New steps in fight against agricultural insect pest

The availability of a natural insecticide to fight an emerging agricultural insect pest is a step closer.

Scientists at AgResearch have been working on a naturally-occurring bacterium Yersinia entomophaga (Ye) that kills caterpillars of the plantain moth (Scopula rubraria), a widespread New Zealand native insect that feeds on a range of plants including plantain. The relatively recent appearance of this and another similar moth (Epyaxa rosearia) in large numbers in plantain crops has given rise to the commonly used name ‘plantain moth’.

AgResearch Senior Scientist Dr Mark Hurst says the insecticidal bacterium Ye could be used as a biopesticide to control at least one plantain moth species, Scopula, populations of which can reach densities of 11,500 larvae m2.

Biopesticides are natural pesticides based on micro-organisms or their bioactives. These are targeted as well as safe for the environment and humans and can provide solutions to many insect pest and plant disease problems. Consumers around the world are demanding sustainable, safe food and New Zealand producers are responding by adopting well-proven alternatives to chemical controls as they become available.

Dr Hurst says early stage laboratory based  research carried out last year during an infestation in the North Island indicated that Ye was an effective tool in killing the Scopula caterpillars.

“Some colleagues investigating the infestation as part of a Beef+Lamb New Zealand project were able to supply caterpillars for an assay and we showed that Ye was very good at killing them. So, we have a control agent and now it’s a case of delivery.”

Dr Hurst suspects a spray could work most effectively and he and colleagues believe that Ye would also be effective against the second species, Epyaxa.

“The caterpillars of both species are surface active meaning the high numbers of caterpillars observed in the field would be readily accessible to a spray application of Ye”.

Dr Hurst expects that within a week of spraying the population could be reduced by 90%.

“It’s very rapid; you see effects within three days of application.”

The bacterium will multiply within the dead larvae increasing the persistence and therefore infection rate of the bacterium in the field.

However, spray application of biopesticides is very challenging because the microbes are vulnerable to UV light and drying, both of which cause rapid microbial death.

Researchers also want to work out how to prolong the spray’s persistence – up to 14 days.

For plantain moth caterpillars however, the facts they are voracious feeders and surface active  should negate the need for persistence and Ye should be able to significantly reduce the population with a single quick acting  spray much like a conventional insecticide.

AgResearch scientist Colin Ferguson says the caterpillars can be controlled by conventional insecticides although none are specifically registered for use against this pest.  They are generally broad spectrum, killing beneficial as well as pest insects and can also have detrimental environmental effects, raise consumer concerns about food safety and affect market access.

“Consequently, although they have little alternative, farmers are showing increasing reluctance to use such pest control measures.  Conventional insecticides also have stock withholding periods that need to be observed and that can interfere with grazing management. Biocontrol, or biopesticides, offer an alternative approach to pest control that can alleviate these concerns,” Mr Ferguson says.

Plantain, once largely neglected as an agricultural plant, is a deep-rooted pasture herb well proven to contain higher levels of trace elements than ryegrasses and can also significantly improve dry matter yield and stock live-weight gains. Field trials have indicated plantain is a high-protein alternative to pasture ideal for finishing lambs.

Plantain is often planted as part of mixed swards along with ryegrass, clover and sometimes chicory. Due to its drought resistant properties it is sometimes sown as a monoculture – a single source pasture. These plantain crops should last a number of years but they provide habitats that allow both species of plantain moth caterpillars to build up to very high numbers. Invasion probably occurs in the first year often going unnoticed but providing a “basis” for populations which increase exponentially the following year.

Development of novel delivery systems for biopesticides is ongoing in the Next Generation Biopesticides (NGB) programme, a joint initiative between AgResearch, the Bio-Protection Research Centre (Lincoln University) and Plant and Food Research with funding from the Ministry of Business, Innovation and Employment, and industry partners Ballance Agri-Nutrients, Foundation for Arable Research, Grasslanz Technology Ltd and Zespri.

Plantain moth damage

Plantain moth damage

About plantain moths

Moths of both species are small, less than 20mm wide, light brown with darker markings on the wings. They belong to a group of moths commonly called carpet moths. When present the moths will often fly up from the crop in front of people, stock or vehicles, sometimes in very large numbers, particularly in autumn.

Caterpillars of both species vary considerably in colour and patterning but are generally brown and small, less than 20mm long. They are known as ‘loopers’ as they raise part of their body off the ground or plant when moving.

Little is known about the biology of the insects but scientists are rapidly learning more.

They appear to have a short generation times and several generations per year occur when conditions are favourable, giving rise to the massive numbers of caterpillars sometimes seen. They are most abundant in late summer and largely disappear from crops in late autumn. Both species probably feed on a wide range of plants.  Epyaxa will also feed and damage clover but Scopula does not appear to. The caterpillars feed on the plant leaves causing small holes which can join up and in severe cases leave only leaf veins.