Effects of seeding date, canola species and pesticides on Lygus bugs in Alberta, Canada
1Beaverlodge Research Farm, Agriculture and Agri-Food Canada, Beaverlodge, Alberta, Canada
2Lethbridge Research Centre, Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada
3Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada
4Lacombe Research Centre, Agriculture and Agri-Food Canada, Lacombe, Alberta, Canada
Correspondence: carcamoh@agr.gc.ca
Plant bugs of the genus Lygus (L. lineolaris, L. borealis, L. keltoni) are common in canola fields throughout Alberta, although, the species complex varies in each region, and require insecticide spraying at the pod stage in localized areas in some years. We conducted field studies from 2001 to 2004 at Lethbridge (south), Lacombe (central) and Beaverlodge (north) to assess the impacts of seeding date (fall, April and May), and canola species (Polish = Brassica rapa cv., Argentine = B. napus cv. Q2) as main plot factors with pesticide spraying (control, fungicide, insecticide, fungicide plus insecticide) as a split factor. Lygus bug numbers were low at all sites most years. However, in at least one of the 3 years at each site Lygus bug populations surpassed the economic threshold of 2 Lygus bugs per sweep. At Beaverlodge plots seeded in April tended to have higher Lygus bugs than those planted in May but at Lacombe in 2002 and Lethbridge in 2004, the opposite trend was observed where plots seeded in May had higher Lygus bug infestations than those seeded earlier in April. In general, B. rapa tended to attract slightly higher numbers of Lygus bugs than B. napus. Yields generally increased with increasing pesticide inputs but detailed economic analyses remain to be done. Correlations examining the relationship between Lygus adult or nymph densities with yield (kg/ha), thousand-seed weights (gm), seed protein (%) or oil content of seed (%) for B. rapa cv. Hysyn110 and B. napus cv. Q2 were performed at three target stages in the above years. The three target stages of “before insecticide”, “after insecticide”, and “mid-pod stage” depended on crop stage as opposed to calendar date between years and sites and were intended to validate the current economic threshold for Lygus bugs in canola which recommends use of insecticide at the early pod stage. No significant correlations were observed between Lygus bug abundance and seed yield at Lethbridge. Lygus bug populations were relatively low at Beaverlodge throughout the duration of the study reaching seasonal maximums of 3.2 bugs per sweep in 2001, 5.2 bugs per sweep in 2002, 3.4 bugs per sweep in 2003, and 3.4 bugs per sweep in 2004, whereas at Lacombe seasonal maximums of 28.0 bugs per sweep in 2002 and 22.5 bugs per sweep in 2004 were observed. Lygus bug densities failed to show a significant relationship with thousand-seed weights and seed protein at Beaverlodge and Lacombe. There were significant, negative relationships between Lygus bug populations and yield and oil content of canola seed and most arose at Lacombe where higher densities of Lygus bugs were observed in plots that enjoyed better seasonal growing conditions. More specifically, data for Beaverlodge and Lacombe was separated into canola species and correlations were examined between Lygus adult or nymph densities and the above harvest variables. Incidents of significant, negative Pearson's correlation coefficients occurred at Lacombe in 2002 in comparisons of Lygus adult densities collected “before insecticide” and yield of Hysyn110 (r = −0.73, p<0.0001, N=47) but also between adult densities collected “after insecticide” and yield of Hysyn110 (r = −0.59, p<0.0001, N=47). Significant, negative correlations occurred at Lacombe in 2002 between adult densities collected “before insecticide” and yield in Q2 (r = −0.56, p<0.0001, N=44) but also between adult densities “after insecticide” and yield (r = −0.68, p<0.0001, N=48). Significant, negative correlations similarly occurred at Lacombe in 2004 between adult densities collected “mid-pod” and yield (r = −0.59, p<0.0001, N=48). These results suggest a trend wherein the longer-season B. napus cv. Q2 suffered more than the shorter-season B. rapa cv. Hysyn110 when Lygus populations were high and canola had good growing conditions. Interestingly Lygus nymph densities failed to show significant correlations in these comparisons with yield, thousand-seed weights, seed protein or oil content of seed. Correlations examining Lygus populations in fall, April or May seeding dates with the above harvest parameters revealed very few significant relationships. There was no relationship between Lygus numbers thousand-seed weights, seed protein, or oil content of seed at Beaverlodge or at Lacombe. There were three incidents of a significant relationship between adult densities and yield and these all occurred at Lacombe in 2002. There was a significant, negative relationship between Lygus adult populations occurring at “mid-pod” and yield at Lacombe in 2002 (r = −0.58, p<0.0001, N=31). Additionally, there was a significant, negative relationship between nymph densities at “mid-pod” and yield at Lacombe in 2002 in both the fall (r = −0.61, p<0.0005, N=32) and April seeding dates (r = −0.76, p<0.0001, N=31). Lygus bugs were not the primary insect pest in canola plots grown in southern Alberta yet results from central and northern Alberta suggest a trend wherein canola species, rather than a specific canola seeding date, was affected to a greater degree by naturally occurring infestations of Lygus bugs. These results suggest that the longer-season B. napus cv. Q2 may suffer more from natural Lygus bug infestations than the shorter-season B. rapa cv. Hysyn110 when grown in the central or northern canola growing regions of Alberta.
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