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Don’t spray lygus bugs before early pod-ripening stage

New research in Alberta has confirmed that lygus bug monitoring and control at canola's early pod-ripening stage is the best time to protect yield and avoid economic loss from lygus. And just as important, it shows that control prior to this stage could reduce yield.

Monitoring for lygus bugs is easier at the early pod stage.

“The early pod stage is when the older juveniles and adults are feeding on the seeds in the upper plant canopy where they can be sampled, so this is the optimal time to get an accurate measure of the risk and determine the need to spray insecticides,” says Héctor Cárcamo, an entomologist with Agriculture and Agri-Food Canada (AAFC) at Lethbridge. “Lygus numbers from surveying at earlier stages can be misleading and spraying at earlier stages does a poorer job of protecting yield, unless other insect pests warrant it, for example the cabbage seedpod weevil in southern Alberta.”

Earlier lygus bug feeding can actually help yield potential
Cárcamo, along with fellow researcher Jennifer Otani at AAFC Beaverlodge and entomologist Jim Jones with Western Pest Management Company of Sherwood Park, Alberta, found that lygus bug feeding activity during the early growth stages can actually help the plant stimulate yield production relative to plants not exposed to lygus bugs at this stage.

The researchers looked at plant growth habits, seed yield and other vegetative and reproductive attributes. The density and duration of lygus bug infestation during the bud through bloom period were manipulated in separate experiments.

They found that canola plants can compensate for lygus bug damage during the early flowering stages. As a result of feeding at this stage and the stimulated branches and pod production, the plant is able to compensate for damage inflicted during the bud to early flower stages.

“Even adding very large numbers such as 10 bugs per plant, between the bud to early flower stages, didn't cause any yield losses,” explains Cárcamo.

A study from 1999 through 2001 at Lethbridge also found that one insecticide application at the correct stage was most effective. Cárcamo found that there was no yield advantage of multiple sprays at bolting, flower and pod stages, nor single sprays at bolting or flowering — compared to a single application at the early pod stage.

Similar results were found in the Peace River region during the same time period by researcher Otani. A single spray at the early pod stage provided comparable yield as spraying multiple times. However, in 2001, yield was similar in Beaverlodge and Dawson Creek in plots treated with a single insecticide application at the bud stage and the early pod stage.

Although the results from a single year of this three year study point to an alternate spraying time for producers in the Peace River region, the bud stage was not the most reliable stage to implement chemical control to protect yield from lygus bugs, says Otani. Over the three years of 1999, 2000 and 2001 when this study was performed in Beaverlodge and Dawson Creek, the highest yield of all treatments compared was consistently observed in plots sprayed with insecticide at the early pod stage.

Table 1. Economic thresholds for lygus bugs in canola (number of adults and older nymphs per 10 sweeps).
Application costs
Canola crop stage*
$/acre
Bud
Early pod (5.1)
Mid pod (5.2)
$8.9
No economic threshold needed
14
12
10
20
17
15
$9.7
16
13
11
22
18
16
$10.5
17
14
12
24
20
17
$11.35
18
15
13
25
22
19
$12.15
19
16
14
27
23
20
$12.95
21
17
15
29
25
21
Canola price
$/tonne
-
$220
$260
$300
$220
$260
$300
$/bushel
-
$5.00
$5.90
$6.80
$5.00
$5.90
$6.80
*Crop stages:
5.1 = Lower pod in the main stem with seeds full size and translucent
5.2 = Seeds in lower pods turning green (Harper and Berkenkamp 1975)

“These results confirm that the economic threshold for lygus bugs in canola, developed by Ian Wise and Robert Lamb in 1998 in Manitoba, hold true for the shorter growing season where there is one generation of lygus bugs. That occurs in the unique Peace River region of Canada,” explains Otani.

Table 2. Canola growth stages
StageDescription of main raceme
0
Pre-emergence
0
Seedling
2
Rosette
2.1 First true leaf expanded
2.2 Second true leaf expanded
2.3 etc. for each additional leaf
3
Bud
3.1 Flower cluster visible at centre of roestte
3.2 Flower cluster raised above level of rosette
3.3 Lower buds yellowing
4
Flower
4.1 First flower open
4.2 Many flowers opened, lower pods elongating
4.3 Lower pods starting to fill
4.4 Flowering complete, seed enlarging in lower pods
5
Ripening
5.1 Seeds in lower pods full size, translucent
5.2 Seeds in lower pods green
5.3 Seeds in lower pods green-brown or green-yellow, mottled
5.4 Seeds in lower pods yellow or brown
5.5 Seeds in all pods brown, plant dead
Canola Council of Canada. Canola Production Manual.

Economic thresholds reflect research
Current recommendations by provincial agriculture departments are based on work conducted in Manitoba. Jay Byer, a cereal and oilseed specialist with Alberta Agriculture, Food and Rural Development, says the recommendations still stick and that the new lygus bug research confirms the treatment timing and economic thresholds.

Monitoring of lygus bugs is also easier at the early pod stage, which typically happens from mid to late July on the southern prairies. Sampling at earlier stages can be ineffective in predicting yield losses in canola, partly because many of the lygus bugs reside lower in the canopy where they are not captured by sweep nets.

Integrated approach beneficial
The research shows the danger in a 'scorched earth policy' when it comes to insect control. Cárcamo says the general bias is to have a field free of harmful insects when having a few insects can be beneficial. But sometimes the environment is better if there are a few insects around. “This is typical of the type of interaction we're exploring in other entomology work,” he says. Other integrated pest management work that Cárcamo and Otani are working on include the effect of cutting alfalfa on lygus bug in canola and the overwintering biology of lygus bug. Researchers have also identified regional differences in the number of generations that occur, with two to three generations of lygus bug occurring per year in the southern prairies and one generation in the north.

In addition, potential biocontrol options are being investigated as part of a major research project that involves several research centres across the country and the international biocontrol institute, CABI-Bioscience. -30-

 


April 28, 2002
By Top Crop Manager

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