Predation rates between S. frugiperda and H. armigera
Both S. frugiperda and H. armigera preyed on their counterparts in all larval combinations except for the 1st instar larvae, and the predation rate increased significantly with increasing life stage of the predator but decreased with increasing life stage of the prey (Tables 1 and 2). More specifically, 5th and 6th instar S. frugiperda larvae preyed on all immature stages of H. armigera, with 5th and 6th instar S. frugiperda consuming a respective 3.3–100% of H. armigera prey (Table 1). However, 2nd and 3rd instar S. frugiperda larvae only preyed upon the same larval stage or younger of H. armigera, with 2nd and 3rd instar S. frugiperda consuming a respective 10.0–76.7% of H. armigera prey (Table 1). Similarly, 2nd through 6th instar H. armigera larvae only preyed on the same stage or younger larvae of S. frugiperda, with 2nd through 6th instar H. armigera consuming a respective 3.3–100% of S. frugiperda prey (Table 2). For the 3rd–3rd, 4th–4th, 5th–5th and 6th–6th, the predation rate of S. frugiperda on H. armigera was 16.7%, 30.0%, 20.0% and 16.7%, respectively, and 10.0%, 3.3%, 10.0% and 6.7%, respectively, for H. armigera preying on S. frugiperda, indicating that S. frugiperda had a competitive advantage over H. armigera (Tables 1 and 2).
Table 1
Predation rates (%) of S. frugiperda larvae on H. armigera larvae at various stages
Larval stage of
predator
S. frugiperda
|
Larval stage of prey H. armigera
|
1st
|
2nd
|
3rd
|
4th
|
5th
|
6th
|
1st
|
-
|
-
|
-
|
-
|
-
|
-
|
2nd
|
53.3 ± 9.1 Abc
|
10.0 ± 5.5 Bc
|
-
|
-
|
-
|
-
|
3rd
|
76.7 ± 7.7 Ab
|
50.0 ± 9.1 Bb
|
16.7 ± 6.8 Cd
|
-
|
-
|
-
|
4th
|
93.3 ± 4.6 Aab
|
96.7 ± 3.3 Aa
|
50.0 ± 9.1 Bc
|
30.0 ± 8.4 Cb
|
3.3 ± 3.3 Dc
|
-
|
5th
|
96.7 ± 3.3 Aa
|
96.7 ± 3.3 Aa
|
86.7 ± 6.2 ABb
|
73.3 ± 8.1 Ba
|
20.0 ± 7.3 Cb
|
3.3 ± 3.3 Da
|
6th
|
96.7 ± 3.3 Aa
|
100.0 ± 0.0 Aa
|
100.0 ± 0.0 Aa
|
76.7 ± 7.7 Ba
|
76.7 ± 7.7 Ba
|
16.7 ± 6.8 Ca
|
Data are means ± SE (n = 30 plates per predator–prey pair combinations). Different uppercase letters indicate a significant difference in predation rate by a larval instar of the predator for the different larval instars of the prey, and different lowercase letters indicate a significant difference among different larval instars of the predator for the same larval instar of prey (χ2 test; P < 0.05).
