Population build-up of apterous aphid
In summer crop, one week after infestation, the population of apterous aphids reached to 7.8, 11.0, 26.5 and 37.8 aphids/ plant at the corresponding infestation levels of 1, 2, 4 and 8 aphids/ plant on 7 days after infestation (DAI). The population increased significantly on 21 DAI and reached the peak in 6th week on 42 DAI with the population levels of 986.0, 2304.9, 3267.3 and 4380.5 aphids/ plant at 1, 2, 4 and 8 AIL, respectively (Table 1). Thereafter, a decline in population was set in and aphid count became zero on 63 DAI (33 DAT) because of the occurrence of an entomopathogenic fungus Lecanicillium (Verticillium) lecanii (Zimmerman). It was also evident that the mean population differ significantly amongst different infestation levels being lowest at 1 aphid level (443.1 aphids/plant) and highest to 8 aphids level (2467.9 aphids/plant). Thereafter, the aphid population built-up took place after a lapse of 8 weeks (119 DAI) but remained in low numbers till harvest of the crop.
Table 1
Population build-up of Myzus persicae (apterous form) in bell pepper at different infestation levels in summer and autumn crop
Sampling period (Days after infestation) | Apterous aphids (number/ plant) at indicted aphid infestation level (AIL) |
Summer crop | Autumn crop |
1 | 2 | 4 | 8 | Mean | 1 | 2 | 4 | 8 | Mean |
0 | 1.0 | 2.0 | 4.0 | 8.0 | 3.8 | 1.0 | 2.0 | 4.0 | 8.0 | 3.8 |
7 | 7.8 | 11.0 | 26.5 | 37.8 | 20.8 | 10.7 | 15.3 | 24.3 | 43.3 | 23.4 |
14 | 21.8 | 32.8 | 61.2 | 73.0 | 48.5 | 23.8 | 36.2 | 49.4 | 116.6 | 56.5 |
21 | 379.8 | 951.3 | 1814.2 | 2371.6 | 1379.3 | 54.0 | 71.3 | 83.2 | 147.8 | 89.1 |
28 | 670.1 | 1568.9 | 2578.4 | 3353.0 | 2042.6 | 144.7 | 198.3 | 214.2 | 274.3 | 207.9 |
35 | 819.6 | 1991.4 | 3034.7 | 3892.1 | 2434.5 | 428.3 | 430.4 | 525.6 | 606.6 | 497.7 |
42 | 986.0 | 2304.9 | 3267.3 | 4380.5 | 2734.7 | 792.5 | 853.6 | 976.3 | 1224.0 | 961.6 |
49 | 510.9 | 1222.8 | 2471.2 | 3805.4 | 2002.6 | 1367.7 | 1472.3 | 1615.8 | 1890.3 | 1586.5 |
56 | 157.7 | 699.7 | 862.6 | 1829.3 | 887.3 | 1890.0 | 2134.1 | 2431.6 | 2738.9 | 2298.6 |
63 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 3037.1 | 3056.2 | 3243.1 | 3520.5 | 3214.2 |
70 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 2485.8 | 2505.9 | 3423.7 | 3029.8 | 3163.8 |
77 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 1815.1 | 1535.2 | 2138.6 | 2232.3 | 1930.3 |
84 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 745.7 | 952.8 | 1033.2 | 1201.1 | 983.2 |
91 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 416.4 | 524.9 | 714.2 | 879.7 | 633.8 |
98 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 180.6 | 302.1 | 435.1 | 478.0 | 348.9 |
105 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 82.3 | 122.3 | 161.3 | 197.0 | 140.7 |
112 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 25.1 | 34.5 | 38.0 | 43.9 | 35.4 |
119 | 2.6 | 1.8 | 0.0 | 3.6 | 2.0 | - | - | - | - | - |
126 | 1.9 | 1.2 | 2.3 | 3.3 | 2.2 | - | - | - | - | - |
