| Country or region name |
Japan |
| Organism group |
insect |
| Order name |
Coleoptera |
| Family name |
Brentidae |
| Species name |
Cylas formicarius |
| English common name |
sweetpotato weevil |
| Substantially same species (synonym) |
|
| Year of invasion or detection |
1903 |
| Native region |
India (Wolfe,1991) |
| Situation of establishment |
Category 4: Settled after 1951, but not distributed all over the
country |
| Taxonomic description |
| |
| Expansion of distribution area |
| In Japan, Cylas formicarius was first noted in Okinawa Prefecture in 1903; the weevil has expanded
its distribution area northward to southwestern islands of Kagoshima Prefecture.
Now they are established throughout Okinawa Prefecture, southwestern islands
of Kagoshima Prefecture, and Bonin Islands of Tokyo Prefecture.
Cylas formicarius has rapidly expanded its distribution area towards southwestern islands
of Kagoshima Prefecture since it was found in Okinawa Islands. The weevils
were found on Yoron Island, a southernmost part of southwestern islands,
in 1915, then on Kikai Island in 1940, and on Kuchino-Shima Island in 1942.
Kuchino-Shima is the northernmost infested area of Japan (Setokuchi, 1990).
They have been established throughout in Okinawa Islands and southwestern
islands of Kagoshima Prefecture by 1950. They have been also established
in the Bonin Islands of Tokyo Prefecture. The weevils invaded accidentally
into Yakushima Island (1997), Tanegashima Island (1990), and even into
the mainland of Kagoshima Prefecture (cf. Kaimon Town, 1965; Kagoshima
City, 1997) (Nishioka et al., 2000; Sugimoto, 2000). In 1995, they unfortunately
invaded into Muroto City of Kochi Prefecture, far north of the established
area (Fujimoto et al., 2000; Komi, 2000). However, they have been eradicated
relatively in short durations for all the cases mentioned above (1-8 years)
by using several control methods: destruction of wild host plants, restriction
of sweet potato cultivation, and detection of the area where the weevils
occurs by the use of sex pheromone traps (Fujimoto et al., 2000; Komi,
2000; Nishioka et al., 2000). Habitat expansions may be mainly due to the
movement of infested roots because weevils, especially female, have generally
poor dispersal ability (Moriya and Miyatake, 2001), though only a few male
weevils can flight over 2 km (Miyatake et al., 1997). Thus, the movement
of roots and vines of sweetpotato from the infested area to the uninfested
area is prohibited by Japan Plant Protection Law.
|
| Environmental impact |
| |
| Economic damage |
| The larvae of Cylas formicarius cause serious economic damage to growers of sweet potato. The infested
roots produce an unfavorable terpene order and taste bitter; thus they
are unpalatable for human food and even for domestic animals food. Adults
feed on sweet potato foliage (Talekar, 1991), but the foliar damage are
marginal. In Okinawa Prefecture and southwestern islands of Kagoshima Prefecture
where the climate is warm, sweet potato can be grown even in winter season.
This makes the sweet potato growers in these areas advantageous over those
who live in other areas in Japan. Thus, the infestation of storage roots
by weevils is an obstacle to the development of agriculture in these warm
regions. The invasion of the weevil into uninfested areas causes devastating
economic damage to sweetpotato growers because sweet potato production
is restricted until the eradication of the weevil. |
| Reproduction |
| |
| Growth |
| Both adults and larvae of Cylas formicarius feed on Ipomoea species such as I. batatas, I. indica, and I. pes-caprae. Especially the sweet potato, I. batatas, is the most favorite plant species. The eggs deposited in the epidermises
of the roots or stems hatch in 3 days; larvae usually take 25 days for
completion of three stadia at 27 C. Adults emerge after 6 days of pupal
stadium at 27 C. Newly emerged adults stay in the root for a few days until
the color of the body turns to black. The developmental zero growth is
14 C and the degree-day requirement for the development from egg to adult
emergence from infested tubers is about 590 DD above 14 C (Yasuda, 1997).
This degree-day requirement suggests that C. formicarius completes about 5 generations per year (Setokuchi, 1990). Although female
adults stop oviposition during winter season (Setokuchi, 1990; Yasuda,
1997), they do not enter diapause and continue to emerge from the host
plants. They overwinter mainly as mature larvae and adults (Yamaguchi,
2000), with both stages suffer relatively high mortality. |
| Countermeasure |
| Emergency control projects in corporation with national government
successfully eradicated the weevils when they were accidentally introduced
into uninfested area such as Kochi and the mainland of Kagoshima Prefectures;
wild plants were removed by chemical applications and burning, sweet potato
production was prohibited, and infested areas were detected using pheromone
traps (Nishioka, et al., 2000; Fujimoto et al., 2000; Komi, 2000). Early
detection of the weevil invasion is crucial for successful control. Sterile
insect technique (SIT) has been applied for the weevil eradication in two
small islets of Japan. Before SIT was applied, the density of the wild
population of Cylas formicarius was controlled to about one tenth of the initial density by applying two
years of mass trapping of mail weevils. In the mass trapping, fiber boards
(5x5cm) loaded with synthesized sex pheromone and insecticide (MEP) was
distributed in the fields at the rate of 5 to 15 board/ha. Pilot project
of SIT was first applied to an uninhabited islet, Kiyama-Zima, of Kagoshima
Prefecture from 1994 to 1996 (Setokuchi et al., 2001). SITs have been then
applied to Kume Island (6,000ha) in Okinawa Prefecture and a small area
(82 ha) of Kikai Island in Kagoshima Prefecture since 1999 and 1994, respectively.
The sweetpotato weevil populations in the target area of both islands have
been almost eradicated (Moriya and Miyatake, 2001). However, this technique
is facing some minor problems: complete artificial diet has not yet been
developed, making mass production of sterile weevils difficult, the weevil
has poor flight ability compared to fruit flies, and gamma irradiation
reduces flight ability and longevity. Mass trapping of male adults by synthesized
sex pheromone is an effective control method. This method is especially
effective in the area where the weevil density is low (Okinawa Prefecture).
Indeed, mass trapping of males by pheromone traps is recommended in Cuba,
(Lagnaoui, et al., 2000). Recently, a new method without chemical application
was developed for the control of the weevils inhabiting in the storage
roots; the roots are fumigated with steam of 47C for about 10 hr. The steam-treated
roots, whose viability and quality do not influenced, can be taken out
from the infested area. |
| Writer's name and affiliation |
| © Written by Suenaga,H. Agriculture and Forestry Section of Oshima
Office, Kagoshima Prefecture. (Written in 2003) |
