Survival, mortality, and cannibalism of fall armyworm and stemborers in single-species combination
At small larval size, FAW had the lowest survival rate while B. fusca had the highest (LR = 26.38, df = 3; P ≤ 0.0001). At medium larval size group, no significant difference was recorded between the four species (LR = 3.01, df = 3; P = 0.39) while at large larval size group, the survival rates of the three stemborer species were significantly higher than those of FAW (LR = 31.66, df = 3; P ≤ 0.0001) (Fig. 1A). Between larval size groups for a given species (Fig. 1A), the survival rate of FAW was significantly lower at large size group as compared to the smaller size groups (χ2 = 22.30; df = 2; P ≤ 0.0001) in opposition to C. partellus where higher survival rate was recorded at larger size group (χ2 = 16.45; df = 2; P ≤ 0.0001). For B. fusca, survival rate was significantly low at medium size group as compared to small and large size groups (χ2 = 6.72; df = 2; P = 03) while no significant difference was recorded between S. calamistis larval sizes (χ2 = 5.30; df = 2; P = 0.07).
In terms of mortality (Fig. 1B), there was no significant difference between the four species at small larval size group (χ2 = 7.57; df = 3; P = 0.06). However, at medium size group, B. fusca presented a significantly higher mortality rate than FAW (χ2 = 22.23; df = 3; P ≤ 0.0001), while at large size group, C. partellus mortality was significantly lower as compared to FAW and B. fusca (χ2 = 25.37; df = 3; P ≤ 0.0001). Between larval size groups for a given species, large size of FAW presented a significant higher mortality as compared to small and medium size groups (χ2 = 16.62; df = 2; P ≤ 0.0001). The mortality rates of B. fusca were significantly higher at medium and large size groups than at small size group (χ2 = 5.17; df = 2; P = 0.04) while they were significantly higher at small and medium size groups than large size group for C. partellus (χ2 = 12.25; df = 2; P ≤ 0.0001). No significant difference was revealed between the different size groups of S. calamistis (χ2 = 1.45; df = 2; P = 0.48).
The cannibalism rate (Fig. 1C) was significantly more pronounced in FAW than in stemborer species at all larval size groups, small (χ2 = 58.76; df = 3; P ≤ 0.0001), medium (χ2 = 46.63; df = 3; P ≤ 0.0001) and large size (χ2 = 105.61; df = 3; P ≤ 0.0001). On the other hand, while the higher cannibalism rate of FAW was recorded at large size group (χ2 = 8.01; df = 2; P =0.02), it was the lower rate that was recorded for B. fusca (χ2 = 11.16; df = 2; P ≤ 0.0001), C. partellus (χ2 = 19.66; df = 2; P ≤ 0.0001) and S. calamistis (χ2 = 18.96; df = 2; P ≤ 0.0001) at that larval size group as compared to small and medium age groups.
Survival, mortality, and cannibalism coupled with intraguild predation in pairing-species combination between fall armyworm and stemborers
For all combinations, FAW survival rate was significantly higher than that of its corresponding competing species at all larval size groups: FAW+Bf (Fig. 2A, Table 2), FAW+Sc (Fig. 2B, Table 2) and FAW+Cp (Fig. 2C, Table 2). There was no significant difference for mortality within species in all combinations at small larval size group: FAW+Bf (Fig. 2D, Table 2), FAW+Sc (Fig. 2E, Table 2) and FAW+Cp (Fig. 2E, Table 2). However, the mortality of FAW was significantly higher in the pairing with B. fusca at medium and large larval size groups (Fig. 2D, Table 2), with S. calamistis at medium and large larval size groups (Fig. 2E, Table 2) and with C. partellus at medium and large larval size groups (Fig. 2F, Table 2). The cannibalism coupled with intraguild predation of stemborer species were significantly higher than that of FAW in all combinations at all larval size groups: FAW+Bf (Fig. 2G, Table 2), FAW+Sc (Fig. 2H, Table 2) and FAW+Cp (Fig. 2I, Table 2).