133 | 2.9 | 3.8 | 5.6 | 6.3 | 4.7 | - | - | - | - | - |
140 | 3.5 | 4.4 | 8.0 | 5.0 | 5.2 | - | - | - | - | - |
147 | 2.3 | 5.6 | 12.7 | 7.9 | 7.1 | - | - | - | - | - |
154 | 4.2 | 6.4 | 10.2 | 9.5 | 7.6 | - | - | - | - | - |
161 | 5.8 | 8.3 | 12.2 | 13.5 | 10.0 | - | - | - | - | - |
168 | 9.6 | 12.2 | 16.7 | 17.1 | 13.9 | - | - | - | - | - |
175 | 16.2 | 18.3 | 19.6 | 22.9 | 19.3 | - | - | - | - | - |
*Mean | 443.1 | 1099.7 | 1764.5 | 2467.9 | | 843.7 | 890.3 | 1144.8 | 1164.0 | |
Interactions- Critical Difference (p = 0.05) | Days of observation (A) 748.0, Infestation levels (B) 528.9 Days of observations x Infestation levels NS | Days of observation (A) 350.6, Infestation levels (B) 175.3 Days of observations x Infestation levels NS |
* based upon analysis from 0 to 56 DAI * based upon analysis from 0 to 112 DAI |
In autumn crop, population of 10.7, 15.3, 24.3 and 43.3 at infestation level of 1, 2, 4 and 8 aphids/ plant was resulted, respectively, on 7 DAI (Table 1). The population of M. persicae resulted in peak of 3037.1, 3056.2 and 3520.5 aphids/ plant for 1, 2 and 8 AIL on 63 DAI whereas, at infestation level of 4 aphids it was delayed by one week to 70 DAI (3423.7 aphids/plant). It was evident that the mean population increased steadily upto 28 DAI (207.9 aphids/plant) and at significantly higher rate during subsequent weeks resulting in aphid peak activity on 63 DAI (3214.2 aphids/plant), being on a par to 70 DAI (3163.8 aphids/plant). Thereafter, M. persicae population experienced a steady decline till the last observation recorded on 112 DAI (35.4 aphids/plant).
It was also evident that the mean population (843.7 and 890.3 aphids/plant) at 1 and 2 aphid infestation levels being on a par to each other and differed significantly to high population levels of 4 and 8 aphids (1144.8 and 1164.0 aphids/plant) which in turn were on a par to themselves (Table 1).
Population build-up of alates aphid
In summer crop, alate formation initiated three weeks after release of aphids (Table 2) with the population of 3.0, 18.8, 17.1 and 14.8 alates/ plant at 1, 2, 4 and 8 AIL, respectively. The alate formation was significantly more at higher infestation level (8 aphids/plant), being on a par to 4-aphid level. Alate formation was significantly low at 1 AIL. At population levels of 2 and 8 aphids per plant, the peak alate formation was observed on 35 DAI with the resultant population of 23.7 and 29.2 alates/ plant whereas, at 1 and 4 population levels, the peak of alates formed was delayed by one week i.e., 42 DAI with the corresponding population levels of 7.3 and 26.4 aphids/plant (Table 2). On the basis of mean alate aphid count, peak activity (21.2 alates/plant) was observed on 42 DAI, coinciding with apterous peak activity (2734.7 apterous/plant), which was also on a par with alate formation observed in preceding two weeks. The crop remained free of aphid infestation from 63 to 105 DAI. Thereafter, sparse alate activity was observed in 112 DAI till the final harvest of the crop on 175 DAT.
