PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Cleobora alive and well in the South Island
Scion is the leading provider of forest-related knowledge in New Zealand
Formerly known as the Forest Research Institute, Scion has been a leader in research relating to forest health for over 50 years. The Rotorua-based Crown Research Institute continues to provide science that will protect all forests from damage caused by insect pests, pathogens and weeds. The information presented below arises from these research activities.
From Forest Health News 140, May 2004.
The introduced eucalyptus tortoise beetle, Paropsis charybdis, has been a thorn in the side of New Zealand eucalypt growers ever since it first caused problems early in the 1900s. Paropsis feeds ravenously on the leaves of certain eucalypt species resulting in growth loss, malformation, dieback, and sometimes death of severely attacked trees. Over the years a number of insect predators or parasitoids have been introduced as potential biological control agents with mixed results, one of the most successful until now being a tiny wasp, Enoggera nassaui , which parasitises the eggs of Paropsis (FHNews 117:1, 130:1).
Another potential control agent, the Tasmanian ladybird Cleobora mellyi , feeds on the eggs and larvae of Paropsis. This insect was deliberately introduced several times into eucalypt plantations in the central North Island, Christchurch, and the Marlborough Sounds during the late 1970s and early 1980s. Although not recovered from the other release areas, Cleobora adults were subsequently found overwintering beneath loose bark in the Marlborough Sounds plantation, a mixed stand of Eucalyptus nitens and Acacia melanoxylon (blackwood). The effect on the Paropsis population was unknown, as there was no follow-up work, and research on Cleobora was abandoned. However, in March 2004 members of the Eucalyptus Action Group of the Farm Forestry Association found a specimen confirmed by John Bain (Forest Research) as Cleobora mellyi in a plantation of Eucalyptus botryoides and E. saligna trees near Havelock.
Shortly after, Dean Satchell (Eucalyptus Action Group) discovered that Cleobora was plentiful on trees at the original release site in Maori Bay in the Marlborough Sounds. Sap-sucking insects on which Cleobora also feeds, such as blackwood psyllids (species of Acizzia), were hard to find, and the black sooty mould fungus (which grows on the honeydew excreted by psyllids) was absent. The blackwood trees were healthy, and the E. nitens crowns were dense, despite moderate chewing damage to foliage.
It is not clear why Cleobora failed to establish in the North Island plantations, where the climate is not dissimilar to that in its natural habitat. In Tasmania the beetles commonly overwinter beneath the loose dead bark of suitable trees, and it was initially thought that such sites may have been lacking in the young eucalypt plantations in the North Island. However, this explanation was invalidated when Cleobora was found overwintering under bark in the Marlborough Sounds stand. It now seems that the survival of the Marlborough Sounds liberations was due primarily to an adequate food supply provided by an abundance of psyllids on the adjacent acacias. It is known that Cleobora does not mate and reproduce readily on a diet solely of Paropsis eggs and larvae.
Although Cleobora may not contribute greatly to the decline of Paropsis populations, it is likely to provide some level of control of other pests, such as acacia and eucalypt psyllids (e.g., the brown lace lerp, Cardiaspina fiscella; FHNews 95:1), scale insects (e.g., species of Eriococcus), and the blackwood tortoise beetle (Dicranosterna semipunctata, FHNews 79:2, 117:2). In addition, a number of new psyllids have become introduced into eucalypt plantations since the early attempts to introduce Cleobora, when only the blue gum psyllid (Ctenarytaina eucalypti) was present. These new insects may provide a more balanced fare for Cleobora, should attempts again be made to spread it more widely within eucalypt plantations throughout New Zealand.
For further information, see What’s New in Forest Research 184:1–4 (New Zealand Forest Research Institute, 1990), New Zealand Tree Grower 25 (2):26–27, or visit: www.nzffa.org.nz
Dean Satchell, Eucalyptus Action Group, and Ian Hood and John Bain, Forest Research
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(Scion is the trading name of the New Zealand Forest Research Institute Limited.)