Impact of Metarhizium rileyi infected larvae on cannibalism behavior in
Spodoptera frugiperda

INTRODUCTION

Cannibalism is the process of killing and consumption of conspecifics which is a taxonomically widespread behavior in phytophagous insects, mostly in lepidopteran species [2]. Cannibalism often accounts for substantial mortality that may influence population dynamics and community structure [3]. Cannibalism may confer direct fitness benefits, in the form of increased survival, developmental rate, or fecundity. Cannibals may also benefit indirectly from the removal of potential competitors [6]. It is often favored under high population densities and when resources are limited but can occur even when food is not limited [6]. This behavior is costly in terms of energy, time, risk of injury, and death for competing individuals that may have repercussions for an individual’s fitness. Cannibalistic insects are most likely to prey on smaller conspecifics. However, they may also attack larger conspecifics. Cannibals can obtain nutrients such as salt, protein, and amino acids by consuming conspecifics [8]. Cannibalism is a frequent behavior of Spodoptera frugiperda accounting for 40–60 per-cent mortality in laboratory culture [1]. On maize, the larvae usually feed on the wrapped leaves of the developing whorl. The density of larvae in wrapped leaves may lead to food competition and then attack individuals weaker than themselves.

MATERIAL AND METHODS

The study was conducted under laboratory conditions during 2021-22 and 2022-23. Later instar larvae of fall armyworm were collected from the field and reared on maize leaves in cavity trays to avoid cannibalism in order to get nucleus culture. The larvae were reared up to pupal stage. Pupae were placed in petri dishes containing sand to simulate natural conditions and were kept in wooden cage (36 x 36 x 36 cm size). When adults emerged, they were provided with 10 per cent honey solution as food for adults. Fresh tender maize leaves were kept in a glass conical flask inside the cage for oviposition. The cut end of the leaf whorl was covered with a wet cotton wad for maintaining turgidity and freshness. Freshly laid egg masses were kept in rearing plastic boxes provided with wet blotting paper at the bottom to protect the eggs from desiccation. After two days when eggs turned to black color, they were provided with fresh maize leaves as food for neonate larvae. Egg masses laid at different days were kept separately. The neonate larvae were released on leaves with the help of a soft hair brush in cavity trays. For the study, the larvae of each instar were taken and placed in a rearing box whose top was covered with muslin cloth in order to facilitate aeration. The food was changed after every 24 hours. One day-old larvae of each instar, based on molting date and the presence of shredded head capsules were taken for the study.

Larvae of S. frugiperda were placed individually in cavity trays and were fed with maize leaves treated with M. rileyi. Five healthy larvae of S. frugiperda were placed into a transparent plastic box containing a wet blotting paper to maintain the moisture and enclosed with a plastic lid having holes to allow ventilation. Each box was considered as one replicate with three replicates per treatment in a completely randomized design. To these boxes, five S. frugiperda larvae after infection with M. rileyi were placed. One day-old M. rileyi treated larvae were used for the experiment. Larval feeding preference and number of larvae cannibalized were observed. Survived larvae were observed till pupation and adult emergence to study the impact of cannibalism on the biology of the insect. Number of normal and abnormal pupae, per cent pupation, number of normal and abnormal adults, and per cent adult emergence were calculated. Observations were calculated using following formulae:

