Vigorous, well-nourished plants can better compensate for insect attack than plants
under nutrient stress, so maintaining relatively high levels of soil fertility seems
appropriate for integrated management of diamondback moth in canola
— Lloyd Dosdall, University of Alberta
Not all insects in your crops are bad. In fact there can be millions of several different species that are actually doing you a favour, says a University of Alberta (U of A) researcher.
While some insects are unwanted pests, there are also dozens of beneficial insects that often prey on those pests, says Lloyd Dosdall, a former provincial entomologist, who is now an associate professor at the U of A.
Dosdall says he isn’t opposed to using insecticides to protect crops, when warranted, but he also says while the pests move in with potential to produce crop damage, in many cases predators to those pests aren’t far behind.
While a fair bit of attention has been paid to biological control agents (insects) that control weeds such as Canada thistle, purple loosestrife and leafy spurge, there are also a number of beneficial predator insects that are very effective in controlling common crop pests such as diamondback moths, root maggots, cabbage seedpod weevil, cereal leaf beetle and wheat midge.
“Often when we think about beneficial insects to crops we first think about honeybees, which help with the pollination process,” says Dosdall. “But even with pollination, there are many other species that help in pollinating crops.”
Butterflies, thrips, wasps, hundreds of fly species and even beetles, are other insects that help pollinate crops as they move from plant to plant, and many are not harmful to the crop itself.
THE GOOD GUYS
Dosdall’s research over the years has shown there are many natural insect predators that help control common crop pests.
In controlling diamondback moths, for example, a small parasitic wasp called Diadegma insulare, commonly found in Western Canada, is very effective in controlling the moth. As the moth is carried into Canada’s canolaproducing areas on wind currents from south of the border, the Diadegma are soon to follow.
“Although the diamondback moth rarely overwinters in the Prairies, we still experience extensive crop losses from infestations almost annually because adult moths are blown here on strong winds from the U. S. and Mexico,” says Dosdall. “The principal natural enemy of diamondback moth is the parasitic wasp, Diadegma. It parasitizes all four larval instars of diamondback moth, and it kills and emerges from the pre-pupal stage of its host.”
CROP GROWTH AFFECTS INSECT DAMAGE
Dosdall’s research shows soil fertility — having a vigorously growing canola crop — has an impact on both the pest and its predator.
Perhaps surprisingly, development of diamondback moth larvae was slowed at low soil fertility levels and occurred faster at higher fertility rates. At low soil fertility levels, diamondback moth adults deposited fewest eggs, and even though their larvae consumed more leaf tissue when nutrients were limited, development progressed more slowly than when fertility was intermediate.
That information may help slow the pest, but isn’t necessarily a good strategy for optimizing crop yield.
However, on the other side of the issue, research also showed the survival and effectiveness of the beneficial parasite improved with higher crop fertility.
“Canola stressed by low nutrient stress, so maintaining relatively high levels of soil fertility seems appropriate for integrated management of diamondback moth in canola.”
Dosdall says in years when there are heavy infestations of diamondback moths, he has seen-fields
levels has poor seed yields, even in the absence of insect damage, and although higher levels of soil fertility can predispose plants to increased infestations of diamondback moth, increased fertility also enhances development and survival of Diadegma insulare,” says Dosdall. “Vigorous, well-nourished plants can better compensate for insect attack than plants under nutrient where there are clouds of Diadegma living in the canola swaths feeding on the moth larvae.
Looking at field spraying statistics, he says in 1995 and 2001 Alberta farmers spent $42 million and $86 million respectively in field-spraying costs to control diamond back-moths. In 2003 and 2005 spraying costs were estimated at $4 million and $3.5 million, respectively. Dosdall credits part of that significant reduction in spraying costs to the effectiveness of the parasitic wasp.
THE ENEMY OF THE CABBAGE SEEDPOD WEEVIL
In another example of an important beneficial insect, he pointed to the parasite that attacks cabbage seedpod weevil.
“The cabbage seedpod weevil is native to Europe and is a serious pest of canola in southern Alberta and Saskatchewan,” says Dosdall. “It first invaded the western Canadian Prairies in 1995, and its invasion garnered considerable attention within the agricultural community because of its economic impact.”
The weevil can cause severe economic losses in canola at several stages of crop development. When overwintered adults invade crops primarily in the bud to early-flowering stages, they feed on buds and often destroy their vascular tissue causing “bud-blasting.”
Plants with severe bud-blasting produce racemes with few pods, and may even fail to flower when weevil densities are high. Larvae feeding within pods causes seed losses of about 18 to 20 per cent. If environmental conditions
are humid after larvae bore exit holes, the pods can be invaded by fungal pathogens that causes more seed damage. When new-generation adults emerge late in the season, they feed upon seeds within green pods to build fat stores for overwintering, causing further reductions in crop yield and quality.
Following the arrival of the weevil in Canada in 1995, there was no evidence of a natural enemy for several years. However, in 2001, specimens of the parasitic wasp, Microctonus melanopus, were found in adult seedpod weevils, and in the same year larvae of the weevil were found to be parasitized within canola pods.
Further examination of weevil-infested canola pods found at least a dozen species of native parasitoids that evidently expanded their host ranges to begin attacking larvae of the cabbage seedpod weevil.
“Some of these, like Necremnus tidius, caused parasitism levels of approximately 50 per cent in some fields,” says Dosdall. “It isn’t clear whether parasitism levels of cabbage seedpod weevil are still likely to increase over time, since the current situation has shown considerable year-to-year variation.”
Along with these insects that attach diamondback moth and cabbage seedpod weevil, root maggots have a very effective predator called Aleochara, a beetle that feeds on the root maggot eggs and larvae. A small parasitic wasp has been found to be effective in controlling cereal leaf beetle, and another parasitoid called Macroglenes is effective in controlling wheat midge.
INVEST IN A SWEEP NET
Dosdall says it is important for producers to know what insects are in a crop — both the harmful and beneficial ones. And he says a few minutes spent with a sweep net can be of significant value.
While he realizes pesticides are necessary for control of insects and weeds, he says producers need to be aware of the impact these products can have on other beneficial insects.
“Pesticide use can be damaging to natural enemy populations in crops (as they are to damaging insects),” he says. “Research shows, for example, that the herbicide 2,4-D killed over 50 per cent of A. bilineata adults (an important insect for controlling root maggots) in one study, and this herbicide or others in the same chemical group can be used to treat volunteer weeds in canola stubble where most A. bilineata adults emerge.”
When synthetic pyrethroid or organophosphate insecticides are applied to crops to control cabbage seedpod weevil, cereal leaf beetle or diamondback moth, they will also destroy Diadegma insulare and other parasitoids in the same field.
“Care must be taken to ensure that insecticides are only applied when necessary, and if applied, are used in a way that minimizes harmful effects on non-target beneficial insects,” he says. “Judicious use of insecticides can help ensure that damage to the environment and to beneficial insects inhabiting the crops is minimized.”
Lee Hart is a field editor for Grainews in Calgary. Contact him at 403-592-1964 or by email at [email protected]