Disease control for grass grub

Press, 7 September 1984, Page 12

Disease control for grass grub

A bacterial disease which kills grass grub larvae, holds the promise of effective and long-lasting control of the pest, without chemicals. The bacteria, causing honey disease, appeared highly specific to grass grub larvae, said Dr Trevor Jackson, a M.A.F. scientist of Lincoln. Researchers believed honey disease could prevent the build-up of peaks in grass grub populations which lead to severe pasture damage. The honey disease bacteria entered grass grubs through soil ingested while feeding. Once in the gut the bacteria multiplied, causing the larvae to stop feeding and, within a month, die. The grub took on a clear honey-coloured appearance — hence the name. The causative bacteria were fairly common in soils in the South Island and the lower half of the North Island, Dr Jackson said. So, if the bacteria was already present in soil, why were grass grub outbreaks still occurring? In a natural situation high densities of grubs would build up before an outbreak of honey disease occurred. Grub numbers would then plummet, leaving large quantities of bacteria in the soil, Dr Jackson said. However, a farmer iSould usually take preventative

action by applying insecticide or cultivating, disrupting the cycling of the disease. Early introduction of the bacteria by artificial means could give control before grub numbers became high, In trials on small plots, introduction of honey disease bacteria gave good control of the grubs with good transmission of the disease, Dr Jackson said. Bacteria were introduced to, the soil via a drench as it was important to avoid desiccation. In future it might be possible to drill the organism into the soil, he said. Biological control using honey disease should mean that grass grub numbers could be kept at levels farmers could live with. Work was still continuing on factors that affect the bacteria. For example, it appeared that droughts could destroy a bacterial population through desiccation. It was also unclear why the bacteria were less common in the north of the North Island, he said. There were numbers of successful biological control programmes being carried out overseas, Dr Jackson said. In the United States, there was a major biological control programme of the Japanese beetle.