Per cent cannibalism =     Number of larvae cannibalized   × 100

                                                   Total number of larvae

Per cent pupation =             Number of pupae formed  × 100

                                                    Total number of larvae

Per cent adult emergence =               Number of adults emerged  × 100

                                                                  Total number of pupae

RESULTS

Per cent cannibalism

When compared to control, cannibalism of 2^nd instar larvae of S. frugiperda was found to be 30.00, 20.00, 13.33, 13.33, 10.00, 6.67, 0.00 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. This contributed to a total of 93.33 per cent in the presence of M. rileyi infected 3^rd instar larvae. The percentage of cannibalism among 2^nd instar larvae of S. frugiperda displayed the following pattern over time: 33.33, 20.00, 13.33, 10.00, 10.00, 6.67, 0.00 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. These cumulative rates amounted to 93.33 per cent in the presence of M. rileyi infected 4^th instar larvae, in comparison to the control. When compared to control, cannibalism of 2^nd instar S. frugiperda larvae was found to be 33.33, 16.67, 13.33, 13.33, 10.00, 6.67 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively in the presence of M. rileyi infected 5^th instar larvae. This contributed to a total of 93.33 per cent. The percentage of 3^rd instar larvae of S. frugiperda that were cannibalized was 23.33, 16.67, 13.33, 10.00, 10.00, 6.67, 0.00 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. This contributed to a total of 80.00 per cent when M. rileyi infected 4^th instar larvae were present compared to control. The percentage of cannibalism among 3^rd instar larvae of S. frugiperda followed a pattern over time: 30.00, 23.33, 13.33, 10.00, 6.67, 3.33, 3.33 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. These cumulative values added up to 90.00 per cent in the presence of M. rileyi infected 5^th instar larvae, as compared to the control. The cannibalism percentages among 4^th instar larvae of S. frugiperda over time was 13.33, 13.33, 13.33, 10.00, 13.33, 3.33, 0.00 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. These cumulative values amounted to 66.67 per cent in the presence of M. rileyi infected 5^th instar larvae, in comparison to the control.

The percentage of cannibalism among 2^nd instar larvae of S. frugiperda was found to be 26.67, 16.67, 13.33, 13.33, 6.67, 6.67, 3.33 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively, adding up to a total of 86.67 per cent in the presence of M. rileyi infected 2^nd instar larvae compared to control. There was a varying cannibalism percentages among 3^rd instar larvae of S. frugiperda across time intervals: 20.00, 16.67, 13.33, 13.33, 6.67, 3.33, 3.33 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. These cumulative percentages summed up to 76.67 per cent in the presence of M. rileyi infected 3^rd instar larvae, compared to the control. The percentage of 4^th instar larvae of S. frugiperda that were cannibalized was 13.33, 13.33, 6.67, 10.00, 6.67, 3.33, 10.00 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. This contributed to a total of 63.33 per cent in the presence of M. rileyi infected 4^th instar larvae compared to control. The results of the data analysis using the two samples t-test showed that there was a significant difference between treatment and control. Cannibalism percentages among 5^th instar larvae of S. frugiperda across different time intervals was 10.00, 10.00, 6.67, 10.00, 13.33, 0.00, 0.00 and 0.00 per cent at 12, 24, 36, 48, 60, 72, 84 and 96 hrs, respectively. These cumulative percentages amounted to 50.00 per cent in the presence of M. rileyi infected 5^th instar larvae, compared to the control. Employing a two-sample t-test for data analysis indicates no statistically significant difference between the conditions being compared (Table 1).

Impact of cannibalism on the biology of S. frugiperda

According to pooled data, when 2^nd instar larvae were released alongside M. rileyi infected 3^rd instar larvae, their larval lifespan was extended by 22.38 per cent as opposed to when they were raised separately (without cannibalism). When released together with M. rileyi infected 2^nd instar larvae, it increased in 2^nd instar larvae by 18.88 per cent over control (Table 2). There were no pupae formed when 2^nd instar larvae were released with M. rileyi infected 4^th or 5^th instar larvae and 3^rd instar larvae were released with M. rileyi infected 5^th instar larvae compared to control. Per cent normal pupae formed was 0.00 per cent in early instar larvae (Fig. 1). When released with M. rileyi infected 3^rd instar larvae, male pupal duration of 3^rd instar larvae increased by 9.30 per cent over control. When 4^th instar larvae were released with M. rileyi infected 5^th instar larvae, it rose by 7.04 per cent over control. When M. rileyi infected 3^rd and 5^th instar larvae were released with healthy 3^rd and 4^th instar larvae, respectively, female pupal duration increased by 10.00 and 7.50 per cent over control (Table 2). There were no adults emerged in 2^nd instar larvae when released along with M. rileyi infected 2^nd or 3^rd or 4^th or 5^th instar larvae and 3^rd instar larvae were released with M. rileyi infected 5^th instar larvae compared to control (Fig. 2). The adult male’s longevity demonstrated an elevation of 13.79 and 10.34 per cent over control during the 3^rd and 4^th instar larval phases, respectively, when introduced in the company of M. rileyi infected 3^rd and 5^th instar larvae. Likewise, the adult female’s longevity exhibited a rise of 10.81 per cent over control during the 4^th instar larval stage upon release alongside M. rileyi infected 5^th instar larvae. Furthermore, it exhibited a rise of 5.41 per cent over control during the 4^th instar larval stage when released alongside M. rileyi infected 4^th instar larvae (Table 2).

