Research Into Black Beetle

Black beetles that eat (and severely weaken or kill) ryegrass plants can be a problem in the warmer parts of the North Island. Populations cycle up and down with warm and relatively dry weather from autumn to spring allowing populations to rise. This coming spring-summer could see a significant increase if conditions suit the survival of adults and early stage larvae. Farmers in susceptible areas need to be aware of this and plan to use the few management tools that are currently available. There has been some recent research showing promising control possibilities but more needs to be done.

Black beetles are a grass-eating pest originally from South Africa and now resident in the warmer regions of the North Island including coastal Taranaki, coastal Gisborne, the Waikato, Bay of Plenty and Northland. The adults cause damage in autumn and spring and the larvae in summer. Most grass species and some crops are susceptible and when numbers increase the effects can be devastating.

Outbreaks occur sporadically and usually follow a warm autumn- spring period. The insects are active at temperatures above 15°C and will grow and reproduce rapidly above 20°C.

The glossy black adult beetles are about 15 mm in size and are usually found in the top 10 mm of soil. The females lay eggs singly in soil (close to the surface) in spring.

The black beetle larvae look a bit like grass-grubs but are larger, with orange spiracles down the length of the body. In February/March they pupate before emerging as adult beetles that overwinter and lay eggs in the following spring. Beetles will fly to new sites under warm conditions in spring and autumn.

Damage can occur on all types of farms but is generally more severe on dairy farms. Adults and larvae feed on many pasture species including annual and perennial ryegrasses, tall fescue, paspalum and kikuyu but not clovers.

Larvae feed on roots close to the surface. In severe infestations, plants are destroyed and the turf can be rolled back. In less severe cases the pastures can become clover dominant or open and are susceptible to weed invasion. Damage to pastures from larvae can appear similar to grass-grub but occurs over summer (January to March) as opposed to autumn/winter. The damage threshold is around 20-40/m² and is lower when it coincides with drought stress.

Adults feed mainly at the base of plants at soil level. Patches of yellowing tillers that pull easily from the pasture may become noticeable in autumn and spring. Any more than 10 adults/m² in pasture are a concern but populations of 30/m² have become common and they can reach as high as 95/m². In maize 1-2 beetles/m² in spring are sufficient to cause an economic loss.

Both beetles and larvae can be particularly destructive to newly sown grasses, and larvae will also feed on brassica roots, maize, kumara and even strawberries.

Until 2011, little research into black beetle control had been done since the late 70’s because of the sporadic nature of outbreaks, although during the 1990’s it was discovered that endophytes in ryegrass could deter adult beetle feeding, leading to reduced egg laying and larval damage.

Dr Warren King, senior scientist with the Innovative Farm Systems group at AgResearch Ruakura, says that black beetles typically had a population outbreak followed by a decline, with 5 to 7 years between cycles. “This made it the perfect organism for avoiding research programmes, because by the time you’ve had an outbreak and then managed to organise research funding the problem has gone,” he says. “However, a series of dry summers and autumns since 2007/8 has meant higher than usual numbers of black beetles particularly on peat and other light soil types.”

Another factor in the increase in beetle populations since 2000 has been the widespread use of the AR1 endophyte ryegrass, according to Dr Alison Popay, also a senior scientist with Innovative Farm Systems at Ruakura. “We knew AR1 didn’t have particularly good resistance to black beetle but that message didn’t get through and it may have helped feed and increase the black beetle population quite effectively,” says Alison. “Also when farmers have black beetle pasture damage they sometimes sow crops of annual or Italian ryegrasses to provide winter forage, but these don’t contain endophyte and so provide the perfect feed for black beetle populations to increase.”

Alison says that ryegrasses containing AR37, Endo 5 or NEA2 endophytes offer the most protection against black beetle. Max P in tall fescue is also effective. “These endophytic fungi produce toxins that deter the adult black beetles from feeding and if the beetles can’t find alternative plants to feed on they will die before they can lay their eggs. Unfortunately most established pastures contain plenty of other species that beetles can feed on and when populations are high it is difficult to prevent larval populations establishing.”

Dr Alison Popay and Dr Warren King have been working on black beetle control for the past three years as part of a research programme that aims to determine what drives the population cycles, investigate tools farmers can use for limiting populations, and provide some possibilities for biological control.

The work has been funded by the MPI’s Sustainable Farming Fund in partnership with DairyNZ, Ballance, RD1, PGG Wrightson Seeds, Agricom and Agri-seeds.

Alison has been running “choice trials” for black beetles – different cultivars of ryegrass with and without AR37 endophyte planted singly with sand in between – and observing which ones the beetles eat. Those that are ignored will be the ones with the most potential for combating beetle infestations.

Another scientist has been looking at the correlation between black beetle outbreaks and the El Niño/La Niña weather patterns to see if they can be used as predictors of problem seasons. Other work suggests that black beetle problems can be more severe in soils where the pH is lower, so it is possible that raising the pH may have some effect on larvae survival. However, it is too early to say whether this will be an effective control measure and full scale field trials are needed.

At Lincoln, scientists have been looking at biological control using nematodes and a bacterium, and Warren King says that while results are very promising they are far from being farm ready. “The three-year programme has come to an end and we are not sure whether further work will be funded,” he says.

In the meantime there are a few measures that farmers can take to limit the growth of black beetle populations and the damage they cause :

• Look at the whole farm platform and determine how much of it is at risk. If the situation is really bad or if you have many paddocks with a lot of susceptible species, it may be wise to plan a summer cropping programme and consider whether you need to purchase supplementary feeds.
• Choose a summer crop that is less susceptible to larvae damage – maize can be useful, also chicory because black beetles don’t like it
• Avoid overgrazing particularly in summer – it will be the final straw for grasses weakened by beetle attack.
• When sowing new pastures or undersowing damaged ones, use endophytes that are effective against black beetle – AR37 or NEA2 are currently the two top ranked ones
• Use seed treated with insecticide so that when plants are emerging in April/May they don’t become food for hungry black beetle adults emerging at the same time

“Even with using all those options effectively there is no guarantee that you won’t have black beetles, but it does reduce the risk of having problems,” says Warren.

Agresearch has created a website resource for farmers – www.pestweb.co.nz – that contains information on how to identify black beetle problems and some management options for dealing with them.