Soybean [Glycine max (L.) Merrill] Shows No Change in Predatory Arthropods Population after Mutagenesis in Northern Ghana

Predatory arthropods can potentially prevent crop damage from reaching economic levels in agroecosystems. This is because they contribute to the delay in pest population build up as a result of diverse interactions with pest populations (Getanjaly et al., 2015). As a result, natural enemies have been used as key components of major pest control programs worldwide (Panizzi, 2013; Chailleux et al., 2013; Yang et al., 2014). For instance, predators and parasitoids are the main pest control methods within soybean ecosystem in parts of the USA and China (Getanjaly et al., 2015; Yang et al., 2014; Underwood, 2017). Studies also show that, the population dynamics of herbivorous arthropods are directly and positively correlated with their natural enemy population (Underwood, 2017; Dib et al., 2010).
       
The relationship between predatory arthropods and their prey can be used as a proxy measure of pest population in an agroecosystem (Griffin et al., 2013; Crawley, 2009). This can be inferred from the law of equilibrium of arthropod population dynamics which states that “the rate at which prey population rise decreases when predator population rises” meaning before predator population could rise there has to be an increase in prey population (including pests) (Begon et al., 2009; Vandermeer and Goldberg, 2013).
       
Soybean is a multipurpose leguminous crop and one of the cheapest protein sources cultivated across the world (Sharma et al., 2016; Gunathilake et al., 2019). Depending on the type and the stage, soybean can tolerate up to 35 per cent defoliation before flowering, however, after seed pod filling, foliage loss above this threshold can cause economic loss (Temple et al., 2009). Due to its ease of controlling arthropods, natural enemies can be used to achieve pest control when managed well without the need to augment with chemical pesticides (Panizzi, 2013; Chailleux et al., 2013; Yang et al., 2014). However, studies elsewhere indicate that mutagenesis can change the properties of some leguminous crops which may also increase its susceptibility to pest attacks (Shekar, 2017; Singh and Sadhukhan, 2019).
   
In line with that, this study was set out to evaluate the attractiveness or otherwise of four newly developed soybean lines (150Gy, 200Gy, 250Gy and 300Gy, through mutagenesis from bombardment with gamma rays) for predatory arthropods which will see these lines as easy food sources. Our hypothesis was that, mutagenesis has not rendered the new lines more susceptible to arthropod pest attacks. If so, there will be no significant difference between the predatory arthropod population of the newly developed mutants and the old line (0Gy). This is because high presence of predator population on a variety would signify high presence of prey population which includes arthropod pests. We specifically sampled for the predatory arthropods associated with the soybean mutant lines compared with the old variety (0Gy) and also measured the indirect impact (if any) of NPK fertilizer on them.

The experiment was conducted at the University for Development Studies (UDS) experimental site (farming for future) in Nyankpala in 2019. The site is located at longitude 0°58’ W and latitude 9°25’N with an altitude of 183 m above sea level. Temperature pattern varies throughout the year with an annual average of about 34°C but could rise to a maximum of 42°C within the year. The landscape within the study area is characterized by natural savanna vegetation with tree density of about 10 to 100 trees ha^-1 (Lawson and Issahaku, 2013).
 
Experimental materials
 
The soybean lines used for this study were acquired from the University for Development Studies Agronomy Department. These lines are still under evaluation and yet to be released as varieties. The newly developed lines are relatively smaller in size compared to the normal soybean seeds. The original variety (0Gy or Jenguman) has high shattering rate compared to the mutant lines. The mutant lines take more than 100-days to mature.
 
Cultural practices and experimental design
 
After field preparation, the site was divided into plots of 4 m x 3 m. Planting was done at 75 cm by 15 cm at a depth of approximately 3 cm within each plot. Soybean lines were sowed at 3 seeds per hill using dibber and covered with soil. In all, there were 48 stands after seed germination per plot. There were 30 plots for the study in total. Manual weeding was done using hoe on the second and the seventh week after sowing to reduce weed competition. NPK-15-15-15 fertilizer was applied at 0Kg/ha and 60Kg/ha to treated fields six weeks after germination. The study used a 5 x 2 factorial design with three replications in Complete Randomized Design (CRD).
 
Predatory arthropod sampling
 
We collected data after the first weed control with sweep net and visual observation. We took twenty sweeps per plot, after which predatory arthropods were either identified using visual cues or sent to the entomology laboratory of the University for Development Studies for identification to the best possible taxon. Data was collected on four occasions at two weeks interval.

In general, there were seven predatory arthropod groups associated with all soybean lines based on morphospecies during the growth period. The total number of predatory arthropods recorded was 1282 belonging to 5 orders (Hemiptera, Neuroptera, Coleoptera, Mesostigmata and Araneae). The predatory arthropods were collected at various stages of growth of the soybean lines (Table 1). There was no statistically significant difference in predatory arthropod diversity and abundance between the control variety (0Gy) and the mutant lines (P > 0.05) (Fig 1). Also, there was no statistically significant difference between soybean lines giving NPK 15-15-15 and those without NPK 15-15-15 (P > 0.05) (Fig 2). Some of the most abundant arthropod pests encountered in the study were wireworms (Melanotus spp.), Soybean loopers (Chrysodeixis spp.) Bean leaf beetles (Cerotoma spp.) and the Fall armyworm (Spodoptera frugiperda).
 

 

 
 

    
Predatory arthropod species that were found to be associated with these soybean lines are known to be beneficial to the plants because they feed on arthropod pests associated with the crop plant. Our study shows that the six predatory arthropods recorded on these soybean lines were also observed on a previous study by Abdul (2006) where ten natural enemies including Ladybird beetle, Predatory spiders, Damsel bug and Greenlace wing were also recorded on soybean. The results of the study also revealed that mutagenesis of the (0Gy) soybean variety did not increase the lines affinity or attractiveness to pest with evidence from the indirect impact on the diversity of predatory arthropods they attract which are statistically similar. Although, arthropod pests were not a big problem in the study, wireworms (Melanotus spp), Soybean loopers (Chrysodeixis spp.) Bean leaf beetles (Cerotoma spp.), the Fall armyworm (Spodoptera frugiperda) and whiteflies (Bemisia tabaci) were observed feeding on the leaves, husks and seeds at various stages of plant as reported elsewhere (Singh and  Singh, 1990; Hartman et al., 2015). In literature such mutagenesis can potentially change some properties of plants and render them more susceptible to serious pest attacks (Narusaka et al., 2003; Scholz et al., 2014).
     
The study further revealed that, the population of predatory arthropods on the NPK applied fields and no NPK applied fields were not statistically significant. Even though there was a slightly high abundance of predatory arthropods on plots given fertilizers compared to those without fertilizers. This finding agrees with findings of another study that reported that the application of fertilizers rarely affected arthropod directly, but influenced the morphological, biochemical and physiological characteristics of host plants hence improving nutritional condition of herbivores (Lu et al., 2007; Shikano, 2017; War et al., 2018; Han et al., 2019). This might have been the case with this study. Under such conditions predators that prey on these herbivores may also benefit indirectly because there will likely be an increase in the population of their prey as a result of the extra vegetative growth through fertilization.

We conclude from this study that, the soybean mutant lines (0Gy, 150Gy, 200Gy, 250Gy and 300Gy) were associated with similar predatory arthropods and that these arthropods diversity were not statistically significant between the different soybean lines. Also, NPK and no NPK applied fields had statistically similar predatory arthropods. We therefore recommend that the best performing line can be given out to farmers without worrying about its attractiveness to arthropods compared to other lines after screening for other performance indicators.