| Efforts at Golden Apple Snail Control in Hawaii | |
Harry Ako and Clyde Tamaru
Department of Molecular Biosciences and Bioengineering and
Sea Grant Extension Service, University of Hawaii, U. S. A
E-mail: hako@hawaii.edu
Abstract
A program of control of the apple snail Pomacea canaliculata has been
conducted in Hawaii over the last 6 years. The apple snails devastate taro production.
Elements of the control program include modified culture methods for growing
the crop, use of carnivorous ducks to feed on the apple snails, and hand picking
apple snails to stock aquaculture tanks. Better than 95% control was achieved.
So much pressure was put on apple snails that their populations declined in
wild wetlands surrounding farms. Experience is showing that control efforts
must be continual and apple snail populations return to their former size when
control efforts are relaxed.
Introduction
The apple snail Pomacea canaliculata is native to South America (Cowie, 1993). It was originally imported to Hawaii by the aquarium trade. This is supported by the fact that most animals in the wild are the yellow color variant. A few individual farmers reasoned that if they placed apple snails in taro patches, at harvest the snails could be sold as food and would provide supplemental income in addition to taro. This proved to be a fallacy. In the taro patches snails will multiply rapidly and will destroy the taro. Moreover, hindsight has shown that the snails will spread throughout the irrigation system and contaminate the wetlands surrounding the taro patches. The same faulty reasoning occurred in many rice growing countries of Southeast Asia (Anderson, 1993; Halwart, 1994; Litsinger and Estano, 1993; Vitousek, et al., 1996).
Taro production is of both cultural and economic significance in Hawaii. Currently taro is a $2-3M industry with 7 million pounds grown annually (Statistics in Hawaiian Agriculture, 1993). This is down from a conservative estimate of 14 million pounds in 1948. However, resurgence in taro growing is occurring. This is fueled by native Hawaiian pride and by the fact that there has been a shortage of poi and taro chips in the State. Poi is an excellent food being hypoallergenic and very easily digestible. There is also a shortage of taro leaves, which are used as cooked vegetables. Legend has it that taro was given by the gods to the Hawaiian people. It served the Hawaiian people well providing food. It is in turn, the responsibility of the Hawaiian people to honor taro, which they do to this day by caring for taro. One rarely sees weeds in a taro patch cared for by Hawaiians. Taro plays a role analogous to the eldest sibling in the traditional Hawaiian family structure. The eldest sibling takes care of the younger siblings. The younger siblings, in turn, honor the eldest.
Taro is traditionally grown in patches filled with water. The water flows from streams or springs in the mountains and is carried through the patches. It then returns to the streams which flow into the ocean. Along the way, it is shared by farmers using a complicated array of ditches. It has been argued that the highly stratified chiefdoms of ancient Hawaii evolved as a means of managing water (Kikuchi, 1976).
Before current studies/control efforts began, the State of Hawaii
made efforts to control the apple snail through use of copper sulfate (Estebenet
and Cazzaniga, 1990). This did not solve the apple snail problem. Other disadvantages
include potential toxicity to crops (Robson and Reuter, 1981), lack of specificity/toxicity
to other aquatic organisms (Birge and Black, 1979; Cheng, 1979), and the possibility
of sterilizing the land. This is an especially sensitive issue to native Hawaiians
who have a reverence for the land because the spirits of their ancestors inhabit
the land of their birth. This being the case, poisoning the land is sacrilegious.
We observed intermediate levels of control. Farmers 'weed' their
patches removing snails constantly. However, once taro plants have rooted, they
cannot walk into the middle of the patches without damaging the root systems
of the plants. Hence, their patches sometimes have bald patches in the middle
of them. Bald patches reduce yield and insult a Hawaiian's efforts to care for
his taro. Snails eat secondary shoots on the taro plants (keikis). These yield
secondary corms and additional planting material for the next cycle. We have
seen taro corms damaged by snails. We have spoken to poi millers who complain
about trimming costs and lowered yields. Taro planting is impossible if no efforts
are made to control the apple snail. Taro tops (huli) from a previous crop are
planted in mud covered with a few inches of water. Farmers have reported being
able to watch their newly planted huli topple one after another in a patch filled
with apple snails. Apple snails chew through the tender stalks of young taro
plants.
Methods
Before and after estimates of population densities of apple
snails were made.