In autumn crop, perusal of data contained in Table 2 revealed that alate formation initiated 3 weeks after release of aphids on 21 DAI with the population levels of 12.7, 21.7, 21.6 and 22.7 alates/ plant at 1, 2, 4 and 8 AILs, respectively. There were two peaks in alate activity, one occurring 6 weeks after release (42 DAI) with the resultant population level of 23.3, 26.4, 28.8 and 34.9 alates/ plant. The second peak also occurred six weeks after first peak i.e., 84 DAI with the alate aphid count of 60.3, 73.4, 64.5 and 99.4 alates/ plant. The alate formation increased steadily up to 63 DAI (27.7, 29.0, 33.2 and 33.2 alates/ plant at different infestation levels), but it increased at faster rate subsequently upto 84 DAI (60.3, 73.4, 64.5 and 99.4 alates/ plant at 1, 2, 4 and 8 AILs, respectively). Thereafter, an abrupt decline was evident. The data also revealed that the aphid infestation levels also influenced the alate formation being significantly minimum (21.0 aphids/plant) and maximum (32.9 aphids/plant) at 1 and 8 AILs, respectively.
Table 2
Population build-up of Myzus persicae (alate form) in bell pepper at different infestation levels in summer and autumn crop
Sampling period (Days after infestation) | Alate aphids (number/ plant) at indicted aphid infestation level (AIL) |
Summer crop | Autumn crop |
1 | 2 | 4 | 8 | Mean | 1 | 2 | 4 | 8 | Mean |
0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
7 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
14 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
21 | 3.0 | 18.8 | 17.1 | 14.8 | 13.4 | 12.7 | 21.7 | 21.6 | 22.7 | 19.7 |
28 | 5.1 | 20.5 | 19.9 | 22.7 | 17.1 | 14.5 | 23.8 | 26.1 | 27.9 | 23.0 |
35 | 5.8 | 23.7 | 22.3 | 29.2 | 20.2 | 16.4 | 22.3 | 19.3 | 21.9 | 20.0 |
42 | 7.3 | 22.2 | 26.4 | 29.1 | 21.2 | 23.3 | 26.4 | 28.8 | 34.9 | 28.4 |
49 | 3.7 | 4.0 | 8.9 | 13.2 | 7.5 | 19.3 | 25.7 | 27.8 | 31.1 | 26.0 |
56 | 7.1 | 13.9 | 13.6 | 26.1 | 15.2 | 15.2 | 24.1 | 27.6 | 34.9 | 25.4 |
63 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 27.7 | 29.0 | 33.2 | 33.2 | 31.8 |
70 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 40.3 | 39.9 | 33.7 | 53.3 | 41.8 |
77 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 49.8 | 66.3 | 61.5 | 72.9 | 62.6 |
84 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 60.3 | 73.4 | 64.5 | 99.4 | 74.4 |
91 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 9.2 | 8.1 | 8.1 | 10.3 | 8.9 |
98 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 6.6 | 5.2 | 5.0 | 7.3 | 6.0 |
105 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 3.7 | 3.9 | 4.4 | 4.2 | 4.1 |
112 | 1.3 | 1.5 | 0.6 | 0.8 | 1.1 | 2.1 | 2.3 | 2.0 | 1.9 | 2.1 |
119 | 1.5 | 1.0 | 0.9 | 0.6 | 1.0 | - | - | - | - | - |
126 | 0.9 | 0.8 | 0.4 | 0.5 | 0.7 | - | - | - | - | - |
133 | 0.2 | 0.8 | 0.4 | 0.6 | 0.5 | - | - | - | - | - |
140 | 0.1 | 0.5 | 0.6 | 0.5 | 0.4 | - | - | - | - | - |
147 | 0.7 | 1.0 | 0.9 | 1.1 | 0.9 | - | - | - | - | - |
154 | 0.4 | 0.7 | 0.6 | 0.7 | 0.6 | - | - | - | - | - |
161 | 0.0 | 0.5 | 0.4 | 0.3 | 0.3 | - | - | - | - | - |
168 | 0.5 | 1.2 | 1.0 | 0.8 | 0.9 | - | - | - | - | - |
175 | 0.8 | 1.6 | 1.2 | 1.0 | 1.2 | - | - | - | - | - |
*Mean | 5.3 | 17.3 | 17.9 | 22.5 | | 21.0 | 26.2 | 26.9 | 32.9 | |
Interactions- Critical Difference (p = 0.05) | Days of observation (A) 5.9, Infestation levels (B) 4.8 Days of observations x Infestation levels NS | Days of observation (A) 5.2, Infestation levels (B) 2.8 Days of observations x Infestation levels 10.4 |
* based upon analysis from 0 to 56 DAI * based upon analysis from 21 to 112 DAI |
It is evident from the Table 1 and Fig. 1 that the peak of aphid population (2734.7 apterous aphids/plant) was achieved after 6 weeks of release (42 DAI) in summer crop and was delayed to 9 weeks (63 DAI) in autumn crop (3214.2 apterous aphids/plant). Despite higher rate of increase in summer crop, the peak population levels were comparatively more in autumn crop. It may be contributed to the aphid population suppression by L. lecanii in summer crop after 49 days of infestation.
