Field evaluation of plants molluscicide against Pomacea canaliculata

S H O R T C O M M U N I C A T I O N


INTRODUCTION
Pomacea canaliculata (Lamarck) is a large freshwater snail. It is listed in the "100 World's Worst Invasive Alien Species" of the Global Invasive Species Group Database 1 . Its feeding habit is, mostly, towards the young stems and leaves of paddy rice (Nylor, 1996), and it could consume 7-24 rice seedlings per day (Oya, 1986), thus, resulting in extreme damage to growing rice. Consequently, the integrity of the rice bowl and food security could be threatened in those countries that depend on rice as their principal staple food and income (Noor et al., 2012). In an attempt to control Pomacea canaliculata, pesticide misuse and abuse by farmers have caused serious economic, social, and environmental impacts, biodiversity loss, and health hazards to rice farming communities (Rejesus et al., 1988).
Solanum mammosum L. and Sapindus saponaria L. are natural molluscicide and their combined effect against Pomacea canaliculata has been evaluated under laboratory conditions by Quijano et al. (2014); those authors demonstrated that there was not a synergism effect in the combination Sapindus saponaria L. and Solanum mammosum L. (50:50, w/w). 1 (ISSG, www.issg.org/database). However, the mixture showed a similar lethal effect to that of Sapindus saponaria L. (100%).
World Health Organization (WHO) recommends that molluscicides should be evaluated in conditions that mimic the closest to natural zone in which the molluscicide will be employed. In this context, the aim of this this study is to evaluate the molluscicidal activity of the mixture of aqueous extracts of Sapindus saponaria L. and Solanum mammosum L. (50:50, w/w) and Sapindus saponaria L. (100%) against the snail Pomacea canaliculata under field conditions.

Extraction
Ripe fruits of Solanum mammosum L. (CIBE012) and Sapindus saponaria L. (CIBE018) were cut into small pieces, dried at 60º C for 24 hours and grinded to get fine particles. Aqueous extracts of each fruit were processed separately by decoction of the vegetal material in proportion of 10% in distilled water during 20 min. The aqueous extracts were lyophilized at 120 x 10 -3 mbar and 47º C below zero.

Field trials
Molluscicide field trials were conducted in Daule, Ecuador located at 1º, 51'36'' S, -79º, 59', 24'' W. The place has a Two plants reported as molluscicides have been tested against the snail Pomacea canaliculata under field conditions. The molluscicide activity of the formulation Sapindus saponaria L. (100%) was compared with the mixture of Sapindus saponaria L. and Solanum mammosum L.

S H O R T C O M M U N I C A T I O N
warm temperature almost all year, for its location on the equator. Forty-eight square-shaped plots were constructed in a paddy field. Plots were constructed with the following dimensions: 0.5 m width × 0.5 m length × 0.4 m height ( Figs. 1 and 2). Molluscicides were applied only once to each plot, afterwards ten snails were introduced to it.

Design of experiment
The formulations Sapindus saponaria L. (100%) and Sapindus saponaria L./Solanum mammosum L. (50:50, w/w) were prepared at five concentrations (10, 50, 100, 200, 400 mg · L -1 ), which were applied in four repeated measurements and three replicates. The arrangement included a negative and positive control which consisted in distilled water and 10 ml · L -1 of endosulfan respectively. The time of exposure of the snails in the bioactive formulated was 24 hours. The individuals that showed no vital signs after tipping with a needle were counted as dead, while snails showing movements or reduced viability were counted as survivors (WHO, 1965a). Fig. 2 Statistical analysis LC 50 (median lethal concentration), LC 90 (lethal concentration for 90 % of the population) values and their confidence limits, probit/log concentration regression equations and slope were calculated by the method described by Finney (1971).
LC 50 values presented in the Table 1 probed that Sapindus saponaria L. 100% was more toxic than the mixture Sapindus saponaria L. and Solanum mammosum L. (50:50, w/w). and LC 90 values indicated that both formulations could not be considered as good molluscicide candidates due to the fact that WHO prerequisites indicate that crude organic extracts should present LC 90 values bellow 20 ppm for direct application in infested water (WHO, 1983b).    (Quijano et al., 2014). Discrepancy between laboratory and field assays may arise because the presence of weed, mud and lower content of dissolved oxygen in the water, factors that reduced the molluscicidal activity of both formulations (Adewunmi and Marquis, 1987;San Martín et al., 2009). These findings suggest that it is not practical to make a partial substitution of Sapindus saponaria L. by Solanum mammosum L., because molluscicidal activity is reduced.

CONCLUSIONS
Results revealed that under field conditions Sapindus saponaria L. 100% have a higher molluscicidal potency than Sapindus saponaria L. and Solanum mammosum L.
(50:50, w/w) against Pomacea canaliculata. However, as large amounts of plant extract are needed to kill the 90% of the snails, one of the prerequisite set by the WHO to consider a plant to be a molluscicide is not satisfied. Another species of plant should be considered to test against Pomacea canaliculata.