Bioshield Grass Grub Control

March 2005
After many years of research AgResearch has found a naturally occurring bacterial parasite of grass grubs, and developed a way of putting it into polymer coated granules that can be drilled into mature pastures when grubs are small and actively feeding (Feb May). Inoculation of the soil generally suppresses grub growth for about five years at a material cost of $100/ha. AgResearch has formed a JV company called Encoate with Ballance AgriNutrients to commercialise the product.

Grass grubs are a major grassland pest throughout New Zealand and are found in about 10% of pastures. They are one of the few native insects that has been able to switch to a new habitat in European style improved pastures, and now cause damage to ryegrass and clover particularly estimated at around $100 million per year.

Research has been going on for decades, more recently into biological control to avoid the problems of persistent pesticides. One day one of Jackson's colleagues brought in some grass grubs that looked rather sick, so they started looking at diseases. In pastures grass grubs are present in densities ten times as great as in their natural habitat. This proximity favours the transmission of diseases between them through overcrowding etc, the Calcutta affect. They are resistant to many soil-borne diseases, but not the bacterium Serratia entomophila, which coincidentally is found only in NZ.

Serratia was found in populations of grass grub that had gone through rapid expansion and then being decimated by the disease, and the challenge was to be able to cause an outbreak of the disease earlier in the population growth cycle since it is the peak populations of grass grub that cause major damage.

Initially researchers did all sorts of things to try to inoculate the soil -- watering solutions on with watering cans etc -- and they found that the bacterium is susceptible to UV light and desiccation so that was ineffective. However, it survives well once in the soil.

The first soil inoculant was in a liquid form but farmers had to have specialised machinery to put it into the soil, and there were difficulties with its stability. So they came up with a way of incorporating it into a biopolymer granule form so that it could be run through a normal seed drill. Topdressing with granules doesn't work because of the sensitivity to UV light and the problem of getting the bacteria into the soil.

AgResearch has formed a JV company Encoate with Ballance Agri-Nutrients to market the product that has the trade marked name BioShield

Cultivation and renewal of pasture generally results in the death of most of grass grubs and it takes several years for the population to build up to levels that warrant control and will sustain survival of the bacterium. So the pastures need to be at least two years old.

Grubs need to be actively feeding so that they ingest the bacteria, and that generally occurs between the end of February and the end of May. Once they have a gut full they stop eating within a day or so and go yellow, and die within five weeks.

Bioshield is generally applied at a rate of 30kg/ha, and this costs about $100 per ha plus application costs. This is more than other control methods, but it should be effective for five years unless there is a drought, which is likely to kill off Serratia can lead to a grub population explosion.

Given the cost it is important to know that grubs are present in sufficient numbers to make it worthwhile. Generally, if there are 120 or more grubs per square metre (4/spade square) thats when inoculation of the soil is worthwhile.

Jackson warns that grass grub could be problem this year early summer was cool and wet, and those are great conditions for grub growth.

AgResearch is also involved in projects using the same approach to local insect problems in Malaysia, Fiji, and South America a bio-protection strategy to find a natural predator or disease and manipulate its numbers so that the target species is kept under control although not necessarily eliminated. It is green technology that underpins sustainability and production of pesticide free produce.