3.2 Other impacts of possums on primary production
The major impact of the possum on primary production is as a Tb vector. The possum's impacts on pastoral farming, horticulture and forestry are of less significance, and the extent of these impacts has not been well quantified in economic terms, at a national level. Eight possums are about equivalent to one stock unit in terms of the pasture feed they consume. Hence, two to three possums per hectare on farmland can represent a significant loss of potential production. In Taranaki, this annual loss is estimated to be about $3,000 for each 100 hectares on dairy farms.
Possums caused significant damage to the new plantations of Pinus radiata in the central North Island, established in the 1960s and early 1970s. At some sites, up to 90% of young pine trees were browsed, and up to 50% may have died following possum damage to the terminal shoots (leaders) of young seedlings. Possums also bend and break terminal shoots and lateral branches in the upper part of young pines. Economic losses certainly exceeded control costs per hectare, and possums were controlled using 1080 and other methods in a number of plantations. Once Pinus radiata reaches 14 years old, possums have little effect on the trees. Areas with new plantings are likely to experience possum damage where possum densities are high. The value of a 5% loss at planting in a Pinus radiata plantation could represent losses at harvesting of between NZ$282 and $840 per hectare, at current prices.
The effects of possums on horticultural production are widespread, but poorly documented. Damage, which is often seasonal and patchy, has been reported to at least 46 varieties of fruit and vegetables. This damage is most common in areas close to trees and native forests.
The other significant damage can be to poplar and willow poles used for catchment protection and erosion control purposes. Young plantings can be protected from broken branches and possum browsing by using protective sleeves.