However, on the basis of corresponding mean seasonal populations recorded in different infestation levels, it was evident that all the infestation levels in summer crop surpassed the mean aphid count as observed in autumn crop, except at infestation level of one aphid (Table 1). It was also observed that the alate formation initiated three weeks after release in both the cropping seasons in all the infestation levels (Table 2) with more number of alates being formed at higher infestation levels of 4 and 8 aphids. The results further revealed that with the increase in the population density (number of aphids per plant), number of alate forms of the aphid also increased which may be due to the effect of crowding as elaborated by Kuo et al. (1999).
Effects of abiotic factors on population build-up of apterous aphids
The correlation worked out between mean population based upon different infestation levels and the abiotic factors of the environment revealed temperature to influence apterous formation positively in summer crop (Table 3) being significant with minimum temperature only (r = 0.5247; p = 0.05). Whereas, in autumn crop, both temperature and relative humidity influenced the apterous formation negatively, which being significant with minimum temperature (r= -0.6425; p = 0.05) and minimum relative humidity (r = − 0.4571; 0 = 0.01) (Table 3).
Table 3
Correlation (r) between abiotic factors and population of Myzus persicae (apterous + alate form) on bell pepper in summer and autumn crop
Weather parameter | Apterous form | Alate form |
Summer | Autumn | Summer | Autumn |
Maximum temperature (oC) | 0.1364 | -0.3133 | 0.6829* | -0.0193 |
Minimum temperature (oC) | 0.5247* | -0.6425* | 0.6604* | -0.4508** |
Maximum relative humidity (%) | 0.0747 | -0.3587 | -0.4613** | -0.5509* |
Minimum relative humidity (%) | -0.0174 | -0.4571** | -0.0837 | -0.5368* |
*: Significant at p = 0.05%; **: Significant at p = 0.01% |
Effect of abiotic factors on population build-up of alate aphids
Both the maximum and minimum temperature in summer crop influenced the alate formation significantly (r = 0.6829 and 0.6604, respectively; p = 0.05), whereas during autumn season it exerted negative influence being significant with minimum temperature only (r= -0.4508; p = 0.01). Relative humidity in both the seasons exerted negative influence on alate formation, being significant with maximum relative humidity (r = − 0.4613; p = 0.01) in summer crop. However, both the maximum and minimum relative humidity influenced the alate formation significantly in autumn crop (r = − 0.5509 and − 0.5368, respectively; p = 0.05) (Table 3).
The present findings are in line with Kurl et al. (2010) who reported potential increase rate of M. persicae to be influenced positively by minimum and maximum temperature and negatively by relative humidity. Konar and Roy (2002), Konar and Paul (2006) and Karim et al. (2011) also recorded the population build-up in potato to be related positively with temperature and negatively with relative humidity. However, the negative relationship with minimum and maximum temperature recorded in autumn crop gets support from the findings of Biswas et al. (2004 and 2005). It can be attributed to the differences in minimum temperature range prevailing during summer and autumn crop which varied from 13.1–26.40C and 10.6–24.50C in respective seasons, being comparatively higher in summer crop. The study revealed that the population of aphid in bell pepper was positively correlated with maximum and minimum temperature in summer crop, whereas negatively correlated with maximum and minimum relative humidity in autumn crop. Similar observations were also reported by Kumar et al. (2019) and Pathipati et al. (2020).
