Control of scale in blackcurrants

Geoff Langford

Scale has been a serious problem for many growers in the past season.  Scale is an insidious problem that is not especially obvious but causes major weakness in bushes and dramatic yield reduction.

I have been interested in what natural predators impact scale because in our gall mite block that never gets sprayed, we never see scale.  I have watched birds like Wax-eyes cleaning up scale insects on my lemon trees but it’s hard to imagine these having much impact on a field scale crop.

It was therefore quite exciting when Molly found a scale crawler that had been fungus infected in an organic block of blackcurrants.  We sent the fungus to Plant Diagnostics and it was identified as Lecanicillium sp.  We used to know this as Verticillium lecanii and I have had it identified before attacking gall mite.  There are commercial products based on this fungus available overseas but I have not found any available in New Zealand.

I’m not sure whether this would have been a useful product for controlling scale in blackcurrants as you usually need high humidity to get it to work and that is hard to get that naturally in a blackcurrant field.  It is interesting that Molly couldn’t find these in a conventionally sprayed block and we wondered if post-harvest fungicides used to control leaf spot could have been the reason.  Getting an understanding of the complex biodiversity in a blackcurrant field and working out what is controlling what, is really difficult.

The key with scale control is understanding the stage they are at.  Once the female adults have laid eggs, they die, but the eggs are protected under the scale body.  At this stage, chemical control is ineffective.  This is true with any species of scale insect.

This past season, the eggs of the dominant species (Parthenolecanium corni or Lecanium scale) hatched quite late in the spring (November-December).  Egg hatch continued over an extended period of weeks, leading into harvest time, making chemical control very tricky.  In addition, we have established that getting spray coverage to the underside of the leaves where crawlers go after hatching is unreliable with our current equipment.

That then leads to the question about what biological control options can be harnessed to help us out.  Reading about scale crawler biological controls suggest ladybirds (especially the little black ladybird (Stethorus), parasitic wasps and lacewings can all be contributors.

One problem I see at present is that the standard chemical being used for scale control is buprofezin (Ovation).  While this is considered relatively benign to many predators, even some species of ladybird beetles, IPM Technologies mentioned that buprofezin prevents reproduction of some Stethorus (black ladybird) beetles.

Looking back at the industry chemical usage reports, we first used buprofezin on 12% of blocks in 2020 (coinciding with Lorsban being phased out). Then in 2021-2022, 40% of blocks used buprofezin. In 2023 our usage went up to 70% of blocks, then back down to 45% in 2024.

We’re concerned that buprofezin, especially used during the growing season, could be reducing a key scale predator in blackcurrant blocks.  We also think that the spring timing and lack of good coverage are two reasons why we are not getting good scale control with our current practice.

We had a major issue with scale in our breeding parents in a shadehouse and I was very pleased with the results we got from oil and wetter combination.  I think this combination has commercial potential if we can get the timing right.  My present thinking is that May after leaf fall might be a good option on blocks where there are high scale populations.

This May is a perfect time to test the effects of various control methods on scale, because we have plenty of scale insects to count! If you want help evaluating the effectiveness of your controls in the field (and are willing to leave even a small untreated control), contact us at Berryworld.