Larvae counting made easy

Cabbage stem flea beetle has proven the greatest challenge for most oilseed rape growers this season - firstly to get then crop established in the dry autumn conditions, and now with larvae damage to petioles and stems.

On the dedicated iOSR trials site - in the high pressure CSFB area of Suffolk - there is a range of trials going on to assess a raft of techniques to confuse and confound the pest, from drilling date, to seed rates, companion cropping and even mowing plots once or twice to remove larvae from growing plants.

But with the sheer numbers of larvae involved, counting infestations is a time consuming and thankless task for the site technical team.

Now, iOSR Trial Site Manager, Max Hazell (below) gives an insight into a simple six-point technique he's been using this season.

• Collect a representative sample of plants, typically around 10 to 15, cut right down to the ground.
• Place the plants on chicken wire suspended above a plastic box.

• Put five to 10 cm of water in the box to drown the larvae, and add a squirt of Fairy Liquid just to help them sink.
• Leave the box in a warm, dry place; ideally a greenhouse.

• After a week to 10 days the crop will have desiccated and the larvae, thinking it's time to leave the plant, drop into the water.

• Sieve the box contents and count the larvae.

"This season we have been capturing numbers between 120-400 larvae from 15 plants, which translates to 8-30 larvae/plant," reported Max.

Even the farmer's healthy looking crop surrounding field was getting an average 28 larvae/plant.

"When you consider the threshold is 5+ larvae per plant the pressure has been immense," added Max. "Every instance I’ve tested this year has been above this level." 

The high level of larvae damage is not only likely to directly affect yield, but the variable growth stages as plants seek to recover can be expected to result in a protracted flowering period.

Protecting against exposure to sclerotinia and prolonging available green leaf area retention for longer is going to be crucial for crops to fill remaining pods. Find out more on the iOSR pages