Ladybirds as a Psyllid Biocontrol

One of the most important insect pests to have invaded New Zealand recently is a bug called the tomato-potato psyllid (TPP). It’s a distant relative of greenfly (aphids) and is devastating tomato crops in glasshouses and in paddocks and also severely damaging potato crops.

The tomato-potato psyllid was first found in New Zealand in 2006.

Over the past three or four years, it has cost potato growers around $100 million. As well as damaging the leaves, the psyllid is a vector for a bacterium causing zebra chip disease. It sucks the plants’ juices and even worse, injects bacteria into the plant. This can reduce yields severely and causes potato tubers when cooked, to be marked internally. This is called a zebra chip and makes them unmarketable.

Insecticides are working but many sprays per season mean that this is a high cost and there are potential environmental problems too.

Andrew Pugh, a Masters student, is studying a ladybird as a predator of the TPP. The southern ladybird (Cleobora mellyi) was introduced from Australia in the early 1970’s to control eucalyptus tortoise beetle, a major pest of eucalyptus trees in forestry plantations.

However, when confronted with TPP, which it has never met before, it apparently goes on a feeding binge, with one adult ladybird consuming up to 100 of these pests each day.

Andrew’s results have recently been published in an international journal with co-workers Dr Dean O’Connell and Prof Steve Wratten, both in the Bio-Protection Research Centre at Lincoln University and have led to a high level of interest.

In recognition of his research, Andrew was recently awarded a prestigious Hicks Scholarship through the University of Waikato. He is hoping to obtain large numbers of female ladybirds, already mated, for a planned release. These are currently being reared by BioForce NZ in Auckland, a company which specialises in producing large numbers of biocontrol agents, mainly for release into commercial glasshouses.

Andrew’s research composes three main experiments:

1. Whether the ladybird can reduce tomato-potato psyllid densities on potato plants in a semi-controlled environment (a glasshouse trial).2. What prey preferences do the ladybirds have in relation to common herbivore pests found in potato crops (psyllid, two aphid species, and also whitefly), and how long does the ladybird live with low psyllid numbers and only with access to buckwheat pollen and nectar (and a combination of those).

So far, the glasshouse trial is underway and he is collecting data. There’s no conclusive results but his theory is that the ladybirds are reducing psyllid densities.

The prey preference trials have been completed and the results found ladybirds did not have a prey preference when presented with a choice of psyllid + green peach aphid (the most common aphid in potato crops and a virus vector/carrier), nor when it had a choice of psyllid + potato aphid (another common aphid and virus carrier), BUT, when presented with psyllid + whitefly it showed an aversion to eating the whitefly. (This could be important if the ladybird gets used for psyllids on tomatoes in glasshouses where if it ate whitefly it could interfere with biological control of the whitefly by parasitioids).

The longevity data, although not analysed at this stage, showed that ladybirds with access to just low psyllid numbers (10 a day) could live for around 6 weeks on average, ladybirds with access to just buckwheat pollen and nectar lived longer than that 8 or so weeks on average, a few lived up to 12 weeks.

Ladybirds with access to low psyllid numbers AND buckwheat pollen and nectar, the majority lived for 12 weeks which is when he terminated the experiment. With no food the ladybirds just lived 1-2 weeks.

Buckwheat is a non-crop plant often planted at a crop edge or amongst the crop to enhance biological control agents by provided nectar and pollen which often increases longevity or the number of eggs it lays (or both).

Research started last November, but experimental problems meant the project didn’t really get going much until February this year. Andrew aims to be finished by January 2013 – including finishing and submitting his Master of Science thesis that the research is for.

Realistically, he says his research is going to lead to more a niche market tool for potato growers, although it has wider applications potentially for glasshouse growers for tomatoes, capsicums.

He says his research is probably aimed more at organic type growers who have been hit the hardest by the psyllid. Currently researchers are trying to secure funding from a range of sources for proper field trials.

Evidence over a number of studies indicated that a predator species that had not come into contact with the prey species previously produced a higher kill rate than a predator that had co-evolved with its prey.

Prof Steve Wratten, who has been supervising Andrew’s research says they were keen to do some open-air trials in Hawkes Bay next summer. “Their tomatoes and potatoes have been absolutely clobbered by the psyllid.

“I’d just like to release them (ladybirds) into a crop with a psyllid infestation and monitor what happens.”

He says a key question that needed answering was whether the ladybirds would stay around on the crop and lay their eggs or whether they would fly away. If they laid their eggs the chances of success would be much higher.“There’s plenty of examples of successful biological control in the world, you just have to give the animals a chance. All we need is a farmer to give us 100 square metres of crop.”

Steve says he is in the ‘have a bash’ school. “The ladybird’s already here, whack it out. It’s not expensive and there are no negatives”.

It would not be possible to use ladybirds in combination with most commercial insecticides.