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.

Watch a video on the biological control of the Eucalyptus tortoise beetle.

Listen to researchers talking about some of the odd places the work has taken them. [RNZ]

Scion will be applying to the Environmental Protection Authority (epa.govt.nz) for permission to release Eadya daenerys from containment in mid-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 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 daenerys, 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 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 predator and prey have evolved 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 daenerys 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 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

Host testing

We need to ensure that Eadya daenerys will not affect the 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. Low numbers also emerged from another pestbeetle from Australia - the small tortoise beetle, Trachymela sloanei.

No Eadya emerged from any of the other non-target beetles tested, although some did contain dead larva inside them that hadn’t been able to complete development.

New Zealand also has around 40 species of native beetles in the same subfamily as Eucalyptus tortoise beetle. They areuncommon, many are tiny, and are thought to live in sub-alpine zones of the South Island. One species, Allocharis tarsalis, veronica leaf beetle, was found and brought into the laboratory for testing. Eadya did attack it on occasion, but the parasitoid failed to develop. 

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 testing environment, and does not mean the parasitoid will do this in the wild. Eadya were not interested in searching plants leaves other than gum leaves, as they would do in Tasmania.

Results to date show that no healthy Eadya offspring were produced from the native veronica leaf beetle or from the beneficial weed biocontrol agent beetles, even though they are in the same sub-family as the Eucalyptus tortoise beetle. Although Eadya will encounter these non-target beetles from time to time, their impact on them should be minimal. We believe the risk to non-target native and beneficial beetles from Eadya is low.

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.

Contact 

Toni Withers, Forest Entomologist, toni.withers@scionresearch.com