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Resistance to Insecticides is good?

Date Added: 24/03/2017

Blog: By Richard Elsdon, Technical Consultant, United Oilseeds

During my working life I have seen various pesticides enter agricultural use and hailed as the answer to our prayers.  They will remove this or that pest and everything will be wonderful!

At first, all that was promised was fulfilled and the targeted problem seemed to either disappear or could be cheerfully resolved with a single application of the appropriate chemical.  However, in due course, usually after some years, the efficacy of the chemical began to wane and either mixtures of pesticides were needed or increased rates of use required, “It isn’t as good as it once was” became generally accepted, as if the manufacturer had in some way produced a weaker version of the chemical.

We have seen the truth of this claim more recently in the area of the control of black grass, aphids, cabbage stem flea beetle and pollen beetle control. Fortunately, the human mind can often find a way to turn a negative into something beneficial.


 Pollen Beetle

Take black grass control as one example. False seed beds have been used to encourage black grass to germinate and then be removed by a broad spectrum herbicide before the cereal crop was planted. By not ploughing, the soil structure has improved in many cases . Sometimes this ‘delay in planting’ has left over wintered stubbles which have benefitted wild life.  Resistance has forced growers to think in a different way.

This change in thinking has required a change in approach for oilseed rape growers as well. And here I want to consider pollen beetle. For quite a long period of time the threshold for spraying the winter crop was about 15 beetles per plant. However, many crops were sprayed almost as soon as the beetles could be seen on the crop, once the day time temperature rose above 15C and the crop began to move from the green bud stage to yellow bud and the first flowers began to open.

We are now being encouraged to adopt a different approach. This has come about from the realisation that crops with lower plant populations will produce more side branches with many more flowers. Losing some flowers will not affect the final yield as the winter crop in particular produces vastly more flowers than it can support through to harvest. To quote the AHDB recommendation where there are less than 30m plants per square metre (m2) the threshold is 25 pollen beetles per plant. How many of you have cut back your seed rate over the years and have about this many plants in the spring?

By not spraying you will leave the pollen beetles long enough for them to turn from mortal enemy to best friend. Once the flowers appear, the beetles, being inherently lazy, will eat some of the pollen in the open flowers and in the process transfers pollen to the style at top of the ovary – pollination done!  In addition, by not spraying the other insects in the crop - and there are many different species present - you will enable the bees and parasitic wasps to go about their business unmolested.

There was a time when pollen beetle control was merely a case of filling the sprayer with a synthetic pyrethroid and driving to the oilseed rape field. Those days are now gone as the pollen beetle is resistant. This resistance may well have prompted another look at what the pollen beetle's actual behaviour in the crop or it may well have been coincidental. The result is that growers now have a better understanding of what is going on and even though insecticides are available, they are being much more selective and cautious in their use.

Perhaps resistance is not all bad after all!

The chart below is the most recent Pollen Beetle thresholds for Winter & Spring OSR from the AHDB

Readers of this series of blogs are eligible to claim Basis Points using PD/55370/1617/g (1 point)

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