Relationship between population of apterous and alate aphids was also worked out in summer and autumn crop, the value of “r” obtained in summer crop was 0.8528 being highly significant (p = 0.001) as compared to autumn crop (r = 0.5634). The phenomenon of formation of alates with increased aphid population also gets support from the findings of Kuo et al. (1999) who reported that with the increase in the population density (number of aphids per plant), number of alate forms of the aphid also increased which may be due to the effect of crowding.
Step wise regression analysis
Stepwise regression analysis between aphid and abiotic factors, given hereunder, revealed that minimum temperature contributed significantly in influencing the population in both summer and autumn crop.
Summer crop: Y = 2526.3-0.515 Tmin R2 = 0.266
Autumn crop: Y = 3206.9–6.67 Tmin R2 = 0.451
Y = 3933.9 − 0.56 Tmin − 0.26 RHmin R2 = 0.507
The contribution of minimum temperature was less in summer crop (26.6%) as compared to autumn crop where it contributed to the extent of 45.1 per cent. In autumn crop, the minimum relative humidity also contributed significantly but to the extent of 50.7 per cent. Contrary to these, Zhang et al. (2008) recorded mean maximum temperature to influence the population build up significantly. However, the significant impact of relative humidity observed in autumn crop is in line with the present findings.
Associated parasitoids
During this study, three species of hymenopterous parasitoids namely, Aphelinus asychis Walker (Hymenoptera: Aphelinidae), Aphidius matricariae Haliday (Hymenoptera: Braconidae) and Aphidius ervi Haliday (Hymenoptera: Braconidae) were found to be associated with M. persicae in bell pepper under polyhouse. Amongst them the aphelinid, A. asychis was recorded to parasitize M. persicae in summer crop (Table 4), whereas braconid parasitoids Aphidius spp. was prevalent in autumn crop (Table 5).
Extent Of Parasitization
During summer cropping season, the weekly data recorded on parasitisation by A. asychis presented in Table 4 revealed that the parasitisation by A. asychis varied from 1.0–13.7 per cent in different infestation levels. A. asychis was first observed in the 28 DAI as black coloured mummies. When the parasitisation was subjected to statistical analysis with respect to level of infestation, it was significantly higher at lower infestation level of 1 aphid (8.0%) which being on a par to 2 aphids level (6.3%). At higher infestation levels of 4 (2.7%) and 8 (2.1%) aphids, significantly lower parasitization was evident. Also the parasitization varied significantly on different observations. The mean minimum parasitisation corresponded to 28 DAI (2.1%), being on a par to subsequent observations i.e., 2.9%, 4.1%, 6.5%, and 8.1% at 35, 42, 49 and 56 DAI, respectively. The parasitization was maximum on 49 DAI with the values of 4.0 per cent at 8 AIL whereas, at infestation level of 1, 2 and 4 aphids it was delayed by one week to 13.7, 11.1 and 4.6 per cent, respectively (Table 4).