Table 2
Predation rates (%) of H. armigera larvae on S. frugiperda larvae at various stages
Larval stage of
predator
H. armigera
|
Larval stage of prey S. frugiperda
|
1st
|
2nd
|
3rd
|
4th
|
5th
|
6th
|
1st
|
-
|
-
|
-
|
-
|
-
|
-
|
2nd
|
36.7 ± 8.8 Ab
|
10.0 ± 5.5 Bc
|
-
|
-
|
-
|
-
|
3rd
|
93.3 ± 4.6 Aa
|
66.7 ± 8.6 Bb
|
10.0 ± 5.5 Cc
|
-
|
-
|
-
|
4th
|
100.0 ± 0.0 Aa
|
90.0 ± 5.5 Aa
|
50.0 ± 9.1 Bb
|
3.3 ± 3.3 Cb
|
-
|
-
|
5th
|
100.0 ± 0.0 Aa
|
96.7 ± 3.3 Aa
|
63.3 ± 8.8 Bb
|
66.7 ± 8.6 Ba
|
10.0 ± 5.5 Cb
|
-
|
6th
|
100.0 ± 0.0 Aa
|
100.0 ± 0.0 Aa
|
100.0 ± 0.0 Aa
|
86.7 ± 6.2 Ba
|
56.7 ± 9.1 Ca
|
6.7 ± 4.6 D
|
Different uppercase letters indicate a significant difference in predation rate by a larval instar of the predator for the different larval instars of the prey, and different lowercase letters indicate a significant difference among different larval instars of the predator for the same larval instar of prey (χ2 test; P < 0.05).
Predation functional responses between S. frugiperda and H. armigera
Both S. frugiperda preyed on H. armigera and H. armigera preyed on S. frugiperda yielded positive P1 values, indicating that the response of either species to prey density was in line with the Holling type Ⅲ functional response (Table 3). The predated proportion increased first, then decreased with as prey density increased, a typical feature of the type Ⅲ predation functional response (Fig. 1). Based on the predation handling time (Th), attack rate (a) and theoretical maximum predation value (Nm) calculated using the Holling Ⅲ equation for S. frugiperda and H. armigera (Table 4), the theoretical maximum predation value of the predator decreased with increasing life stage of the prey. In addition, the highest theoretical maximum predation value of S. frugiperda (71.43) was obviously higher than that of H. armigera(38.46).
Table 3
Logistic regression describing the proportion of 6th instar predator larvae (S. frugiperda or H. armigera) preying on 1st–3rd instar larvae of the other species
Predator species
|
Larval stage of
prey
|
Parameter
|
Estimated value
|
SE
|
T-value
|
Pr(>|z|)
|
S. frugiperda
|
1st instar of
|
P0
|
-0.1382
|
0.2568
|
-5.3820
|
0.0000
|
|
H. armigera
|
P1
|
0.0896
|
0.03324
|
2.6950
|
0.0122
|
|
|
P2
|
-0.0025
|
0.0011
|
-2.3580
|
0.0262
|
P3
|
0.0000
|
0.0000
|
2.1670
|
0.0396
|
2nd instar of
|
P0
|
-1.9691
|
0.7408
|
-2.6580
|
0.0133
|
H. armigera
|
P1
|
0.3781
|
0.1703
|
2.2200
|
0.0354
|
|
P2
|
-0.0215
|
0.0104
|
-2.0630
|
0.0492
|
P3
|
0.0003
|
0.0002
|
1.8880
|
0.0702
|
3rd instar of
|
P0
|
-3.0660
|
0.3804
|
-8.0600
|
0.0000
|
H. armigera
|
P1
|
0.8138
|
0.2237
|
3.6380
|
0.0012
|
|
P2
|
-0.0895
|
0.0303
|
-2.9490
|
0.0067
|
P3
|
0.0029
|
0.0011
|
2.6660
|
0.0130
|
H. armigera
|
1st instar of
|
P0
|
-0.1370
|
0.3875
|
-3.5360
|
0.0020
|
|
S. frugiperda
|
P1
|
0.1856
|
0.0828
|
2.2420
|
0.0359
|
|
|
P2
|
-0.0012
|
0.0047
|
-2.6260
|
0.0157
|
P3
|
0.0001
|
0.0000
|
2.6730
|
0.0143
|
2nd instar of
|
P0
|
-1.7780
|
0.2440
|
-7.2860
|
0.0000
|
S. frugiperda
|
P1
|
0.1036
|
0.0479
|
2.1640
|
0.0406
|
|
P2
|
-0.0027
|
0.0021