Several methods of apple snail control were tried (Tamaru et al., 1999). One
involved changing the method of growing taro. Taro is normally grown in a few
inches of water. A new method was developed in which ditches were dug around
the sides of the patches and taro was grown in mud. The reasoning was that snails
would migrate to the water on the sides of the patches and would be hand collectable.
The strain of taro grown at that site was a hardy strain tolerant of lower water
quality.
Another method that worked involved the use of cayuga ducks.
The method requires that ducks be imprinted on humans. Ducks were trained to
walk out of their cages onto the back of a 4 wheeler in the morning. They would
be driven to a taro patch and would feed on apple snails until they were either
moved to another patch or returned to their cages for the night. If the ducks
are not caged when they are not watched, they will be killed by feral and others
dogs running free in the area.
A third method involved hand collection and aquaculture. Calls went out to the community that apple snails were being purchased at $2/kg. Schoolchildren hand picked snails from harvested patches or from ditches. Sometimes snails were attracted to an area using chicken feed as bait. About 300 snails/bait/2 hr may be trapped. Snails were then aquacultured in tanks. Growth is about 5 g in a month for animals starting out with a mean weight of 8 g. The feed conversion ratio is about 1 meaning that 1 g of feed produces about 1 g of snail meat. The cost of feed is $0.30-0.40 per pound of snail produced. Each 100 gallon aquaculture tank can produce 70 pounds of snails per month and prices to farmers of aquacultured snails was about $8/kg.
Taste tests done by a professional chef and his staff suggested
that wild snails are not gourmet items. They were characterized as 'crunchy'
and had a mean length of 20 mm and mean weight of 4 g, substantially under the
preferred market weight of 15 g. However, aquacultured snails were much better
accepted and would be purchased by the chef if available in constant supply.
The taste panel characterized cultured snails as 'tender', 'chewy', 'juicy'
and 'sweet'. Aquacultured snails have a length to weight ratio of about 25 mm
to 15 g. Aquacultured snails of this weight are young animals with a higher
fat content. This was determined by fatty acid analysis. The excellent physiological
condition of aquacultured snails suggest that glycogen content should be high.
This explains the characterization as 'sweet'.
First efforts at apple snail control were completed near the
end of 1996 in the Keanae region on the island of Maui. This area is the second
largest taro growing region in the state and dominates State Department of Agriculture
statistics for this region. Increases in production in 1998 and 1999 can be
attributed to success in apple snail control.
Snail densities decreased significantly as a result of control methods. Decreases were more than 95%.
| Patch | Snails/90 cm2, before control efforts | Snails/90 cm2, after control efforts |
| Patch 1 | 15.4 | 0.1 |
| Patch 2 | 10.4 | 0.5 |
Discussion
As seen above, the combination of control methods was very effective. The ducks
worked very well to eat small snails, the growing of taro in mud allowed farmers
some income while apple snails were still a problem, and hand picking and aquaculture
added the final touch to the control method. On hindsight, limited attention
would probably have not been as effective as using the combination of methods
described here.
These methods were transferred to other communities via visits between community
members. The use of ducks became very popular on the Waipio on the island of
Hawaii. The apple snail problem in Hanalei was controlled for several years
by strict quarantine but eventually apple snails became a problem. As the area
is a wildlife reserve, ducks are not allowed (because they might interbreed
with wild ducks) and the hand picking/aquaculture approach is being tried. Another
interesting aspect of the control methods is that so much pressure was put on
snails on farms that they disappeared in the wild wetlands surrounding the farms
also.
But the story does not end here. Two years after the major control efforts,
populations of apple snails increased again, and community-wide control efforts
had to be re-instituted. Our own data support estimates in the literature that
if only two snails survive control efforts and breed with mortality among the
young, there will be tens of millions snails at the end of the year. Hawaii
is now in the third major cycle of apple snail control. Perhaps the mind set
needs to change and it should be expected that control of pests like the apple
snail needs to be thought of as continual.
With this in mind, several preliminary experiments were tried. One used ornamental
carp or koi as biocontrol agents. This worked well in the laboratory as the
carp ate the apple snails. Fish have been used in Asia (Halwart, 1992). Saltwater
was tried because a stream infested with apple snails was cleared out when the
hatchery effluent changed from freshwater the saltwater. A two hour exposure
to saltwater was ineffective in the laboratory. Pyrethrin was tried at a 0.01%
level in the laboratory. It was ineffective. However, 50 mg/L hydrogen peroxide
was 90% effective after 24 hr exposure.
References
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