Biological and ecological studies of fall armyworm,Spodoptera frugiperda (J.E. Smith) and its control:a case study from maize field in Phitsanulok, Thailand

Abstract

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a polyphagous insect, and a native pest to tropical and subtropical America. This insect has recently reported its first appearance in the Africa continent and caused a huge infestation since 2016. This transboundary pest has continued to spread across Asia and become a new invasive species in Thailand which mainly affected maize. Since the S. frugiperda occurrence, the baseline information of this insect biological aspect and its distribution in various conditions is essential. Therefore, to expand the knowledge and support for planning the efficiency management, this study was organized into three experiments which were conducted in the laboratory of the National Biological Control Research Center (NBCRC) (16°44'10.5"N 100°11'37.0"E) and the experimental field (16°44'08.9"N 100°11'38.7"E) of the Faculty of Agriculture, Natural Resources and Environment, Naresuan University, Phitsanulok Province, Thailand.

The first experiment was performed in the laboratory to assess the biological characteristic and the parameters of the fertility life table of S. frugiperda on three maize (Zea mays L.) cultivars (field, sweet, and waxy maize) under controlled conditions (30±2°C, 55±5% RH, and a 12 h of photoperiod). Results suggested that larvae were developed through six instars on all the maize cultivars. The duration of larval stage when fed with sweet maize (10.83±0.14 d) was shorter than those when fed with field maize (11.28±0.05 d) (P<0.05). More than 70.5% of them were transformed into the pupal stage. The life cycle duration lasted 28.11±0.40, 27.16±0.37, and 28.41±0.34 d on field maize, sweet maize, and waxy maize, respectively. Significant differences among host plants were not observed for the different development durations. Sex ratio (female:male) was varied between 0.83:1 when reared on field maize, 1.07:1 on sweet maize, and 1.18:1 on waxy maize. The survivorship curve of S. frugiperda exhibiting a type I. The highest values of net reproductive, R0 (220.41±5.88), innate capacity of increase, rc (0.23±0.001) and finite rate of increase, λ (1.25±0.002) obtained on sweet maize, were not statistically different from other cultivars. The mean generation time (Tc) differed between waxy maize and sweet maize (P<0.05) which was 26.36±0.43 d on waxy maize and 23.80±0.24 d on sweet maize. Those results have indicated the potential and suitability of host-plant on S. frugiperda immature development and adult fitness.

The second experiment was conducted at the field level to describe the population dynamic of S. frugiperda and its affecting factors. The field maize (Nakhon Sawan 3) trial was conducted during the dry season (October 2019-February 2020) and repeated in the rainy season (July-October 2020). The highest number of S. frugiperda was observed during the whorl-stage of maize, 0.85 and 1.25 larvae per grid in the dry and rainy season, respectively. Meanwhile, the lowest number of S. frugiperda was observed during the post whorl-stage of maize, 0.71 larvae per grid for the dry season and 0.70 larvae per grid for the rainy season. The highest percentage of S. frugiperda infestation was recorded during the whorl-stage of maize, 21.95±3.91% for the dry season and 23.06±3.75% for the rainy season, while the lowest percentage was recorded during the post whorl-stage of maize, 7.47±2.60 and 0.90±0.00% for the dry and rainy seasons, respectively. The actual yield loss due to S. frugiperda was recorded at 1.94% in the dry season and 2.48% in the rainy season. Two species of parasitoids were identified and associated with the pest, of which Chelonus sp. (Hymenoptera: Scelionidae) was larval parasitoid, while Telenomus sp. (Hymenoptera: Braconidae) was egg parasitoid. According to correlation analysis between the S. frugiperda population and its affecting factors, parasitism was the major influenced factor on the population dynamic of S. frugiperda, contributing to 21.77% of the variation in pest abundance. The climatic conditions (temperature, humidity, rainfall) recorded during the field experiment did not influence pest abundance. However, this is a primary report of population dynamic of S. frugiperda and its parasitoids from the small-scale maize field in northern Thailand, which a long-term observation from the various community are necessary.

Finally, the third experiment was conducted in the laboratory to evaluate the efficacy of 12 insecticides from the different mode of action (MoA) and 3 biopesticides against third instar larvae of S. frugiperda under the laboratory conditions (25±2°C, 75±5% RH, and a 12 h of photoperiod). Results indicated that spinosad caused the highest mortality of 100% at 1 d after treatment application, followed by emamectin benzoate, and chlorantraniliprole which caused mortality of 100% at 2 and 3 d after treatment application, respectively. At 7 d, the minimum reduction was still noticed in biopesticides consisting of Bacillus thuringiensis var. aizawai (15.83%), Metarhizium anisoplia (15.00%) and Beauveria bassiana (11.67%), however, they had effective on adult emergence which was below 76% on all biopesticides treatments compare to control. The median lethal time (LT50) value of 2.02, 10.73, and 33.75 h was recorded on spinosad, emamectin benzoate and chlorantraniliprole, respectively. These data can be used as a guideline for planning the integrated pest management (IPM) for S. frugiperda under smallholder farmer conditions.

In conclusion, S. frugiperda has its particular feature which varying depend on the host-plant and the environmental conditions, therefore, the information presented here would greatly provide a comprehensive knowledge of S. frugiperda and could be useful information for the preparation of the efficiency management techniques for this critical crop pest. Further study by combination and implement on the field will be helpful.

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