Table 4
Parasitization of Myzus persicae by Aphelinus asychis on bell pepper in summer crop
Sampling period Days after infestation | Per cent parasitization at indicated aphid infestation levels (AIL) | Mean |
1 | 2 | 4 | 8 |
28 | 2.5 (1.8)* | 3.6 (2.0) | 1.3 (1.5) | 1.0 (1.4) | 2.1 (1.7) |
35 | 5.7 (2.6) | 2.9 (1.9) | 1.8 (1.6) | 1.3 (1.5) | 2.9 (1.9) |
42 | 7.6 (2.9) | 5.2 (2.3) | 2.2 (1.8) | 1.4 (1.5) | 4.1 (2.1) |
49 | 10.3 (3.3) | 8.6 (2.8) | 3.4 (2.0) | 4.0 (2.2) | 6.5 (2.6) |
56 | 13.7 (3.8) | 11.1 (3.3) | 4.6 (2.3) | 2.9 (2.0) | 8.1 (2.9) |
Mean | 8.0 (2.9) | 6.3 (2.5) | 2.7 (1.9) | 2.1 (1.7) | 4.74 (11.2) |
Interactions-Critical Difference (p = 0.05), Days of observation (A) 0.6, Infestation levels (B) 0.5, Days of observations x Infestation levels NS |
*Figures in parenthesis are the square root transformed values |
Parasitisation by braconid parasitoids, A. matricariae and A. ervi in autumn crop presented in Table 5 revealed their activity to initiate in 63 DAI. As in-situ observations could not differentiate both the species, thus weekly data was recorded on parasitisation by both the species as a group and was restricted to by observing the golden- brown coloured mummies. It was observed that parasitisation by braconids varied significantly at different dates of observation and the differences with respect to levels of aphids infestation being non-significant. The mean parasitisation by A. matricariae and A. ervi at different aphid infestation levels varied from 0.7–61.4 per cent with the seasonal mean of 33.7 per cent. The mean parasitisation was minimum on 63 DAI (0.7%) which increased steadily upto 77 DAI (2.0%), thereafter an abrupt increase was evident on 84 DAI (34.2%). The peak of mean parasitization was recorded on 105 DAI (61.4%), being on a par to observation on 98 DAI (61.1%) and 112 DAI (58.0%). On the day of peak activity of Aphidius parasitoids, the parasitization to the extent of 56.9, 56.2, 62.1 and 69.1 per cent, respectively, was recorded at aphid infestation level of 1, 2, 4 and 8, however, the variation amongst infestation levels being non-significant.
Table 5
Parasitization of Myzus persicae by Aphidius spp. on bell pepper in autumn crop
Sampling period Days after infestation | Per cent parasitisation at indicated aphid infestation level (AIL) | Mean |
1 | 2 | 4 | 8 |
63 | 0.6 (4.4) | 0.6 (4.4) | 0.5 (4.0) | 0.9 (4.0) | 0.7 (4.1) |
70 | 0.4 (3.6) | 0.9 (5.4) | 1.1 (6.0) | 0.9 (5.3) | 0.8 (6.3) |
77 | 2.3 (8.6) | 0.7 (4.8) | 2.2 (8.5) | 2.8 (9.6) | 2.0 (8.3) |
84 | 3.2 (10.2) | 35.1 (35.7) | 42.6 (39.7) | 56.0 (48.5) | 34.2 (41.7) |
91 | 47.0 (43.1 | 50.0 (45.0) | 48.9 (44.4) | 58.3 (50.5) | 51.1 (46.2) |
98 | 56.9 (49.0) | 56.2 (48.6) | 62.1 (52.0) | 69.1 (56.4) | 61.1 (51.5) |
105 | 55.6 (48.2 | 62.1 (52.0) | 61.1 (51.4) | 66.9 (55.0) | 61.4 (51.7) |
112 | 58.4 (50.1) | 56.1 (48.5) | 57.0 (49.0) | 60.6 (51.4) | 58.0 (49.8) |
Mean | 28.1 (30.3) | 32.7 (31.6) | 34.4 (33.0) | 39.4 (34.9) | 33.7 (32.5) |
Interactions-Critical Difference (p = 0.05), Days of observation (A) 5.4, Infestation levels (B) NS, Days of observations x Infestation levels NS |
*Figures in parenthesis are arc sine transformed values |
In this study, the three hymenopteran parasitoids were recorded from M. persicae, including one Aphelinidae, A. asychis and two Braconidae, A. matricariae and A. ervi in India under polyhouse conditions. Earlier, Rameseshiah and Dharmadhikari (1969) reported finding these parasitoid species on M. persicae under field conditions, and later on additional information on the taxonomic aspects of the parasitoids was published by Hayat (1980). All three parasitoids have been used as important bio-control agents and are commercially produced and released for the management of aphids including M. persicae in different parts of the world (van Lentaren et al. 1997; Wei et al. 2005; Kos et al. 2008; Schirmer et al. 2008; Sanchez et al. 2010; Acheampong et al. 2012; Gavkare et al. 2013). Of the three, A. matricarae has also been reported to parasitize different species of aphids in India, under field conditions (Akhtar et al. 2011).
