During early summer last year our collaborators, Nic Venter and Blair Cowie from the University of the Witwatersrand (Wits) in South Africa, along with Holly Cox (Auckland Council), undertook a survey for the natural enemies of woolly nightshade (Solanum mauritianum) in Uruguay. Field surveys were undertaken in areas with the highest known abundance of woolly nightshade, predominantly in the southeast of Uruguay between the Punta del Este and Aiguá regions.
Image: Holly Cox surveying woolly nightshade.
The surveys focused on the assessment and collection of three promising candidate biocontrol agents: a stem-galling weevil (Collabismus notulatus), a stem-boring weevil (Conotrachelus squalidus), and a flower-bud-feeding weevil (Anthonomus morticinus). The flower-bud-feeding weevil is already undergoing host specificity testing at Wits, but it was targeted for re-collection to boost the genetic diversity of the laboratory colony. Local Solanum species were also surveyed to gain better insights into the insect assemblages associated with close relatives of woolly nightshade to help identify more generalist species.
Twelve insect species were found to be associated with woolly nightshade during this survey trip, but at least three (two beetles and a bug) are known pests. Unfortunately, the stem-boring and stem-galling weevils were not encountered, despite surveying sites where they were previously collected. Although only one of the three target species was collected (the flower bud weevil), two new insect candidates – a flea beetle (Epitrix sp.) and an unidentified leaf-tying moth in the family Gelechiidae – were imported into the containment facility at Wits for further assessment.
The flea beetles were highly abundant on woolly nightshade in Uruguay and were very damaging to plants at some of the survey sites. Approximately 200 adults were imported into containment at Wits in December. These flea beetles are gregarious, mostly clustering on the underside of woolly nightshade leaves and causing extensive ‘shothole’ feeding pits. Although the beetles were not found on most local Solanum species surveyed, they were present and feeding, albeit to a lesser extent, on American black nightshade (Solanum americanum) (native to NZ), when growing in close proximity to woolly nightshade. In addition to this, as well as being difficult to rear (no eggs, larvae or new adults have been observed), adult feeding on potato (Solanum turberosum) in preliminary adult no-choice feeding tests ruled the flea beetle out as a candidate biocontrol agent.
The leaf-tying moth appeared to be the most abundant insect on woolly nightshade in Uruguay at this time of the year, being found at nearly all sites and on plants of varying ages. However, the level of damage was variable, with the foliage of some plants heavily tied (c. 50–70 % of the leaves) and others showing only a few ties. “Leaf ties appear very damaging, causing large sections of the leaf to desiccate and later necrose,” explained Nic. “Because of the moth’s high abundance and apparent damage to woolly nightshade, leaf ties containing larvae were collected for assessment of the moth’s host specificity. However, unfortunately host range assessments were quickly abandoned due to extensive feeding and complete larval development on potato,” he added.
This means that the flower-bud-feeding weevil remains the only candidate agent for woolly nightshade. Approximately 70 adult weevils were collected and imported into containment to supplement the colony at Wits. The field surveys of other Solanum spp. that did not turn up this weevil species provides good evidence of its narrow field host range. These small, black weevils are typically found nestled within the flowers and buds of woolly nightshade and are believed to be the main insect responsible for the reduced flowering, and hence fruiting, of woolly nightshade in Uruguay. Adults feed on the flower petals and anthers, as well as on the young leaves. Eggs are laid into new flower buds, and the larvae entirely destroy the bud as they develop.
Extensive host range testing with the flower bud weevil on crop plants in the Solanaceae has yielded positive results so far, indicating low non-target risks to various prominent agricultural species such as eggplant (Solanum melongena), potato, tomato (Solanum lycopersicum), and chillis/peppers (Capsicum annuum).
“Testing of New Zealand native poroporo species (Solanum aviculare and S. laciniatum) has been severely delayed but remains the focus of this collaborative project, in addition to conducting further surveys in the native range,” said project leader Angela Bownes. “Future surveys will focus on plant pathogens as candidate biocontrol agents for woolly nightshade in New Zealand, alongside the continued search for new insects of interest,” she added.
This project was jointly funded by the National Biocontrol Collective and the Ministry for Primary Industries’ Sustainable Food and Fibre Futures Fund [Grant #20095] on multi-weed biocontrol.