Table 2
Results of GLM analysis comparing larval survival, mortality and cannibalism/intraguild predation between fall armyworm vs stemborer species in multi-species combinations at different development stages
Combinations | Larvae development stages |
Small | Medium | Large |
LR | df | P | LR | df | P | LR | df | P |
Survival |
FAW+Bf | 45.41 | 1 | ≤ 0.0001 | 82.85 | 1 | ≤ 0.0001 | 28.03 | 1 | ≤ 0.0001 |
FAW+Sc | 47.72 | 1 | ≤ 0.0001 | 30.75 | 1 | ≤ 0.0001 | 6.95 | 1 | 0.008 |
FAW+Cp | 54.45 | 1 | ≤ 0.0001 | 28.87 | 1 | ≤ 0.0001 | 25.30 | 1 | ≤ 0.0001 |
Mortality |
FAW+Bf | 0.22 | 1 | 0.63 | 4.6 | 1 | 0.04 | 6.05 | 1 | 0.01 |
FAW+Sc | 0.86 | 1 | 0.35 | 6.73 | 1 | 0.009 | 10.35 | 1 | 0.001 |
FAW+Cp | 0.02 | 1 | 0.88 | 11. 82 | 1 | 0.0005 | 12.20 | 1 | 0. 0004 |
Cannibalism/intraguild predation |
FAW+Bf | 82.47 | 1 | ≤ 0.0001 | 49.11 | 1 | ≤ 0.0001 | 26.95 | 1 | ≤ 0.0001 |
FAW+Sc | 62.41 | 1 | ≤ 0.0001 | 28.73 | 1 | ≤ 0.0001 | 10.35 | 1 | 0.001 |
FAW+Cp | 49.14 | 1 | ≤ 0.0001 | 22.88 | 1 | ≤ 0.0001 | 28.90 | 1 | ≤ 0.0001 |
Comparison of survival, mortality, and cannibalism and/or intraguild predation in single- versus pairing-species combinations
For this comparison purpose, the survival, mortality, and cannibalism and/or intraguild predation rates were calculated for the overall larvae regardless of the species present in each test for pairing-species combinations. All stemborer single-species combinations exhibited a significant higher survival rate than their respective pairing-species combinations with FAW at all larval size groups (Fig. 3, small: LR = 202.62; df = 6; P ≤ 0.0001, medium: LR = 146.56; df = 6; P ≤ 0.0001 and large: LR = 127.78; df = 6; P ≤ 0.0001).
At small size group, stemborer single-species communities presented a higher mortality than their respective pairing-species combination with FAW (LR = 24.86; df = 6; P = 0.0003). At medium size group, while FAW had lower mortality, B. fusca single-species had significantly higher mortality as compared to its pairing-species combination counterpart mortality (LR = 28.69; df = 6; P ≤ 0.0001). At large size group, S. calamistis and C. partellus single-species combinations exhibited significantly lower mortalities as compared to the mortalities of their pairing-species combination counterparts (LR = 35.14; df = 6; P ≤ 0.0001).
The cannibalism coupled with intraguild predation in pairing-species combination was significantly higher as compared to cannibalism in single-species combinations at all larval size groups (Fig. 3, small: LR = 349.81; df = 6; P ≤ 0.0001, medium: LR = 356.83; df = 6; P ≤ 0.0001 and large: LR = 246.41; df = 6; P ≤ 0.0001).
Life-time survival, relative growth rate, mortality, and cannibalism of fall armyworm and stemborers across larval instars in single-species combination
Across larval instars, the three stemborers species presented similar survival evolution while significantly higher than those of FAW species from third to sixth instar (Fig. 4A, χ2 = 308.17; df = 5; P = 0.0001). The FAW presented the greater relative growth rate as compared to those of stemborers at third and fourth instars with an optimum of 15mg/day (Fig. 4B).
The Fig. 4C present the evolution of mortality rate of the different species across larval development instars. At L1 instar, S. calamistis presented a high mortality rate as compared to other species (χ2 = 26.93; df = 5; P =0.0001). However, at L5 and L6 instars, the mortality rate of FAW became significantly lower while those of stemborer species were still high L5 (χ2 = 37.94; df = 3; P =0.02), L6 (χ2 = 98.23; df = 3; P =0.03). The cannibalism rate (Fig. 4D) was significantly higher in FAW than in stemborer species from third to last instar, L3 (χ2 = 56.7; df = 3; P =0.01), L4 (χ2 = 34.92; df = 3; P =0.002) L5 (χ2 = 95.78; df = 3; P =0.03) and L6 (χ2 = 123.54; df = 3; P ≤0.0001).
Life-time survival, relative growth rate, mortality and cannibalism coupled with intraguild predation across larval development instars in pairing-species combinations
In pairing-species combination between fall armyworm and stemborers, none of stemborer species survived at the last instar to pupate (Fig. 5A1-A3). The survival rate of either FAW or stemborers significantly decreased along larval development instars (χ2 = 42.52; df = 5; P =0.001) and the survival of FAW was significantly higher than those of stemborers in each combination (χ2 = 57.13; df = 3; P =0.004). B. fusca and S. calamistis reached 0% at L4 instar and C. partellus at L5 instar while about 10% of FAW survived at last instar to pupate. The growth rate of stemborer species did not reach the optimum before they disappeared in the combinations (Fig. 5B1-B3). The optimum of FAW growth rate ranged from 40 to 50mg/day across the different combinations and it has mostly decreased after stemborer species were no longer present in the combinations (Fig. 5B1-B3). In each pairing-species combination, the mortality rate of stemborers was significantly (χ2 = 32.34; df = 3; P =0.001) higher than those of FAW along larval development instars before the optimum and thereafter, the mortality rate of FAW come over those of stemborer species (Fig. 5C1-C3). on the other hand, the cannibalism coupled with intra-guild predation of stemborers in each combination was significantly higher than those of FAW along larval development instars (χ2 = 54.21; df = 3; P ≤0.0001). However, when they finished in the system, the cannibalism of FAW presented exponential trend (Fig. 5D1-D3).