Effects of late season flea beetle feeding on canola yields

Project Code PRR06-110

Project Lead

Julie Soroka - Agriculture and Agri-Food Canada

Objective

To develop economic thresholds for flea beetle infestations late in the year, allowing canola producers to make informed decisions on late season flea beetle management

Summary of Results

Flea beetles (Phyllotreta cruciferae and P. striolata) are the most economically damaging insect pest of canola in Canada. Although having one generation per year, flea beetle adults appear in considerable numbers twice during the canola growing season: overwintering adults emerge in spring (May – June) and the new generation adults emerge in late summer (end of July – September). Flea beetles are, however, considered to be an economic threat only in the spring due to heavy feeding of overwintering adults on canola seedlings. Canola is most susceptible to flea beetle damage during the seedling stage and beetles can completely destroy a newly emerging crop. Thus, flea beetle control typically targets spring infestations and relies mainly on canola seed treatments, with foliar insecticide sprays applied in some cases, if warranted.

In recent years, flea beetle populations have been unusually high in many areas of the prairies late in the summer, causing canola growers to question the need and economics for late season flea beetle control. This project was undertaken to determine the effects and economic risks of late season flea beetle feeding on canola seed yields. The goal was to quantify crop loss levels and conditions when control measures might be necessary, allowing growers to make informed decisions on late season flea beetle management. The work was conducted by Agriculture and Agri-Food Canada research scientists at the Saskatoon Research Centre experimental farm in Saskatchewan.

Field trials were established during the 2006-2008 summer seasons to monitor late season flea beetle infestations on canola at different stages of maturity in open and caged field crops. Two canola cultivars and two seeding times were asessed, i.e. early (mid-May) and late spring (beginning of June) seeded crops. Three scenarios were compared: in one the (open) plots were allowed to be infested by natural late season flea beetle populations and half of each plot was then sprayed with foliar insecticide and half left untreated; in the second, 1.0x1.0x1.5 m "cube" screen cages were placed over early and late seeded canola and half of the cages were infested in early August with 16,000 to 26,000 flea beetles collected from the sorrunding areas; in the third, 60 cm long sleeve cages were placed over individual canola plants and each infested with 100 flea beetles. Seed yields and 1000 seed weights were measured from sprayed and unsprayed plots and from infested and uninfested cages at harvest.

Noticeable levels of natural infestations of flea beetles were present in late summer in all three years, although overall numbers were low for the duration of the project. The high temperatures in July of both 2006 and 2007 resulted in rapid maturation of the canola, rendering it insensitive to damage by the beetles. In 2008, canola matured slowly, and infestations of flea beetles occurred when late-seeded canola seed was still turning colour, leading to considerably higher flea beetle feeding damage.

Overall, application of insecticide to naturally infested plots or infestation of cube and sleeve cages with flea beetles did not have any impact on canola seed yields or weights, indicating that significant flea beetle damage to canola late in the season is an uncommon occurrence. More specifically, within seeding dates, flea beetles had no detrimental effects on early-seeded canola plots in any of the study years. Occasionally, flea beetles did affect seed yields of late-seeded plots of individual cultivars in some years. Seeding date had the greatest influence on harvest parameters, with earlier seeded crops usually outyielding the later ones, mainly because maturing canola is much less susceptible to flea beetle damage. This study concludes that two elements appear to be necessary for significant damage to canola from late season flea beetle feeding to occur: a) presence of very high numbers of flea beetles and b) immature canola pods at the time of infestation.

This study identified the 5.2 growth stage (seeds in lower pods green) as a critical cut off point in canola development that determines the crop's susceptibilty to losses from late season flea beetle damage. Once canola is past the 5.2 growth stage, it becomes resistant to injury from flea beetles. Even at 5.1 - 5.2 growth stages, when seeds in lower pods are still green, numbers higher than 100 flea beetles per plant, and for some cultivars higher than 350 per plant, may be necessary to cause significant yield reductions. This is a useful indicator to guide growers in determining weather flea beetle control might be economically warranted before harvest.

As part of this project, a survey was conducted among producers and agronomists to assess the current level of awareness and practices adopted to address this pest issue. The survey questionare, developed in co-operation with the Canola Council of Canada, was distributed to canola producers, field scouts, extension personnel and agronomists in the region. The 130 survey respondents represented a broad geographic area, including all four provinces of western Canada and one American state. The survey results support and complement the findings from this study. Almost all repondents perceived flea beetles to be a serious pest issue in the spring only. In most years and most places growers do not feel that late season flea beetles are an issue for their canola crops. Although the majority of respondents had noticed the presence of flea beetles late in the season, the damage was considered to be infrequent in both severity to pods and area affected. In only a few cases did beetle damage lead to late season application of insecticides by growers. Time and cost constraints prevented some respondents from taking action even when flea beetle damage to canola pods was noticed.

The results from the research and survey in this project demonstrate that late season damage from flea beetle is of minor concern. A unique set of circumstances, such as late pod maturity (canola at a growth stage younger than 5.2) and very high numbers of flea beetles would need to arise to cause damage significant enough to warrant late season insecticide spray applications. The best recommended protection against late season flea beetle damage, is therefore, to seed canola at mid-May or earlier. By using such science-based information to make management decisions, canola growers may eliminate unnecessary insecticide applications late in the season, thus minimizing pesticide risk to humans and environment.

For more information about this project, please contact Dr. Julie Soroka.

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