|
-1.2830
|
0.2116
|
P3
|
0.0000
|
0.0000
|
1.1180
|
0.2748
|
3rd instar of
|
P0
|
-2.2347
|
0.7147
|
-3.1270
|
0.0043
|
S. frugiperda
|
P1
|
0.7597
|
0.3088
|
2.4600
|
0.0209
|
|
P2
|
-0.0817
|
0.0358
|
-2.2800
|
0.0310
|
P3
|
0.0025
|
0.0012
|
2.1260
|
0.0431
|
P0, P1, P2, P3 are the intercept, linear, quadratic and cubic coefficients, respectively, of the logistic regression analysis equation for predator consuming different larval stage of prey. |
Table 3 (see end)
Table 4
Attack rate (a), handling time (Th), and theoretical maximum prey consumption (Nm) of 6th instar larvae of S. frugiperda preying on H. armigera and H. armigera preying on S. frugiperda at different prey stages
Predator
|
Stage of prey species
|
Model
|
a
|
Th (d)
|
Nm
|
R2
|
S. frugiperda
|
1st instar of H. armigera
|
III
|
b1N0
|
0.014 ± 0.001
|
71.43
|
0.871
|
|
2nd instar of H. armigera
|
III
|
b2N0
|
0.031 ± 0.002
|
32.26
|
0.818
|
|
3rd instar of H. armigera
|
III
|
b3N0
|
0.033 ± 0.011
|
30.30
|
0.875
|
H. armigera
|
1st instar of S. frugiperda
|
III
|
b4N0
|
0.026 ± 0.003
|
38.46
|
0.732
|
|
2nd instar of S. frugiperda
|
III
|
b5N0
|
0.035 ± 0.005
|
28.57
|
0.795
|
|
3rd instar of S. frugiperda
|
III
|
b6N0
|
0.059 ± 0.006
|
16.95
|
0.746
|
In the best-fit type III model, a = bN0, b1 = 0.011 ± 0.002, b2 = 0.042 ± 0.010, b3 = 0.011 ± 0.003, b4 = 0.007 ± 0.002, b5 = 0.014 ± 0.005, b6 = 0.049 ± 0.015 and N0 = initial density of prey. |
Interspecific competition between S. frugiperda and H. armigera on maize plants
When 1st instar larvae of S. frugiperda and H. armigera co-infested maize tassels, the survival rate was 75.6% and 57.8%, respectively, with no significant differences (F1,4 = 5.815, P = 0.073). When the age of the two pests differed (1st instar larva of S. frugiperda with 3rd instar larva of H. armigera; 1st instar larva of H. armigera with 3rd instar larva of S. frugiperda), the older pest had a significantly higher survival rate than the younger (3rd instar FAW: F1,4 = 62.515, P = 0.001; 3rd CBW: F1,4 = 35.591, P = 0.004), but the survival rate of 1st instar S. frugiperda larvae (44.4%) was higher than that of H. armigera (23.3%) (Fig. 2), indicating that S. frugiperda larvae had greater survivability.
Population abundances of the two pests in local maize fields
Significantly more adults of S. frugiperda were trapped in 2021 than in 2019 and 2020 (F2,6 = 13.232, P = 0.006) and similarly for H. armigera (F2,6 = 90.371, P = 0.000) (Fig. 3). However, the number of S. frugiperda adults compared with H. armigera trapped each year did not differ significantly (2019: F1,4 = 0.000, P = 1.000; 2020: F1,4 = 6.429, P = 0.064; 2021: F1,4 = 1.877, P = 0.243; Fig. 3A). On maize at the tasseling stage, the number of S. frugiperda larvae per 100 maize plants was significantly higher than that of H. armigera each year (2019: F1,28 = 60.982, P = 0.000; 2020: F1,28 = 21.815, P = 0.000; 2021: F1,4 = 12.213, P = 0.001). In addition, the number of S. frugiperda larvae per 100 maize plants in 2021 was significantly higher than in 2019 and 2020 (F2,42 = 7.118, P = 0.002; Fig. 3B).