Host Plant Selection Affects Biological Parameters in Armyworm, Spodoptera litura (Lepidoptera: Noctuidae)

Armyworm, also known as common cutworm, Spodoptera litura Fabricius (Lepidoptera: Noctuidae) has achieved the significant position as a destructive insect pest in cotton growing areas of Pakistan (Saeed et al., 2010; Ahmad et al., 2007). It has a wide range of hosts from about 25 genera and 14 families including fodder crops, economically important cash crops, vegetables, weeds, various horticultural and ornamentals plants (Simpson et al., 2002; Raubenheimer and Simpson, 2003; Lee et al., 2003; Ahmad et al., 2013). This pest has a vigorous migratory ability (Fu et al., 2015) and is widely distributed throughout the tropical and temperate climatic conditions (Tenywa et al., 2018). This pest cause damage in the form of monstrous patches from one to another host plant, causing inconsiderable economic losses (Tuan et al., 2014; Zhang et al., 2006). S. litura damage the host plants by feeding leaves, brackets, newly emerged seedlings, tender shoots and fresh seed kernels (Yinghua et al., 2017).

Both species, Spodoptera exigua, Hübner (Lepidoptera: Noctuidae) and S. litura are economically to their high reproductive rate and heavy losses. Due to the absence of common hosts, S. litura may select the grasses for the reproduction and survival. Although it has been a sporadic pest of cotton in Pakistan for many years, it is becoming gradually a very important insect pest of various agricultural crops due Due to the absence of common hosts, S. litura may select the grasses for the reproduction and survival. Although it has been a sporadic pest of cotton in Pakistan for many years, it is becoming gradually a very important insect pest of fodder crops in recent years (Ahmad et al., 2008). The common cultivated fodder crops, mostly affected by the S. litura are corn, Zea maize L. (family: poaceae), alfalfa, Medicago sativa L. (family: fabaceae), barseem, Trifolium alexandrinum L. (family: fabaceae) (Khan et al., 2011; Ahmad et al., 2013). Alfalfa, being a perennial crop is mostly affected by the S. litura as alternate host (Kaur, 2012; Agrell et al., 2003).

S. litura not only reduce the quality of various crops but also lower the quantity. S. litura damage the immature cobs of maize at milking stage by making galleries (Naz et al., 2003). Same trend is observed in cabbage flower due to which fungus develops and stunting in growth occurs (Bhatia and Gupta, 2003). This pest has 1-2 generations on sesbania and completely destroys the plant leaves (Ghaffar et al., 2002).

Number of studies have reported S. litura selection of various host plants and mode of damage under different environmental conditions in India (Christou et al., 2006; Xue et al., 2010), Pakistan (Ahmad et al., 2007; Chaudhry et al., 2017) and China (Kaur, 2012; Zhu et al., 2000, 2005; Qin et al., 2004), including other Asian countries. From the last many years, it is reported that this pest has established high immunity against the large groups of pesticides (Saleem et al., 2008; Nathan and Kalaivani, 2005; Sang et al., 2013) and also have minute range of susceptibility for the non-conventional growing crops (Wan et al., 2008). Due to the resistance, S. litura can survive from the area where application of pesticides carried out (Kaur et al., 2014).

In this study, we determined the oviposition preferences, development and survival of larval and pupal stages of S. litura on the different host plants (cabbage, alfalfa, sesbania and maize) to determine its host preferences.

