First assessment of predation by the arthropod natural enemy
complex on the insect pests of canola-cropping systems in
southern Alberta in Canada: example of diamondback moth
(DBM, Plutella xylostella)
as a paid intern, Master's thesis, university of Alberta and AAFC Canada at LRC (2012)
Diamondback moth (Plutella xylostella) is a serious worldwide pest of Brassica cropping systems that is generally managed with insecticides. However, its ever-changing genome and ability to become resistant to new chemicals has led the scientific community and a posteriori by the farmers to use IPM programs and the help of natural enemies to curb moth populations. Parasitism of diamondbacks has been well studied at the expense of other sources of mortality. This one-year study focuses on the arthropod predators of diamondback moth in south Alberta in canola-cropping systems, to improve biological control of this pest in the prairies.
Inventory of potential predators of DBM
Pitfall trap filled with anti-freeze
The first questions I wanted to answer and add to the study were: what are the potential DBM predators living in canola in southern Alberta and how abundant are they ?
I used pitfall traps, sweep sampling and visual sampling from May to the end of July, though only pitfall traps gave consistent results.
I found that a lot of taxa of potential predators of DBM were present in the canola field (and also in other habitats), but only one predator actually stood out from the others by its very high season-long relative abundance: Pterostichus melanarius (Coleoptera, Carabidae). The overall result was: Coleoptera (Carabidae, Staphylinidae) > Lycosidae > Phalangiidae > Thomisidae > Formicidae
I used pitfall traps, sweep sampling and visual sampling from May to the end of July, though only pitfall traps gave consistent results.
I found that a lot of taxa of potential predators of DBM were present in the canola field (and also in other habitats), but only one predator actually stood out from the others by its very high season-long relative abundance: Pterostichus melanarius (Coleoptera, Carabidae). The overall result was: Coleoptera (Carabidae, Staphylinidae) > Lycosidae > Phalangiidae > Thomisidae > Formicidae
The impact of agricultural practices on predation of DBM
Cage that excluded ground predators
This crucial part was the initial objective of the study that was given to me. It aimed to answer these questions: Do the canola species (Brassica rapa or Brassica napus), the crop density (High density, low density) have an impact on DBM predation in the field ?
Canola was seeded in the spring following an experimental design including 4 blocs of 4 plots each, each plot received one of the two treatments of canola species and densities. Later in the season, 4 exclusion cages (one that was totally open, one that was exclusively open to ground predators, one that was exclusively open to foliage predators, and one that was fully enclosed) were set up in each plot. They contain a single canola plant supporting a certain number of second-instar DBM larvae, that were eventually exposed to natural predation until they pupated. Two runs were carried away during the season, enabling me to test the cages and to conclude that the cages were inefficient under rainy and humid conditions (drowned or diseased larvae influenced the results), contrary to dry conditions during which predation was witnessed.
Back in the lab, the number of surviving pupae was counted and a three-way anova (relating the split-plot design) showed that predation was higher in plots that harbored Brassica rapa (Polish canola) than in ones with Brassica napus (Argentine canola). It also demonstrated that ground predation was significantly higher than foliage predation, maybe because Albertan canola contains abundant ground predators, but very few foliage predators in general.
However, the experiment doesn't showcase why predation seems to be moderately higher in Polish canola (10% of the canola production in western Canada) than in Argentine canola (90% of the production).
Canola was seeded in the spring following an experimental design including 4 blocs of 4 plots each, each plot received one of the two treatments of canola species and densities. Later in the season, 4 exclusion cages (one that was totally open, one that was exclusively open to ground predators, one that was exclusively open to foliage predators, and one that was fully enclosed) were set up in each plot. They contain a single canola plant supporting a certain number of second-instar DBM larvae, that were eventually exposed to natural predation until they pupated. Two runs were carried away during the season, enabling me to test the cages and to conclude that the cages were inefficient under rainy and humid conditions (drowned or diseased larvae influenced the results), contrary to dry conditions during which predation was witnessed.
Back in the lab, the number of surviving pupae was counted and a three-way anova (relating the split-plot design) showed that predation was higher in plots that harbored Brassica rapa (Polish canola) than in ones with Brassica napus (Argentine canola). It also demonstrated that ground predation was significantly higher than foliage predation, maybe because Albertan canola contains abundant ground predators, but very few foliage predators in general.
However, the experiment doesn't showcase why predation seems to be moderately higher in Polish canola (10% of the canola production in western Canada) than in Argentine canola (90% of the production).
Role of predators and rain in the biological control of DBM
Pterostichus melanarius catching a DBM larva
I decided to add a third objective to the study to answer what follows: what predators can actually kill DBM? How many prey can they consume per day ? what stage do they prefer ? does rain have an impact on DBM and / or predators ?
I used small plastic containers containing 1 predator at a time to do the predation rate experiment. Then I used bigger plexiglass cages containing a canola seedling, DBM larvae and a predator, to carry out the rain / predator experiment.
Interestingly enough, I pointed out that the effect of rain can dramatically reduce the moth larval abundance in the field yet concurrently increase the ground predation as another source of mortality. Pterostichus melanarius had a predation rate of 9.5 larvae killed per day on average, which led us to believe that the combined effect of rain, its highest relative abundance from June to July and its preying capacity could have an important negative impact on P. xylostella and perhaps prevent the need to spray insecticides. Some other species had high predation rates, but are in too low numbers in the field to have a significant impact on DBM as biological control.
I used small plastic containers containing 1 predator at a time to do the predation rate experiment. Then I used bigger plexiglass cages containing a canola seedling, DBM larvae and a predator, to carry out the rain / predator experiment.
Interestingly enough, I pointed out that the effect of rain can dramatically reduce the moth larval abundance in the field yet concurrently increase the ground predation as another source of mortality. Pterostichus melanarius had a predation rate of 9.5 larvae killed per day on average, which led us to believe that the combined effect of rain, its highest relative abundance from June to July and its preying capacity could have an important negative impact on P. xylostella and perhaps prevent the need to spray insecticides. Some other species had high predation rates, but are in too low numbers in the field to have a significant impact on DBM as biological control.
More about this research...
Pardosa groenlandica (Lycosidae) eating a DBM larva
This end-of-Master's-degree project allowed me to entirely manage and remodel my work (problem statement, objectives, experiments...) and I want to thank all the people that helped me (Dr. Carcamo, Dr. Dosdall, Carolyn Herle, Robyn Geers...). It also allowed creating a very comprehensive and thorough novel study that was never done in such conditions. A peer-reviewed paper is on its way to be published (submission in 2013).
For more information, please contact me.
For more information, please contact me.