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March 2004. Samoa is continuing work on the fruit piercing moth. Three parasitoids are present: Telenomus luculus, Ooencyrtus crassulus and Trichogramma australiscum and are being mass produced in the lab and released in the field. Egg parasitisation surveys are continuing and the results are always below 50%. Consequently, fruit damage is high. Are there other parasites to introduce or lures to test?
Palau also asked about Erythina as E variegata has been introduced in recent years. The tree is common in Samoa where it is used as a living post. The link between Erythrina and outbreaks of the moth is well established. Paul Cochereau undertook what is probably the most definitive study on Eudocima fullonia and its relationship with Erythrina in New Caledonia. This work is reported in Cochereau P (1977) Biologie et ecologie des populations en Nouvelle-Caledonie d’un piqueur de fruits: Othreis fullonia Clerck. Trav. et Doc. L’ORSTOM No.71. 322p.
Research on lures
DPI&F mentioned that there are possibilities for area-wide control of E fullonia on Erythrina in the Pacific (to
supplement the biological control) using Gemstar (NPV based larvicide) or Bt. A funding organisation is needed to support the work. ACIAR had intended to review the fruitpiercing moth situation in the Pacific (last year) to determine whether the earlier work by CSIRO needed revisiting.
DPI&F has been working on attractant baits for fruitpiercing moths, as reported previously on Pestnet. Trials in commercial crops are in their 3rd year and, in citrus, it appears that up to 70-80% control can be achieved (assuming the insecticide in the baits always does its job). Control levels may come back somewhat if moth populations are extremely high. The baits work as decoys so are most effective before fruit are fully ripe. Trials with baits in the Pacific would be a good idea.
Research in the Pacific: a summary
In the Pacific islands, FPM populations are commonly kept under biological control by the parasitoids, Ooencyrtus and Trichogramma. Pheidole megacephala) and Chrysopid (lacewing) larvae are important predators that, perhaps, have an even larger impact on the populations than parasitoids, as they feed on large numbers of eggs, both parasitized and non-parasitised. Levels of damage to oranges in those years amounted to 30-35% of fruits, but this was in a very poorly maintained orchard (at Asau, Savaii) and production levels were very low.
However, two types of environmental disasters induce outbreaks of the moth: cyclones and droughts. Both have a significant impact on the host plant, which results in less suitable food for the FPM larvae, increasing mortality, lowering the number of adult moths, which then produce less eggs. This then affects the survival and reproduction of the parasitoids. Their population increases when the moth population increases, but there is a lag of 1-2 generations of moths (some 1-2 months), before the parasitoids have ‘caught up’ again and restored the balance. In the meantime, 4-7 months after the ‘disaster’, an outbreak of the moth can be expected.
With cyclones this may not be such a serious problem, since they destroy much of the fruit crop, which would not have recovered by the time the outbreak occurs. Following droughts, however, there can be losses of 100% of oranges in Samoa.
Since the last 10-15 years, Samoa has embarked on a crop diversification program, which has seen the development of a number of other fruit. Several of these are soft-skinned, and are easily penetrated by FPM.
However, they are also easily pierced by moths with a weak proboscis, that are not strong enough to pierce an orange skin. Thus, these crops are subject to attack by a much larger number of species.
For the commercial cultivation of those crops, the grower should consider covering the trees with nets.
The relationship between Erythrina and FPM is also an interesting one, and one that was considered in the 80’s. Eradication was discussed, but was deemed highly unfeasible in the hot and humid Samoan climate;
Erythrina was already abundant, and there were at least three species that were larval hosts. In the ensuing years, when land became available for farming, many farmers began using Erythrina as live fence posts. These are regularly pruned, which promotes growth of young shoots (which provide the best food source for FPM larvae).
Eradicating Erythrina from Samoa remains a highly unfeasible option. Restricting or prohibiting its use by legislation could be an option, but this raises the issue of enforcing it – always a difficult undertaking – but possible if there is a genuine effort that is maintained over a long time. Other countries where Erythrina is not so widespread would, perhaps, not have such a problem, but then they would not have a big problem with the moths anyway – unless there are other host plants (i.e. Menispermaceae).
As mentioned earlier, the use of nets for commercial farmers (for carambola, guava, etc.) is a good option, which would probably pay for itself in the first year. For subsistence and semi-subsistence farmers and home-gardeners, the initial investment would likely make this option unfeasible. For high value fruits, another option is bagging (with paper bags) to protect the fruit from fruit flies – very cheap. Perhaps a lure would be useful as an alternative. But how would the expense of a lure be financed?