Larvae of S. litura were collected from farmer fields from Muzaffargarh (29.5069° N, 70.8536° E) and Sargodha district (32.0740° N, 72.6861° E) and were brought into the laboratory of department of Entomology, University of Sargodha, Sargodha. Artificial diet was prepared from wheat germ based according to the method described by Sorour et al. (2011). The artificial diet contained 90 g of wheat flour, methyle-hydroxy benzoate (2g), streptomycin (0.25g), yeast (10g), vitamin C (2 capsule), wheat flour (90g), ascorbic acid (3g) and sorbic acid (3g). Flour was sterilized for 30 min before using. Diet was provided daily up to the pupation stage for the homogenous and less resistant population. Pupae were collected on the daily basis and kept in separate rearing cage for the adult emergence. Adults were released in separate rearing transparent cages (2x2 ft square) provided with moist cotton and honey solution (9:1%) to feed the adults. Towel paper strips were provided for egg laying. The strips having egg batches were replaced daily and eggs per batch were recorded under the microscope, (OPTIKA-SZM2, Italy). Standard rearing conditions of 27±2°C, 65±05% RH with 16:8 lights to dark ratio were followed (Ahmad et al., 2007; Arif et al., 2009; Gupta et al., 2005). The F3 generation of reared culture of S. litura was used in this study.

Four host plants were used in this study including common name jantar/sesban (Sesbania sesban (L.), lucern/alfalfa (Medicago sativa L.), maize (Zea maize L.) and cabbage (Brassica oleracea L.). These plant species were planted in research area at College of Agriculture, University of Sargodha, Sargodha. No pesticides were applied in the field. Fresh leaves of each host were collected to feed the S. litura. The leaves were first washed with distilled water and then dried at room temperature prior to feeding the larvae.

Newly hatched larvae were reared on artificial diet. Second instar larvae were used in the experiment. Larvae were starved for 24 h prior to experimentation. The experimental arena was a petri dish containing 1 larva/plate. There were three replications with 5 larvae in each replication. The data for larval length and weight were recorded daily using measuring scale and digital weight balance. After pupal formation, the pupal weight and length was recorded. Adult emergence ratio was also determined to check the effect of host plant.

For each treatment, three pairs of adults were separated and released into clean cages separately. The towel paper was hanged to facilitate the egg laying, number of egg batches and total number of eggs were recorded daily. The honey solution was provided as discussed above.

One-way analysis of variance (ANOVA) was performed to test the significance of host plants on the developmental biology, and fecundity of S. litura, by keeping host plant as main factors. Means were separated by Tukey HSD all pairwise comparison test. All the analyses were performed using SPSS 20.0 software.

Overall, larval development was significantly affected by the host plants (P<0.05) and was longest on cabbage (8.86 days) followed by alfalfa (8.00 days) and shortest on maize (7.00 days). Maximum larval length was found on cabbage (3.74 cm), followed by alfalfa (3.54 cm) and minimum on maize (2.93 cm). In case of larval weight, similar trend was found. However, the pupal development was also affected significantly (P<0.05) by hosts. Pupal developmental period was longest on artificial diet (17.00 days), followed by maize (13.66 days) and the shortest on cabbage (9.66 days). Pupal length was also significantly different (P<0.001) by host and was longer 2.05 cm on cabbage, followed by 1.93 cm on artificial diet and shortest 1.34 cm on maize. Pupal weight was significantly different by host and was found heaviest (0.089 g) in cabbage treatment and was the lightest (0.034 g) on maize (Table I).

The means±SE in the same column followed by same letters are not significantly different at P > 0.05.

Number of egg masses and total eggs oviposited by S. litura females on host plants differed significantly (egg masses: F = 174.0; df = 4, 74; P < 0.001; total eggs: F = 348.0; df = 4, 74; P < 0.001). S. litura oviposited greater and almost the similar number of egg masses (21-22.6) on alfalfa and sesbania compared to other treatments.

The lowest number of egg masses (9.3) were found on maize. However, the total number of eggs laid by three females were found greater on cabbage (7366.7) followed by 5250.0 on alfalfa and the lowest 2316.0 on artificial diet (Fig. 1).

Adult emergence rate was also significantly affected (F = 28.41 and P > 0.001) by host plants and was maximum on cabbage (93.11%±6.73) followed by alfalfa (87.5%±4.61). While minimum adult emergence percentage 68.44%±2.59 was found on maize (Fig. 2).

