GLOBAL – Mild winters will require extra vigilance if extensive grassland damage at the hand of pasture pests is to be avoided, farmers are being warned.
Two species, Leatherjackets and Black Beetles, are causing concern in Britain and New Zealand respectively, and while not new to either country, their presence is growing at an alarming rate.
Wintering underground, both species eat at plant roots. Leatherjackets – the grub of crane fly or ‘daddy long legs’ - have spread from their central and western localisation in Scotland, bringing the threat of ‘severe and visible damage' to grass swards.
This is according to Scottish Rural College Ecologist Professor Davy McCracken who conducts an annual survey in the central and western stronghold of the larvae.
This year, one field broke the record with 13.6 million grubs per hectare.
Summarising this year’s results, Professor McCracken said: “While over 90 per cent of the fields sampled contained more than 0.6 million grubs per hectare nearly 60 per cent of the fields harboured populations of over two million per hectare.
“Densities like that, if left untreated, are likely to result in severe and visible damage to the grass sward or any spring crops sown after the grass.”
The survey, on-going since the 1970s, shows long term trends and indicates a rise in Leatherjackets over the past 14 years. SRUC analysts link this to climate change.
Also benefitting from tight surveillance are northern Kiwi farmers who have been given a prior warning of a pest explosion.
Should New Zealand’s current winter season bring warm temperature, farmers have been told to expect a Black Beetle problem.
This warning comes from AgResearch scientist Dr Kumar Vetharanium who has built a model to forecast likely population increases.
AgResearch Biocontrol expert, Dr Alison Popay says recent weather has suited the beetles as they prepare for hibernation.
“This means that the adult black beetles have plenty of time to feed and build up fat reserves to help them through the winter,” she explained.
“If warm conditions continue through autumn and spring conditions are right, some farmers could be facing another serious black beetle outbreak next summer.”
However, the overall impact will be slight with only the far northern areas such as Waikato and the Bay of Plenty expected to be hit.
What’s more, the weather may oblige in curbing insects below the grass.
“Forecasters are predicting about 50 per cent chance of an El Niño event July – Sept,” said Dr Popay.
“While Waikato farmers may not like the cold wet conditions it may bring, it should help reduce overwintering black beetle adult populations.”
"If warm conditions continue through autumn and spring conditions are right, some farmers could be facing another serious black beetle outbreak next summer."
Damage can be minimised by using a tailored endophyte when renovating pasture and keeping Black Beetle friendly plants like paspalum and other summer grasses out of pasture.
“Legumes are not attacked by black beetle. Also consider crops like chicory, which are not a good host for black beetle and can help break the pests’ lifecycle.”
For British farmers wishing to minimise Leatherjacket damage, Dr McCracken advises that populations are surveyed to inform chemical applications.
“Grub populations can vary, not only from area to area but also within an area and from field to field on the same farm,” said Dr McCracken.
“It is essential that an assessment of leatherjacket densities is conducted prior to deciding whether any insecticide application is necessary.”
Those considering cultivations have pre-ploughing insecticides available. The SRUC has said insecticides are more effective under grass than in new sown spring-crops.