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Media Release - Landcare Research - Wasps

23 May 2002

Rain puts dampener on wasp season The wasp season is now coming to an end, and scientists say unusually heavy spring rain has averted what could have been a plague year.

Wasp numbers in the South Island's beech forests were expected to be high this season. Conditions were ideal for nest formation, with dry September weather and little competition between founding queens after three seasons of relatively low wasp numbers. However, the wasps suffered a soggy setback in late spring. New rainfall records were set in many areas during a warm but unsettled December. NIWA records show many regions received at least twice their average December rainfall. Parts of Marlborough and Nelson recorded the wettest spring ever. This rain drowned some nests, and hampered wasps' ability to forage for food. Landcare Research scientist Dr Jacqueline Beggs says until this year, it was not known that December rainfall could have such an impact on the newly established nests. "We have been studying the beech forest wasps for 14 years now, but we had not seen anything like this December rainfall. Dr Beggs says the December rain's main effect was to wash away the honeydew that wasps feed on.

"Wasps need to collect sugar for an energy kick before they go out and get the protein they also need. So instead being able to catch insects, the wasps just sat around and waited for sugar day after day. "That reduced the growth rate of nests, and the number that actually survived." Dr Beggs says this season there was an average of nine nests per hectare of beech forest. The highest numbers recorded were in 1993, with 17 nests per hectare, and 1998, with 16.

But wasp numbers are still far too high. "New Zealand has the highest wasp densities in the world. In forests in their native Europe, average nest densities are around three per hectare. "At nine nests per hectare, our beech forests are still humming with wasps. These wasps are excellent hunters, and each one reduces the food source of our insect-feeding birds. "Our models show that unless we can reduce wasp densities to just two nests per hectare, wasps will continue to have significant negative impacts on native birds and insects." Dr Beggs says the lower wasp numbers this season have been most evident to city dwellers. "In urban Nelson, wasps were hardly seen this season. Only now at the end of the season are we starting to see a few."


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