Pasture pests – know your enemy

MEDIA RELEASE

2 March 2009

Pasture pests – know your enemy

Ballance Agri-Nutrients is gearing up to lead the charge against a range of pasture grubs that are about to enter their most destructive months.

Given the similarities between many of these grubs, the best place to start is to know a little more about them and how they affect farming operations, says Murray Lane, Ballance Technical Extension Officer.

‘This is where we come in. We can play an important role in helping farmers to identify the various pasture pests, and therefore show farmers how to deal to them appropriately,’ says Murray.

‘The key thing is to know the difference between the pests, a little about their lifecycle, and how to best manage their population, bringing greater understanding to the community.’

He says massive insect flights occur at various times of the year. In Northland during early autumn it’s black field crickets. Every 5-10 years both Northland and Waikato get flights of Black Beetle. October and November in Waikato, Takapau Plains, Canterbury and many other areas, NZ Grass Grub adult (Brown Beetle) flights occur. This is when large numbers of ornamental trees and shrubs are defoliated.

‘Recently, three wheelbarrow loads of beetles were swept up from the entrance to an Otorohanga store. Everyone thought they were Black Beetle, because they were black. They were in fact Tasmanian Grass Grub beetle… black for sure, but narrower and with ridging on its back.’

Intervention and control starts with being able to accurately describe the lifecycle of the pests, says Murray.

‘Farmers should know that right now is prime time for drilling Ballance Bioshield into high-risk pastures – those that have light soil, and were cultivated two or three years ago. (This trend showed up during monitoring of Waikato pastures last season by Ballance, and has been widely reported in AgResearch papers since 1985.)

‘Now is the most effective time. The grass grub larvae are most active over the next 8-10 weeks as they develop from second into third instars. Being more active they will move through more soil, thereby increasing the chances of picking up the Bioshield bacteria.’

Murray says NZ Grass Grub can be controlled on a per paddock basis. ‘They do not move across the landscape in an unstoppable wave like locusts. What we do in a paddock determines the population in that paddock.’

This is distinctly different to Black Beetle. The Waikato has been treated to a lot of Black Beetle damage in recent years, likely related to the continued use of rye grasses with no endophyte (annuals and Italians), or with endophyte toxins that fail to prevent Black Beetle adults feeding through the winter.

‘Knowing a little about the Black Beetle life cycle will aid control. Most of the damage to existing pasture occurs in January to early February, as the larvae develop from second into third instars. At this stage they are large, very large, up to 3cm long, at least twice as big as the biggest grass grub larvae. The damage declines mid February and into March because the larvae have stopped feeding and are pupating into adults.

‘In early March the adults emerge, keen to eat anything green. Adult beetle damage is then seen in newly sown pastures – those sown with untreated seed are generally wiped out.

‘The adult Black Beetle over-winters on the surface (quite different to the grass grub), offering a chance to control them, reducing numbers of eggs laid in November, and hence reducing pasture damage in January. The best way to control them is to plant perennial ryegrasses with endophytes that produce toxins that prevent adult Black Beetles from feeding over winter. The weaker they are, the less likely they are to survive the winter, so fewer eggs are laid.

‘Back to the wheelbarrows of Tasmanian grass grub. They can be incredibly damaging. The current flights signal egg laying is occurring. The resultant larvae cause extensive pasture damage through April-June.

‘However, they are the easiest to control of all the grubs mentioned. Their weak point is that the larvae come to the surface to feed. Foliar insecticides applied in April to early May are very effective.

‘You will know when to spray. Act quickly, soon after the first damage is evident, for the rest of the pasture on the whole farm has been known to disappear.’

The grubs look very much like the NZ Grass Grub, Murray explains, but have a black skullcap. The NZ Grass Grub has a brown skullcap.

‘Ballance has qualified technical sales representatives throughout the country who can help farmers make the right decisions. Anyone in doubt should give them a call on 0800 222 090.’

Attached to this email is a file outlining the respective life cycles of NZ Grass Grub, Black Beetle, Tasmanian Grass Grub and Porina.

ENDS

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