Fitness cost analysis

Combined data analysis revealed that the most significant reduction percentage (51.59 %) in pupal weight (111.96 mg) occurred in 2^nd instar larvae when introduced alongside M. rileyi infected 3^rd instar larvae. This was succeeded by a reduction of 47.64 per cent in 3^rd instar larvae (122.03 mg) when released in the presence of M. rileyi infected 5^th instar larvae, as compared to the control (243.50 mg). On the other hand, the least notable reduction percentage (35.41 %) in pupal weight was observed in 5^th instar larvae (153.21 mg) when released alongside M. rileyi infected 5^th instar larvae. This was followed by a reduction of 38.94 per cent in 4^th instar larvae (144.20 mg) when introduced alongside M. rileyi infected 4^th instar larvae (Table 3).

DISCUSSION

Interestingly, our findings suggest that, no larvae consumed the disease-infected larvae in the presence of food. They did not even step near to the disease-infected larvae. In some cases, i.e., when there was no food, larvae had gone near disease infected larvae and tried to feed on them even though they were covered with fungal spores (Unpublished data). Our results suggest that healthy larvae avoided cannibalizing on disease-infected larvae, rather they cannibalized the healthy larvae at a higher rate. This may be because disease-infected larvae became less active until their death. After their death, fungal spores start coming out of the larvae. So, the healthy larvae avoided coming near to the larvae which were infected because of the fungal toxins released by the disease-infected larvae. In the majority of the cases, larvae cannibalise on healthy larvae which are actively moving and, behaviorally attacking and defending, posing threat of encounter. Rarely they will feed on conspecifics that are already injured, partially fed or disease-infected. Sometimes they even feed on disease-infected individuals when the pathogen is virus.

Varied level of cannibalism was noticed among different instar larvae of S. frugiperda. Cannibalism was increased when different instars of healthy and diseased larvae were interacted. Cannibalism was at a greater extent among smaller healthy larvae in the presence of larger diseased larvae, compared to cannibalism among equal aged larvae. This might be because when the healthy larvae are smaller than disease-infected larvae, fungal toxins released from the bigger ones may be at a greater extent due to larger surface area. It was also observed that, cannibalism was highest in early instar larvae rather than late instar larvae. Deaths due to cannibalism occurred gradually and regularly so that there was a decreasing number of insects in a situation becoming decreasingly crowded and less encountered by the conspecifics. Results are in line with the findings of [4,5,7] who documented that cannibalism was prevalent among larvae in a group when the pathogen was present compared to when it was absent.

CONCLUSION

Cannibalism had a more pronounced impact on early instar larvae in contrast to their later instar counterparts in the presence of entomopathogen. The findings of our study carry significant implications for the management of S. frugiperda. Given that S. frugiperda larval populations are influenced by the entomopathogenic fungus M. rileyi, the application of EPF spray could potentially lead to an increase in cannibalistic behavior among larvae that remain unaffected by M. rileyi.

FUTURE SCOPE

Investigating the population dynamics of S. frugiperda larvae in field conditions while coexisting with M. rileyi infected larvae stands as a significant objective for upcoming research endeavors.

CONLICT OF INTEREST

The authors declare no conlict of interest.

ACKNOWLEDGMENT

The authors are highly thankful to the Department of Entomology, College of Agriculture, University of Agricultural Sciences for all the support.

REFERENCES

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