Economic impact of eucalyptus tortoise beetle (Paropsis charybdis) in New Zealand
By Robert I. Radics, Toni M. Withers, Dean F. Meason, Toby Stovold, and Richard Yao, August 2018.
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A number of tree species in the genus Eucalyptus are grown in New Zealand on a small scale. However, the economic value of this resource is not known. The economic impact of damage caused by a pest, the eucalyptus tortoise beetle (Paropsis charybdis), to Eucalyptus species in grown in New Zealand is also not defined.
The current approach
The standing Eucalyptus crop in New Zealand was valued in terms of projected yield and other ecosystem services. Also, the cost of Paropsis charybdis damage to Eucalyptus forests was estimated along with the costs and benefits of chemical and biological control of this pest.
The total planted area of Eucalyptus species in New Zealand was estimated to be 27,598 ha with a standing volume of 8.1 million m3, with a conservative asset value of $671 million. This could be increased in the future if higher value products (e.g. wood flooring or ground-durable poles) are produced from the existing Eucalyptus estate and future plantings. Paropsis charybdis shows a strong feeding preference for Eucalyptus nitens, which is the major species grown for the production of wood chips for paper making. Paropsis charybdis finds many species in the eucalypt sub-genus Symphyomyrtus palatable but all show different susceptibility to the pest. We know the proportion of susceptible species in different stands will differ between regions. Regional forest inventory data from the Ministry for Primary Industries (MPI 2016) was examined and combined with Scion in-house species-site matching knowledge to estimate the proportion of Eucalyptus species in each region that might be palatable to P. charybdis. From this exercise, the weighted average across New Zealand of Eucalyptus plantations susceptible to P. charybdis was estimated at 60-75%. Therefore, $402-$503 million worth of Eucalyptus stands have a high potential of being damaged by P. charybdis.
Benefit : Cost of managing Paropsis charybdis
Damage caused by P. charybdis in terms of yield loss can reach $10,000 ha-1 in the case of low- severity, $30,000 ha-1 for medium-severity and $60,000 ha-1 following high-severity attack at the end of a 40-year rotation. The value of the damage is lower ($1,600 ha-1 low-severity, $4,800 ha-1medium-severity, and $9,700 ha-1 high-severity) for shorter (15-year) rotation pulpwood plantations. However, in the absence of effective chemical control, the rotation period of a severely damaged stand will need to be extended to obtain the same volume at harvest as an unaffected stand.
There are approximately 15,300 ha of vulnerable E. nitens within short-rotation pulpwood plantations in New Zealand and the potential yield loss due to P. charybdis damage is estimated at $10 million per year. The current management method involves chemical control by aerial spraying with insecticides once or twice per year. This costs $160 ha-1 per year for plantations >40 ha. Chemical treatment was found to be uneconomical for small plantations or woodlots (<10 ha) at an estimated $340 ha-1 per year. The current chemical control costs an estimated $1.0–$2.6 million/year and the Net Present Value of the pest control of all susceptible Eucalyptus species is $30-$38 million in New Zealand over a 40-year rotation.
Comparison of biological control with chemical control
Effective biological control would reduce damage caused by P. charybdis with no on-going costs once the agent is established. In contrast, chemical control involves on-going costs that vary based on the size of the plantation. In most situations, biological control was found to be more cost effective than chemical control. Large plantations (>40 ha) will need to be protected by chemical control when damage is severe but this is an uneconomic method for plantations <10 ha, and not economically justifiable yearly when damage is light to moderate. Thus, small growers are reliant on biological control to realise the value of their woodlots. Effective biological control will prevent an average yield loss of 4.1 m3 ha-1 per year in susceptible Eucalyptus stands, which is equivalent to $417 ha-1 per year in value. Effective biological control with Eadya daenerys could prevent $5.8- $7.2 million in losses per year for the current Eucalyptus spp. stands established in New Zealand.
Eucalyptus in the context of ecosystem services
Exotic planted forests in New Zealand (including Eucalyptus forests) provide important environmental benefits. These include carbon sequestration, habitats for taonga species, shelter, shading and avoided nitrate leaching. Such benefits are not considered in market transactions but their values can be approximated using environmental economic valuation techniques. The quantifiable environmental value of existing Eucalyptus plantings was estimated to be about $11 million per year. However, these environmental values should be considered as indicative only as the value of these ecosystem services can vary substantially across space and time, as well as across tree ages and forest management practices.
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