Two New ‘World-First’ Agents Ready for Combat
Two new biocontrol agents have recently been approved for release: the privet lace bug (Leptoypha hospita) and the Japanese honeysuckle stem beetle (Oberea shirahatai).
The Environmental Protection Authority (EPA) granted approval to release the privet lace bug in May, with Waikato Regional Council as the applicant on behalf of the National Biocontrol Collective. This is the first agent to be used against Chinese privet (Ligustrum sinense) both in New Zealand and worldwide.
Only two of the four privet species naturalised in New Zealand, Chinese privet and the less widespread but larger tree privet (Ligustrum lucidum), are considered serious problems at present, particularly in the North Island. Tree privet can grow above the canopy of many native trees and the smaller Chinese privet prevents native seedling regeneration, making both species environmental pests. Privet is also a problem in places that are not intensively managed such as along roadsides and railway lines or in hedgerows. Tolerant of a range of growing conditions, privet grows equally well in dry stony ground and the heavier soils found around the Waikato. The leaves and dark purple berries are poisonous to humans and other animals. Many people, especially in urban areas, believe that privet is responsible for allergic reactions, but the evidence for this is a bit controversial. Possibly the perfume rather than the pollen is an irritant for these people. The berries are attractive to birds, which are great at spreading the seed around to new places.
Although manual control can be effective (but expensive) and herbicide application is an option in some areas, biological control is the only reasonable way forward for improving widespread control of these species.
The privet lace bug was identified as a good prospective biocontrol agent by scientists in the USA, who started working on a privet biocontrol project ahead of New Zealand. The lace bug adults and nymphs pierce and suck the sap from the privet leaves damaging the leaf tips, leading to defoliation and reducing the vigour of the plant. “In its native range the lace bug reportedly attacks a range of privet species in addition to Chinese privet and host-range testing has indicated that other Ligustrum species present in New Zealand are potential hosts, so it will be interesting to see which additional privet species it can survive on in field conditions in New Zealand,” said Quentin Paynter, who has led the New Zealand work.
“We have been fortunate to be able to ‘ride on the coat-tails’ of the Americans with this project as they had already done quite extensive host testing on plants in the Oleaceae family, which includes olives, when we got interested in the project. The only native plant that needed testing here was Nestegis, the only New Zealand plant in the Oleaceae family, and fortunately this was not attacked by the lace bug. “We also tested the ornamental lilac (Syringa spp.) varieties that are grown in New Zealand, but the survival of the lace bug on these was very poor so it is expected that there will be only limited spillover non-target attack, if any at all, on lilac if there is privet nearby,” said Chris Winks, who has been working alongside Quentin, looking after the lace bugs in the Auckland containment facility.
“The early indications are that the lace bug is relatively easy to rear, which is definitely a bonus,” said Quentin. “Mass rearing is underway with the first field releases scheduled to get underway this spring,” he added. It is not certain how many generations would occur in the field in New Zealand, but in the field in China the lace bug is present as both immature and adult stages for 7 months of the year from early spring. Given the observations in its native range, at least two generations are likely in New Zealand.
Meanwhile, the Japanese honeysuckle stem beetle gained EPA approval for release in July, with the Hawke’s Bay Regional Council as applicant on behalf of the National Biocontrol Collective. This use of this agent is also a world first, and it is hoped that the beetle will complement the recently released Honshu white admiral butterfly (Limenitis glorifica), which attacks the leaves of the plant. The butterfly was released last season at two sites and we hope this spring to be able to find evidence of establishment and possibly begin to redistribute it.
Adult stem beetles lay eggs by boring a hole into the stem and sliding an egg down the stem, just underneath the bark below the hole. From here a larva emerges, feeds initially on the cambium bark material then crawls up to the hole and burrows into the pith at the centre of the stem, to feed and mature. Leaf and shoot growth above the larval feeding site becomes stunted and often dies as the larva feeds. Larger, older stems 5–15mm in diameter are preferred by adults laying eggs and for larval feeding. The beetle larvae take up to 2 years to develop through to adult stage and during this time they can do a lot of damage, killing entire stems. Adult beetles were recently collected in Japan, and brought back to our Lincoln containment facility by Hugh Gourlay in June, with the aim of trying to start a colony here in New Zealand. Stem borers are notoriously difficult to rear because the larvae are quite particular about the size and texture of the stems that are available, as well as taking so long to develop. Efforts to rear the stem beetle to date inside containment have found the Japanese honeysuckle beetle to be no exception. “We have to keep the honeysuckle in a healthy condition for 2 years in pots and this is not always easy, especially when they have been damaged by the beetles,” explained Hugh.
“We are anticipating that mass-rearing may prove to be too difficult and that it might be best to repeat what we did with the Honshu white admiral: establish the beetle at one or two safe field sites from which we can later harvest for redistribution,” said Quentin. One advantage of this species over the white admiral is that it will at least breed in captivity, so there will be the potential to maintain a small ‘back-up’ colony. “The beetles I brought back from Japan have laid eggs and we now have some larvae developing on Japanese honeysuckle plants growing in our containment facility,” said Hugh. “Because they are quite tricky to collect in large numbers in the field in Japan, we will only have a relatively small number to work with once the usual 30 are sacrificed for disease testing,” he added. At the last count, Hugh had around 100 larvae to form the basis of a population. Once the beetles have been confirmed to be free of disease, Hugh will apply for permission to remove them from containment. We will likely need to collect more beetles from Japan next year to boost numbers and augment the genetic diversity of our population.
“We won’t really know when field releases can begin until we know how well our attempt to “rephase” them to Southern Hemisphere conditions goes and see how many adults we can rear through,” cautioned Quentin. Because the larvae take so long to develop and it is difficult to keep potted plants alive in glasshouses, we are hoping to cage potted plants in the field to allow more natural development of the larvae and plants. The cages will ensure that emerging adults can find each other and mate, and be collected up more easily for release at new field sites.
Hugh is also experimenting with rearing the larvae on an artificial diet. Overseas studies on similar beetles reared on artificial diet suggest they develop faster and can take only 1 year to develop to an adult. It may take some time to develop a successful artificial diet approach, but if Hugh can crack this it may eventually allow us to rear larger numbers of beetles indoors in addition to harvesting from field sites.
Both projects were funded by the National Biocontrol Collective. Japanese honeysuckle stem beetle rearing has also been supported by Landcare Research’s Capability Funding.