If porina moth larvae are a problem on your land, read on. One of the most useful approaches is to prevent the moths from laying new eggs according to this article from Christchurch Polytech's Holger Kahl and his organic course students.
Firstly, the key to tackling any pest problem is to know and understand your pest’s lifecycle and therefore its vulnerabilities. There are several websites that have detailed information on the timings of reproduction, emergence and over-wintering etc. Also try HortNet
The moths are highly attracted to light, only flying at dusk, and are only active over a very specific time period (unfortunately there are three types of Porina moth, Wiseana cervinata, W. signata and W. umbraculata, and each of them have slightly varied times of activity and it also varies depending on regional and environmental conditions, but for a vague estimate, they will be active October to November). This could provide the opportunity for light traps, perhaps involving strong lights and yellow sticky cards? This may be difficult over a large area of land, but if the reproductive cycle can be broken, the results will be well worth the effort. You may have to get fairly inventive here. To save the costs of sticky traps, perhaps honey on cardboard, or some kind of adhesive that will remain tacky to trap the moths will work. Battery powered lights or torches may be an option too.
As yet there doesn’t seem to be a pheromone trap on the market for this moth, the research seems to be concentrated on orchard crops, not pastures.
The eggs remain on the surface of the pasture for a day or two and this provides us with a small window of opportunity. Monitoring of moth activity should indicate when eggs are beginning to be laid. Once this is known, action can be taken. The most obvious option seems to be crushing or squashing to eggs by compaction. This can be achieved with the use of heavy machinery or preferably, using animals to "mobstock", that is, get them milling about in an enclosed area to compact the earth and hence squash the eggs. Eggs hatch in 3-5 weeks in moist conditions.
This the stage when most damage is done. The larvae feed on the aerial parts of the plant, eating the foliage off at ground level. They live in tunnels 15-50cm under the surface of the pasture. Feeding occurs mostly at night. This is another stage when they are vulnerable to trampling/compaction. The larval stage lasts from December to August.
Pupation occurs under the surface in small silk lined tunnels, taking about a month. Then the adults emerge, ready to start the whole thing again!
A Place to Start
I think the most likely place to start would be with preventing the moths from laying new eggs. Once the eggs are laid, it is very difficult to tackle the problem as the larvae are largely below surface level. I’d start with monitoring moth activity and trapping as many as possible using lights and something sticky.
This is safe to humans and accepted under most if not all organic standards.
There may be possibilities for using B.T (Bacillus thuringiensis) a biological control that is ingested by the larval stage of the Lepidoptera (Butterfly) family. This works by paralysing the digestive tract of the caterpillar, causing death within 12hrs to five days.
Changing what is growing in the pasture may deny them a food source and reduce populations. I couldn’t seem to find much information on which specific cultivars they like and dislike, but it has been documented that they feed on "pasture grasses, clovers and lucerne", which leaves few other options! Try investigating the possibility of herbal leys which are unpalatable to the Porina. Unfortunately I have no information on this, but seed suppliers or consultants in this area may have some info for you. This will also aid your stock’s nutritional intake and improve your soil structure, so is worth looking into anyway. There is also the possibility of cultivating the soil, i.e. ploughing it up, to expose the larvae to birds. This may have some effect on reducing populations.
This is probably not practical on a large scale, but consider using a biological resource to help with your pest control. The best options are either pigs or chickens. Pigs can totally turn the soil over, exposing the larvae, either to eat themselves, or for the birds to eat. Chickens will peck and dig at the soil, eating whatever treats they find too. I don’t know for sure whether either of these will definitely find the Porina palatable, but it is worth a try! These will have the added bonus of recycling kitchen waste and producing meat, as well as eggs in the case of the chickens.
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