The development of insect species is influenced by the conditions of the local environment they find themselves (Sibly & Atkinson, 1994; Nylin & Gotthard, 1998; Huang et al., 2021).
In fact, an increase in temperature speeds up and increases growth of the affected insects (Chown & Gaston 2010; Karuppaiah & Sujayanad, 2012). In this study, the fecundity of FAW was relatively high perhaps due to the relatively high optimum environmental conditions in the laboratory. In a similar study, comparable fecundity of female S. frugiperda was recorded. The female moths laid 1,169 eggs under temperature and humidity conditions of 26 ± 2℃, 75 to 80% RH and L12:D12 photoperiod (Sharanabasappa et al., 2018a). High fecundity and shorter incubation period enhances the exponential population growth of FAW within a short time. This implies a low action threshold that requires early and sustained implementation of FAW management strategies to avert the likelihood of economic injury which comes at a high cost especially to resource poor farmers.
Frequent insecticide application to keep FAW population below the economic injury level may result in insecticide resistance (Yu et al., 2003; Guedes et al., 2019; Gutiérrez-Moreno et al., 2019; Boaventura et al., 2020). Although high fecundity of insect pests may not be good for crop protection, when managed properly, high fecundity could be harnessed for mass production of egg parasitoids for release in biological control programmes (Altieri & Ni holls, 2003). This could be successful if FAW larval cannibalism is reduced to allow more larvae to develop into adults to lay eggs for the production of parasitoids in laboratories for release into farms.
Each larva went through six instar stages over a period of 16 to 18 days. In a similar study under similar environmental conditions, Janwa et al. (2021) reported that the total larval duration of FAW was 15 to 19 days. Longer larval periods would always have a detrimental effect on maize plants since the larval stage of the FAW is the most destructive, where physical damage occurs. A long feeding period by FAW larvae on host plants would likely result in low yields and in some cases complete yield loss (Boote et al., 1993; Overton et al., 2021). Some yield losses may results from loss of plant leaf surface area for photosynthesis and carbon dioxide absorption (Welter et al., 2019). It has been established that as FAW larvae grow, the amount of food they consume increase (Agboyi et al., 2019). This would be a source of worry if FAW larvae live longer on host plant leaves as the potential of crop damage would increase. On the contrary, where larval parasitoids prevail, a longer larval period may result in higher parasitism - an advantage for the parasitoid population in searching for host larvae.
The second instar larvae of FAW are greenish brown with small head capsules. The head capsule size measured in this study is similar to findings from earlier studies (Bhusal & Bhattarai, 2019). Coupled with their small size, they are difficult to detect early in the field without aided eyes. The difficulty in detection is further complicated by the minimal colour contrast between plant leaves and the first and second larval instars. The close similarity between the colour of FAW larvae and maize leaves serve as camouflage for the larvae and limits early detection and management.
While this study found pupal duration to be 8–14 days, previous studies under laboratory conditions (26 ± 2℃, 75 to 80% RH and L12: D12 photoperiod) showed a mean pupal duration ranging from nine to twelve days (Sharanabasappa et al., 2018b). Meanwhile, an earlier work reported a mean pupal duration of 8.54 days (da Silva et al., 2017). Adult morphological characteristics were also similar to those reported by Sartiami et al. (2020). Generally, adult FAW have a relatively short lifespan. Typically, a few minutes after emergence, FAW moth move around in search of food, mate resulting in oviposition by female and both males and females die off within days. In this study, females had relatively short pre-oviposition and oviposition periods and lived relatively longer than their male counterparts. Even when starved, females lived longer than males (Viña, et al., 2003). This can be explained on the basis of the fact that normally, male insects use more carbohydrates due to their activities i.e. courtship rituals, sexual activity, feeding and mating (Yuval, 2006; Howell, & Knols, 2009). According to Arrese, & Soulages, (2010), when the insects are not feeding, they rely on their reserved energy for all their activities but that of the male moth may be less than the reserves of females because females use less carbohydrate resources.
Findings from this study have demonstrated that the life cycle of male FAW is relatively shorter than that of females. This is in line with those of Huang et al. (2021) who reported a total life cycle of 30–40 days. The generation period for FAW is likely to reduce with the current worldwide phenomenon of global warming (Chimienti et al., 2021) resulting in more generations per year. Apparently, there is a 1:1 FAW generation period per month with the most destructive stage having the longest lifespan period.
In conclusion, this study provides baseline information about the basic biology of FAW, an alien invasive insect pest in Ghana. The species has become a major pest of maize which is an important crop in Ghana. Establishing the biology of this pest will inure to the benefit of sustainable management of FAW in the country. The FAW is likely to be more devastating over time as global temperatures are projected to rise further (Karsten et al., 2015). To forestall this potential devastating damage, compatible IPM approaches are needed at the early stages of FAW infestation.