As the invasion of FAW, the number of insecticide applications on corn has increased especially in tropical areas, which may increase the resistance of FAW to insecticide and harm the environment and non-target organisms. Chemical selection, determiting the timing and times of insecticides application are crucial for effective management of FAW. The economic benefits of corn were not always proportional to the frequency of applications. Economic threshold levels for FAW control have not been determined with precision (Overton et al. 2021). Therefore, we explored the dynamic Economic threshold to provide guidance for farmers against FAW larvae in tropical Asian where
a distinct lack of FAW yield loss data.
In this study, we firstly selected low-risk, selective insecticides with high performance. Chlorantraniliprole had the better performance, followed by emamectin benzoate. A similar result was found for the susceptibility of eight populations of FAW larvae from seven provinces of China to insecticides (Zhao et al. 2020). Chlorantraniliprole is an efficacious insecticide with systemic activity and a report indicated that it is toxic to the eggs of Lepidoptera (Hannig et al. 2009). In our study, 9 days after insecticide application, the number of FAW larvae had returned to the level before application. However, in the north of China, chlorantraniliprole maintained excellent efficacy for more than 14 days (Lira et al. 2020). The differences maybe due to the higher temperature, humid and the infestation of FAW. Emamectin benzoate can also be selected to control FAW in practical production and application due to its lower price and good performance. In addition, biological control and alternative use of chemical insecticides are also necessary to avoid the resistance of FAW to chemical insecticides. In general, chlorantraniliprole is a valuable option for IPM because of its safety for key beneficial arthropods and non-target organisms (Hu et al. 2019).
There were differences in FAW intensity and duration of infestation between temperate zone and tropics and this could translate to yield losses. Surveys in our study showed that overlapping generations occur in the tropics, with all FAW developmental stages found on all plant stages. Population dynamics in our research demonstrated a bimodal pattern with peak numbers occurring typically during the vegetative and tassel formation stages. This similar to the maize, yield loss from FAW varies with plant growth stages (Buntin, 1986), and is typically most vulnerable to damage during seedling emergence, early-whorl stages. At the same time, although the corn leaves at the reproductive stage were not suitable for the growth of early FAW larvae, the silk and kernel tissues were suitable and had a positive effect on survival (Pannuti et al. 2016). In tropics where do have continual persistence FAW population, insecticide applications should be based on the infestation level observed during the whole plant growth stages.
We found that FAW infestation caused up to 45% yield loss without insecticide application. But it is difficult for farmers to control the frequency of insecticide application. Yang et al. (2021) reported that the frequency at which farmers applied insecticides increased to 6.4 in response to FAW larval infestation pressure in tropical areas. Economic threshold levels for FAW control have not been determined with precision (Overton et al. 2021), particularly in Asia tropics for sweet corn. Most EILs and ATs have been established in the USA, with several also reported in South America and Africa. And also there were significant variability in the manner EILs and ATs are reported, with a lack of consistency in pest pressure metrics, plant growth stages. The economic injury levels (EILs) are greatly affected by the yield loss of corn (Soliman et al., 2012). Yield loss caused by FAW differed with growth stage of corn (Buntin, 1986). Therefore, our research analyzed the relationship between yield and population density at four different stages of corn growth. Our research showed that the responses of corn to the injury levels of FAW larvae differed with the stage of corn growth. In the vegetable and silking stages, one FAW larva led to more corn yield reduction, which was similar with the results of Gross et al. (1982). Although younger plants have stronger defence response to recover from defoliation, total defoliation occurs frequently in younger plants may be greater (Morrill & Greene 1974, Kiesselbach & Lyness 1945). Especially in the early whorl stage of corn, FAW larvae attack the growing point, which may lead to plant death (Jaraleño-Teniente et al. 2020). At the early silking stage, the feeding of the fall armyworm will affect maize pollination (Gross et al 1982). These may be the reasons for higher yield loss caused by FAW. Our research showed that the ETs of FAW control were 7.77–13.10, 17.47–33.25, 24.89–46.52, and 12.44–21.78 per 100 plants at V4–V6, V8–V10, V12–V14 and VT–R1 stages of corn growth. Because EIL and ET change with the fluctuation of market price, corn yield and cost of FAW control, the EIL and ET calculated by us were dynamic (Barrigossi et al. 2003). Farmers choose ET according to the actual situation. Jaramillo-Barrios et al. (2020) reported that the action thresholds (ATs) were 1.8 and 1.8–2 larvae per 10 plants, respectively for 0–20 and 20–40 day plants (Respectively corresponding to V4-V6 and V8-V10 growth stage of this study). The ETs of this study lower than the ATs proposed by Jaramillo-Barrios et al. (2020). In this study, we placed FAW larvae on caged corn to simulate natural infection, which eliminates the yield loss caused by other pests. Therefore, the yield loss caused by FAW was lower, and the ETs was higher. However, the management cost reported by Jaramillo-Barrios et al. was higher than this study, while the price of corn was lower than this study, so it was higher than the economic threshold of this study.
Nevertheless, for fresh corn, the factors that affect the economic benefits of corn are not only the corn yield but also the ear weight, smell, colour, etc. (Lyu et al. 2021), which requires further experiments to determine its ET.