There was a significant difference in male (F=26.1 and P>0.00) and female (F=12.1 and P>0.00) adult life span feeding on different host plants. Female adults lived more days compared to male feeding on hosts. However, the adult life span was more; 10±0.29 days for female and 8.80±0.34 days for male followed by alfalfa and sesbania when were fed on cabbage leaves compared to other hosts. The shortest period of both male (4.6±0.31 days) and female (6.6±0.42 days) adults were observed when they fed on maize (Fig. 3). The host suitability was found maximum on cabbage, Alfaalfa and least on artificial diet (Fig. 4).

The current study results show that larval food directly affects the pupal size and weight and duration as well adult life span on all four host plants. The results are in line to Xue et al. (2010) and Ahmad et al. (2013), they reported that the food directly affects the larval and pupal developmental period along with length and weight of larvae and pupae. Their findings depicted that during the larval feeding activity, specific storage proteins play an important role in fecundity and metamorphosis of insects. Few work has been reported that focused on the process of protein storage at larval and pupal stages (Telang et al., 2002). Our results showed that S. litura preferred the cabbage, lucern (alfalfa) and jantar (sesbania) leaves in comparison to maize and artificial diet, these results supported by Wakil et al. (2011) and Maung (2016). Current study results are in line with Zhu et al. (2000); Chen et al. (2002), Seema et al. (2004) and Courdon et al. (2002) they stated that due to the variation in nutritional values, larval weight and developmental period may differ. Differences in pupal survival, adult sex ratio, longevity, and fecundity may also be affected by temperature and other environmental conditions (Zhu et al., 2000; Chen et al., 2002; Seema et al., 2004).

Similarly, the adult emergence ratio was higher on cabbage leaves than other diets and the adult life span was also higher in this treatment. The results about female longevity and the number of eggs laid is also in accordance to Mabrouk et al. (2001) and El-Awady et al. (2009) who reported that S. litura laid maximum number of eggs feeding on cabbage leaves. Selection of the suitable host plants for feeding help the insects to maintain the population up to several generations and multiplication (Lee et al., 2003). The information about the host plants for insect pest development led to the useful implementation for the integrated pest management (IPM) tool (Greenberg et al., 2001; Saeed et al., 2009; Rezapanah et al., 2008). To understand the insect behavior and for the management tools this basic nutritional ecology can enhance our appreciation of insects’ adaptation to new food resources. The destruction ability of S. litura for host plants represented by the fitness of the pest (Tuan et al., 2015). The fitness can be determined with the help of pest life table, because life table provides complete and inclusive exposure about the pest developmental stage, reproduction of new progeny and its existence (Jha et al., 2012). Integrated pest management program can be established by the life tables as an important ecology-based implementing tool (Tuan et al., 2014; Ahmad et al., 2018). Under the different laboratory circumstances for the life history facts, S. litura was reared on diverse host plant materials reported by Xue et al. (2010) and Shahout et al. (2011). To understand the effect of host plants diversity on the ecosystem of pest is very essential for the population management strategies (Greenberg et al., 2001).

Cabbage crop is a potentially good candidate trap crop for oviposition by the S. litura. However, the effectiveness of trap crops depends on the relative attractiveness of the plant species (Badenes-Perez et al., 2004), the proportion of the field occupied by the trap crop (Banks and Ekbom, 1999), duration of the trapping effect (Cook et al., 2007), insect migratory and host-finding behaviors (Shelton and Badenes-Perez, 2006), and the relative planting times of trap crops. Trap crops at the field edge can prevent the pest from reaching the crop (Rea et al., 2002). The use of trap cropping in agriculture is not well established because this approach is more knowledge-intensive than is pesticide use. However, in least developed countries, pesticides can represent an unacceptable extra cost (Khan et al., 2001; Gurr et al., 2017) and generally lead to many environmental problems.

The results of our study will be helpful to determine the population dynamics and preferences for various biological parameters in different host regimes and ultimately will aid in developing the suitable management practices for the control of this notorious pest.

These findings of this study will be useful in understanding the biology of S. litura that could be utilized in various management protocols including integrated pest management (IPM)

The research work is the part of PhD study of first author and is funded by UOS funded project.

The authors declare no conflict of interest.