Tradescantia Beetles Gain Momentum
Typical stem beetle larval damage (adult inset).
Tradescantia (Tradescantia fluminensis) ranks among the top targets for environmental weeds in New Zealand.
This is because it forms a dense mat and suppresses native regeneration in forest remnants, and once established it is extremely difficult to eradicate. Any broken stems are capable of resprouting and forming new plants. Herbicide needs to be applied multiple times and can be damaging to beneficial plants. Happily, surveys in the native range of the weed in Brazil identified many natural enemies that prevent it from forming the dense mats that are so harmful here. Since tradescantia is also a major problem for many home gardeners there are likely to be a lot of very happy people if biocontrol can be successful.
Following the successful release of the tradescantia leaf beetle (Neolema ogloblini) in March last year, a second beetle species, the tradescantia stem beetle aka “Knobbly” (Lema basicostata) , has now joined the attack against this weed. The stem beetle has been liberated this autumn at sites in Northland, Auckland, Bay of Plenty, Waikato and Wellington, with further releases planned for next spring. The stem beetle is a similar size (4-5 mm) to the leaf beetle but is black and has bumps on the elytra (wing cases), hence the nickname. Unlike the leaf beetle which feeds on the foliage, the larvae of this species mines the stems of the plant.
“While the two beetles are expected to co-exist happily and be complementary in their damage, we are initially releasing them at different sites to prevent any competition during the critical establishment phase, and to enable the impact of each species to be assessed,” said Quentin Paynter. “We are really excited about the stem beetle because in Brazil, its larval mining was associated with fairly dramatic wilting and collapsing stems, and it is absolutely demolishing the plants that Chris Winks is mass-rearing it on in Auckland.”
Since the original release of the leaf beetle in Auckland, a further 28 releases have been made around New Zealand. Damage to tradescantia plants has been observed already at a number of release sites checked to date, so establishment is looking promising. Typical damage to the leaves to look for includes notches along the edges through to skeletonised leaves where the beetles have grazed all the green tissue off the leaf surface. “The adult beetles can be hard to spot as they tend to drop to the ground when disturbed but the slug-like larvae and ‘styrofoam-like’ pupal cases may be visible on the leaves,” said Chris.
It is not known how fast the two beetles will breed in New Zealand but it is hoped that there will be at least two or three generations each year in warmer areas. “Biomass samples of tradescantia have been taken from a range of sites prior to the release of the beetles so that later on we can determine how well the beetles are suppressing tradescantia. Once establishment of the leaf beetle has been confirmed (i.e. survived two winters), a more intensive monitoring programme will begin,” confirmed Simon Fowler.
A third beetle which attacks the tips of a range of tradescantia leaves, nicknamed “Stripy” (Neolema abbreviata) has not been released as yet. However, despite only having a small number of gregarine-free adults to work with initially, mass rearing is going well and releases are expected to begin next spring. Permission to import and release a fourth agent, a Brazilian fungus (Kordyana sp.), is next on the agenda for tradescantia (see: White Smut in Shining Armour?).
This project is funded by the Department of Conservation, National Biocontrol Collective, and the Ministry of Science and Innovation under the Beating Weeds Programme. This project would not have been possible without assistance from Professor Pedrosa-Macedo and colleagues at the University of Parana, Brazil.