Ginger Biocontrol Edges Closer to Fruition
Ginger weevil lifecycle. Image - CABI Europe-UK
Research aiming to biologically control wild ginger (Hedychium gardnerianum) is now in its fifth phase and prospects of success remain promising. Since our last update (Issue 61), the team at CABI Europe-UK have continued to make progress.
The large red and black weevil (Tetratopus sp.) is still looking good following further host-range testing. Despite some feeding on test plants in the Zingiberaceae by adults, egg-laying and feeding by larvae was only observed on white ginger (Hedychium coronarium), yellow ginger (H. fl avescens), kahili ginger (H. gardnerianum), and importantly the hybrid species that is weedy in New Zealand and Hawai’i (thought to be H. gardnerianum x H. coronarium). The weevil appears to have a preference for Hedychium spp. both in the lab and in the field. The larvae feed in the rhizomes and can cause spectacular damage. Field observations and other evidence suggest that their developmental rate is slow and that there is only one generation per year. This has meant it is taking a long time for CABI to build up a sizeable captive population of the weevils for study and testing. They are now making efforts to see if development can be speeded up by feeding the larvae on a highly nutritious semi-artificial diet. This technique has worked well for other insects, such as purple loosestrife root-feeding weevil, which developed in just 2–3 months instead of 1–2 years. Unfortunately the ginger weevils were not interested in feeding on the prototype diet offered, and some experimentation will be required to identify a suitable formula.
The ginger frit fly (Merochlorops dimorphus), which is a stem-miner, is also still shaping up well. Some good advances have been made in understanding its life cycle, helping overcome initial difficulties rearing the fly in containment. “It takes about 2–3 months to develop from an egg to the adult stage and the fly makes its first appearance in spring when it emerges from the tip of the ginger stem,” said Djami Djeddour, the lead scientist for the project. A second generation makes its appearance later in the summer. In the fly’s native range, the winter is sufficiently cold to induce a dormant phase in their life cycle but it is not clear whether they over winter as adults or pupae in the ginger stem. Testing to date suggests that the fly will only complete its life cycle on kahili ginger and the hybrid species of interest to New Zealand and Hawai’i. None of the other test plants offered have been attractive to the fly. Although the fly has been challenging to rear, the team at CABI are optimistic about establishing a large culture so they can complete host testing and are well placed for mass rearing down the track.
On the flip side the gregarious moth (Artona spp.) is now looking like a less promising potential agent. Larval choice tests have shown that while they had a clear preference for yellow and white ginger they only fed to a limited degree on the hybrid ginger species of interest to New Zealand. There was also some feeding on plants outside of the Zingiberaceae such as canna lily (Canna indica). Unfortunately the larvae proved to be heavily parasitised with few making it through to adults, so oviposition tests to see what plants the moths will actually lay eggs on could not be carried out, and the colony died out. “Maintaining good breeding populations is critical to successful host testing and all of the most promising ginger agents have proved challenging in that regard,” explained Djami.
Work to identify and study other insects found during recent surveys in India is also continuing and may yield more potential candidates. But for now the red and black weevil and the ginger frit fly are the front-runners and it is hoped that host testing of at least one of them can be completed in the next year.
This project is funded by the National Weed Biocontrol Collective and the Nature Conservancy of Hawai’i.