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To Spray or Not To Spray? (PCN Winter 2015) DEC 22 2014 | Consumers and Producers | Pulse Crop News

This article appeared in the Winter 2015 issue of Pulse Crop News.

Nevin Rosaasen, Research Economist, Alberta Agriculture and Rural Development

Determining farm specific economic thresholds for pest management and the role of beneficial insects and fungi

If you have ever found yourself waffling on the decision to spray pesticides due to not enough or too much information, rest assured, you are not alone. When scouting fields, you are faced with identifying a myriad of leaf diseases and thousands of insects. Today’s farm business managers wear many different hats, and mycology and entomology may not be your area of expertise. The decision to spray a fungicide or insecticide can be complex, including direct “will it pay” and indirect “other benefits” elements.

Do yourself a favour: do your assessments beforehand so that when crunch time comes, you can feel confident that your choice both pays in the short term and respects the longterm “health” of the beneficial insects and fungi on your farm. Last minute choices at spraying time stand a good chance of missing the mark on one or both of these key results.

Determining your economic threshold of when it pays to spray is not as cut and dried as a simple insect count, a specified severity of infection rate or basing the decision solely on the weather forecast. All of these variables are important in making the most informed decision. The decision should be made on a case of profitability, and putting more dollars in your jeans. The most important first step is determining your true cost of spraying, a number that is vastly different from your neighbour’s. The best way to tell whether it pays to spray is by using a partial budget.

First, determine your farm specific economic cost of spraying the pest that you are trying to control. Determining your costs is the left hand side of the ledger of a partial budget and should include not only the cost of the pesticide, but all variable costs including fuel, time, labour of both the sprayer operator and water hauler. Fixed costs include depreciation on the sprayer, chemical handler, water truck, tanks, etc. Divide these by all the acres that you spray over the estimated lifetime of these assets to give you a rough fixed cost per acre. Knowing what it costs you to spray is essential before evaluating the potential benefit, loading up the sprayer and hitting the field.

On the direct benefit side of your partial budget, evaluate how the decision to spray may affect your yield. Factors to consider include the expected yield savings and price of the crop you are protecting (use realistic yield estimates and conservative estimates for price). You may also want to net off the crop lost from compaction and tramping – roughly two per cent crop loss on 100 foot booms.

The net of these gives insight into the “direct” profitability of spraying pests. If it is negative, here is where you stop. If you have defined a threshold, you would know where you would start. However, a few other considerations should weigh into your assessment.

The role of beneficial insects and fungi in our cropping systems are becoming increasingly evident with developing research. Many insects, beyond bees, play important roles as pollinators, predators and are often keeping economically detrimental insect populations in check. Parasitoid wasps, beetles, daddy-long-leg spiders and fungal pathogens can potentially reduce certain insect outbreaks to a level under your economic threshold for spraying. Before you close the door on your evaluation, consider that there may be benefits of maintaining healthy insect populations that may give you a different result than your direct budget.

Research from Australia has shown that canola yields are reduced over 20 per cent if the distance from a honeybee hive is over one kilometre. Remember that honeybees, although efficient pollinators, are only one of the hundreds of insects that pollinate your crop. While there are relatively safer products for bees, many will greatly reduce the populations of many of these other beneficial insects including parasitoid wasps and other small flies that play a significant role in pollination.

Determining an exact economic value on beneficial insects is a difficult task. There is no hard number or formula for the number of dragonflies, parasitoid wasps or fungal colony forming units that producers can realistically use in determining their economic benefit. Understanding that your field is a complex ecosystem that when shocked with a fungicide or insecticide, may have unintended consequences on the beneficial insects or fungi and subsequent infestation levels is key. When infestation rates and benefits of spraying outweigh the costs, producers need to act to protect their profitability.

However, when you find that deciding to spray or not to spray is based on costs and benefits being nearly even, letting nature run its course may indeed have longer-term net benefits for your integrated pest management and ultimately, your longer-term profitability. The best way to remain informed is to follow pest outbreaks, read about the role of beneficial insects and fungi, and get out to your local field days and crop walks to rub shoulders with entomologists, pathologists and other researchers.

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