Mass rearing of FPM parasitoids that are already established in the field (such as in Samoa) is not a good idea. First, mass rearing is only possible on FPM eggs – which are difficult to obtain. Second, under ‘normal’ conditions (most years) the parasitoids maintain the FPM population under adequate control – data collected over 5-6 years showed that many of those eggs collected from the field were already parasitized. Research (unpublished, unfortunately) to determine life table data of eggs in Tonga showed that only 3% of all eggs deposited in the field actually developed into larvae, and although first instar larvae were found occasionally, later instars were extremely rare, perhaps indicating the presence of an effective larval predator. One cannot reasonably expect to lower the egg survival rates – for any additional releases to have an impact one would need to release thousands each week on a specific site. And it is impractical to continue rearing them over the years whilst awaiting an environmental ‘upset’ to occur.
Control strategies should be considered on a ‘per crop’ basis. An early step in such consideration would be to
determine if and what other moth (and other pest) species attack the fruit (when immature and ripe). The market value of the crop (at various times of the year) would also play a role in deciding what control options are feasible.
On the question of specificity of the parasitoids
There are other moths that attack fruit. There are ‘fruit piercing moths’ and the ‘fruit sucking moths’. The first group is capable of piercing through (relatively) tough skins of fruit with their more heavily sclerotized proboscis (sucking mouth parts), whilst the proboscis of the latter is not as strong to allow piercing of tough fruit skin. Of course, a species that cannot pierce the skin of an orange may well be able to pierce the skin of a carambola fruit – so the difference is more linked to the kind of fruit than to the moth. It becomes more complicated when one observes both categories of moths attacking fruit with tough skins, such as orange. However, in those cases the ‘fruit sucking moths’ use the holes pierced by ‘fruit piercing moths’ to access the juice of the fruit.
As for a relationship between Erythrina sp. and the abundance of the fruit piercing moths), it is clear that the presence of low numbers of host trees do not provide sufficient food to allow the build up of large populations of the moth. The converse may be true, however. The trees provide opportunities for the moth (its larvae, eggs and pupae) to ‘escape’ from predators and parasitoids, and survive to adulthood), even though parasitization and predation ratios can be very high. To put it another way: even if there is 98% mortality of immature stages, there is still 2% that will develop into adult moths. If only a few trees are present, and there are 10000 immature insects, this number would yield 200 adults. If 100 times as many trees are present, this would then allow the development of 20000 adults – presuming the mortality remains the same, 98%. So, it is much better to have fewer host plants.
And on the value of Erythrina as fence posts
Erythrina variegata and E subumbrans are excellent species for fence posts: they are nitrogen-fixing trees that are easily planted from cuttings; their soft wood is easily pruned and can sustain heavy pruning; the leaves provide a good mulch; they provide quick regrowth; and cattle feed on the leaves. Hibiscus tiliaceus does not have all these qualities.
Other comments
A question was asked if BTK (Bacillus thuringiensis kurstaki) has been tested against fruit-piercing moth – it has been extremely useful in the eradication of painted apple moth in Auckland. Many of the secondary moths need an initial puncture, either from Othreis or fruit fly to allow them to insert their proboscis and suck. These hangers on are often those that gather on over-ripe and damaged fruit.
CSIRO carried out studies on these moths in the Pacific for several years and from reports two parasitoids that were introduced have lowered the abundance and reduced the impacts by the moths except after cyclones and drought. There has a recent outbreak in southeastern Queensalnd following drought, the first for several years. Numbers will undoubtedly decline if our climate returns to normal!
Nets with a small mesh, smaller than usually used, are recommended (ca. 8 mm). They need be be placed over the trees about 1 month before ripening and can be removed and packed for storage for the following years. They have the advantage of keeping fruit bats and birds from attacking the fruit, as well as the moths, if they are mounted appropriately and kept well clear of the outer foliage bearing fruit. Both small and large plantations can benefit from using these nets and they have been shown to be cost effective in the longer term.
A paper by Don Sands and Wilco Liebregts, presented at the 2nd International Symposium on Biological Control of Arthropods, Davos, Switzerland, 12-16 Sept. 2005, provides information ton the parasitoids of PNG and elsewhere in Pacific Island countries.
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