutter bees, neither of which are native to North America, and bumblebees, which are native.
Honeybees and leafcutter bees increase yields of various field crops. Bumblebees are used in greenhouses to pollinate crops like tomato, resulting in larger and more uniform fruit, and saving greenhouse operators the cost of hiring people to manually pollinate the plants.
Richards has experimented with using commercially reared bumblebees for pollinating field crops. The crop he worked with was cicer milk vetch because bumblebees are the best pollinator for this particular forage legume. He says, “The crop did extremely well and we did get increased seed yield close to the bumblebee colonies. However, the seed yields tended to decrease as you moved away from the colonies, and it cost a lot of money to place commercially reared bumblebee colonies in the fields. The added value of the cicer milk vetch seed would not have been enough to compensate a grower for the purchase cost of the colonies, so it was not practical.”
Other native insect pollinators in Canada include a few thousand bee species as well as butterflies and moths. Richards notes, “Other native pollinators do play a role in crop production, but we don’t know how much of a role. We don’t understand their basic biology well enough and we certainly don’t know how to manage them.”
Experience shows native pollinators can play a big role under the right conditions. Richards cites an example. “During the 1940s and early 1950s, and maybe earlier than that, we used to get tremendous yields of alfalfa seed on the Canadian prairies; 1000 kg/ha was not uncommon. The seed producers grew their crops in the prairie in long narrow strips and relied on native bees to forage in those areas. Then we started to increase the field size, increase pasture grazing, and cultivate more land. The seed yields dropped significantly, and growing alfalfa seed on the prairies almost disappeared. Alfalfa seed production moved into the parkland areas of Alberta, Saskatchewan and some of Manitoba. They started with smaller fields mainly surrounded by trees; there were a lot of native bees living in the trees and around the fields. However, as we cleared more land and made the fields larger, we again destroyed the nesting sites for all those bees, and again alfalfa seed yields dropped.”
With the introduction of commercial leafcutter bees, alfalfa seed production has once again improved, and we are again self-sufficient in seed.
Many native bee species nest on or just under the ground surface, and those nesting sites disappear with repeated cultivation for crops. Grazing intensity affects both the survival of nesting sites and availability of food for bees. Richards’ research on native bumblebees shows very heavily grazed pastures have the fewest number of flowering plants and the fewest number of bees.
Loss of habitat is clearly an important issue for native bees, but Richards explains bee populations face additional threats. Diseases and some predator insects and mites can have serious effects on honeybees and leafcutter bees. Although native bees are known to be subject to diseases and mites, not much is known about the impacts on the bee populations.
Another challenge influencing bee populations is the use of insecticides. Richards says, “Each year in Canada and around the world significant numbers of honeybee colonies are lost due to mistakes in insecticide application. … For the species that have been tested for insecticide effects, the native bees as well as honeybees, leafcutter bees and bumblebees are very susceptible.” In one French study on honeybees, even insecticides considered relatively safe for bees had harmful effects. At one-tenth of a lethal dose, the insecticides caused the bees to forget how to communicate where a food source was located, and shortened the bees’ life spans.
|Aleochara can help control root maggot, eating on average 23 maggot eggs per day.
Photo courtesy of Bruce Barker.
Tips to benefit from beneficial insects
Both Dosdall and Richards note farmers are becoming increasingly interested in doing what they can to reduce impacts on beneficial insects. Although our lack of basic biological knowledge about beneficial insects makes it difficult to know exactly how to encourage specific species, there are some general things a grower can do.
Managing your insecticide applications is important. Apply an insecticide only when very necessary. If there is a need to use an insecticide, try to choose one that is less likely to harm beneficial insects. For example, Dosdall says, “Seed treatments can be detrimental to insect pests but they leave the natural enemies alone by and large.” In some cases it may be possible to use a trap crop to lure the pest into a small area where the insecticide can be targeted.
The timing of insecticide applications can also help to reduce impacts. Try to avoid spraying a crop when it is flowering. “If it is necessary, for example, to spray canola for cabbage seedpod weevil, always spray just before the crop comes into full flower. And if it is absolutely necessary to spray a flowering crop, then it should be done late in the day, when the pollinators are less active,” says Dosdall.
Richards recommends farmers let nearby beekeepers know when they will be applying an insecticide so the beekeepers can move the colonies out of the area for a few days.
Reducing habitat disturbance and increasing plant diversity could favour beneficial insects. Richards suggests light grazing of native pastures and retaining natural areas to encourage native bees. Dosdall says, “One of the practices that is really taking hold in Europe is leaving wild areas next to fields. There is a program to compensate farmers who put part of their agricultural land into hedgerows, which has been shown to increase the number of flowering plants. Most of the parasitoids that attack insects are wasps, so they can feed on crops like canola that have lots of pollen and nectar, but in a crop like wheat there is not much for them to feed on. If we provide nutrients by leaving wild areas nearby, it helps parasitic wasps.”
Longer crop rotations and intercropping may also encourage beneficial insects and hinder insect pests.
Studies have found zero tillage benefits insects that decompose organic matter in the soil. Dosdall thinks zero till could favour natural enemies because many of these insects overwinter in crop residues. Richards says zero tillage is not likely to help native bees because the nests of ground-nesting bees already have been disturbed by previous equipment passes.
Growers being more aware of the beneficial insects on their land could help when planning pest control practices. The Canola Council of Council is currently preparing a handbook of the natural enemies of canola insect pests. If funding becomes available, Dosdall is interested in starting a databank to provide photographs and information to help farmers identify beneficial insect species.
There is a long way to go to fill the gaps in the knowledge of how to encourage beneficial insects, but interest in this aspect is growing. For instance, the Canola Council of Canada and the Alberta Canola Producers Commission have been funding Dosdall’s research on natural enemies of cabbage seedpod weevil, root maggot and diamondback moth. As well, initiatives like the North American Pollinator Protection Campaign and Canadian Pollinator Protection Initiative are working to increase funding for pollinator research and to raise awareness among policy makers and the pubic of the important role pollinators play in consumers’ daily food supply.