The biological control of Eucalyptus tortoise beetle
Protecting our Eucalyptus trees
Eucalyptus plantations are a recognisable part of New Zealand’s diversified forestry industry. They provide pulp and timber, with additional benefits in farm forestry, honey production, firewood and carbon sequestration. Eucalypts are also widely used as amenity trees in urban areas.
The eucalyptus tortoise beetle (Paropsis charybdis) has been a pest of eucalypts in New Zealand for over 100 years. It causes significant damage to shining gum (Eucalyptus nitens) as well as other gums such as the coastal grey box (Eucalyptus bosistoana), that hold potential for producing ground durable wood.
Proposed biological control
Scion (NZ Forest Research Institute Ltd) and the New Zealand Farm Forestry Association want to introduce a new biological control agent into Aotearoa from Australia to control the larvae of the Eucalyptus tortoise beetle.
The biological control agent is a natural predator of Eucalyptus tortoise beetle, cannot interbreed with native species and will not affect human health.
We are seeking feedback on people’s views on the release this biological control agent. Issues raised during consultation will be incorporated into our submission to the Environmental Protection Authority(EPA) in October 2018.
Rapid population growth
There are no native Paropsis beetles in New Zealand and therefore no natural enemies to attack them. This enables populations of eucalyptus tortoise beetle to grow at an alarming rate. The beetle produces two generations a year with both larvae and adult beetles eating large amounts of young eucalypt leaves for many months of the year.
Host-specific biocontrol agents that target the egg stage have been introduced to New Zealand from Australia, but are insufficient to manage down the numbers of the pest in springtime. The beetle feeding damage can completely stop the trees from growing.
Scion scientists have been studying a specialist parasitoid wasp, Eadya sp., in their native habitat in Tasmania. These small wasps have evolved to only attack the larval stage of the life cycle.They attack just a few species of eucalypt leaf-feeding beetles, and nothing else. Scion entomologists have imported this parasitoid into a secure containment facility at Scion in Rotorua for safety testing to see if they attack beetle larvae from different species.
What is biological control?
Biological control (or biocontrol) is is an effective and environmentally sustainable method of managing unwanted pests. Natural enemies are used to reduce pest populations in countries where they have been unwelcome invaders. It often involves the importation of a specialist insect parasitoid from the pest’s native environment where they have evolved closely together. It is one of the most important alternatives to chemical pesticides.
What is a parasitoid? A parasitoid lives most of its life attached to, or inside a single host. If it cannot find its host, it will die without being able to reproduce.
Sustainable and economic. Successful biocontrol can substantially limit pesticide use, and sometimes removes the need for pesticide application at all. It sometimes takes a few years to see these benefits.
Eadya sp wasps (Eadya). Adult parasitoid wasps are about 10 mm long with a black body and orange head. The parasitoid specifically hunts for Eucalyptus leaf beetle larvae on gum leaves, stings them and lays an egg inside them. Only one parasitoid develops within each larva.
The parasitoid larvae feed within the host beetle larvae for about 21 days. They then emerge, killing the host, and pupate out of sight within the soil. After hibernating as a pupa for about ten months, the adult parasitoid hatches in springtime. Only one generation is produced per year.
Evaluating Eadya within containment
We need to ensure that Eadya will not impact on larvae of any native or beneficial beetles before recommending its release as a biological control agent to New Zealand. This is called host testing.
Importing Eadya into containment has allowed our entomologists to assess its host specificity against a range of beetles to determine whether any other beetles could be at risk.
Eadya emerged from approximately half of attacked eucalyptus tortoise beetle larvae, which is expected given natural parasitism rates in their Australian native range.
Most importantly for assessing risk, the tests revealed Eadya only emerged from invasive pest leaf beetles of eucalypts .No Eadya emerged from any of the other non-target beetles tested, although some did contain dead larva inside them that hadn’t sufficiently completed development.
Scion experts also observed the behaviour of Eadya closely during host specificity testing. They saw that Eadya sometimes did attack other non-target beetle larvae. This can happen within the artificial confinement of a small petri dish, and does not mean the parasitoid will do this in the wild. Eadya were not interested in searching other plants leaves, only gum tree leaves, as they would do in Tasmania.
Results to date show that no healthy Eadya offspring were produced from any of the beneficial beetles (the weed biocontrol agents) that were exposed to Eadya, even though some are related to the eucalyptus tortoise beetle (in the same sub-family).
Frequently asked Questions
Q: The parasitoid is a wasp, so will it sting me?
A: No, the stinger on the Eadya parasitoid is designed only to sting a soft little beetle larva and cannot pierce human skin.
Q: Will this parasitoid wasp become a pest like the German wasp?
A: No, Eadya parasitoids are not that kind of wasp. Eadya only feed on Eucalyptus leaf beetles and are you are unlikely to ever notice them as they do not form colonies.
Toni Withers, Forest Entomologist, email@example.com