A. asychis and Aphidius spp. prevalent in summer and autumn crop, respectively, were also recorded earlier by Gavkare (2012) under protected environment in India. He recorded the per cent parasitization by A. asychis to vary from 2.35–38.56 per cent, whereas, in case of A. matricariae and A. ervi varied from 2.87–58.23 per cent. Parasitization by Aphidius spp. recorded in present studies is in line with the findings of Gavkare et al. (2013). However, the parasitization by A. asychis, being low in present studies can be attributed to the difference in observational period. Gavkare et al. (2013) recorded the prevalence during December to February, whereas in present studies the parasitization was recorded during June-July.
Parasitization by A. asychis being significantly more at lower infestation levels and that by Aphidius spp. being irrespective of the population abundance (Fig. 2) can be attributed to the difference in functional response of both the species as recorded by Byeon et al. (2011). They also observed Aphidius spp. to parasitize less number of M. perisicae at higher aphid density as compared to A. asychis. The present record of these three promising parasitoids of M. persicae under polyhouse conditions is an important piece of information in developing a bio-intensive strategy for the management of this aphid. However, further studies are required to develop methods for mass rearing these parasitoids, as well as suitable release techniques for use in Indian polyhouses for the effective management of M. persicae in green peppers.
On the basis of correlation worked out with infestation levels of M. persicae and parasitization during summer and autumn season, it was observed that A. asychis had a significant negative functional response (r= -0.9091) to M. persicae population, whereas braconids had significant positive functional response (r = 0.9803).
Effect of biotic and abiotic factors on parasitization:
The correlation between parasitization by hymenopteran parasitoids and abiotic factors presented in Table 6 revealed that except maximum temperature all other environmental factors affected the parasitization positively but non-significantly in summer crop. Also, same trend was observed in autumn crop, but it being significant with minimum relative humidity (r = 0.6990; p = 0.01). The relationship between aphid population and per cent parasitization revealed a negative relationship being significant in autumn crop (r= -0.9494; p = 0.05) depicting the parasitoids to affect the population build-up adversely (Table 6).
Table 6
Correlation (r) between parasitisation by hymenopteran parasitoids of Myzus persicae and abiotic factors on bell pepper in summer and autumn crop
Parameter | Summer | Autumn |
Maximum temperature (oC) | -0.6134 | -0.0948 |
Minimum temperature (oC) | 0.0514 | 0.5110 |
Maximum relative humidity (%) | 0.5077 | 0.5077 |
Minimum relative humidity (%) | 0.6990 | 0.6990** |
Aphid population | -0.7801 | -0.9494* |
*: Significant at p = 0.05%; **: Significant at p = 0.01% |
Correlation between aphid population in different infestation levels of 1, 2, 4 and 8 aphids and parasitization presented in Table 7 depicted negative relationship in both the cropping seasons being significant at 2 and 4 aphids infestation level at (p = 0.01) in summer crop with the corresponding values of r = -0.8214 and − 0.8198, respectively. Whereas in autumn crop, the correlation between aphid population and parasitisation was found to be highly significant (p = 0.05) in all the infestation levels depicting the parasitoids to exert influence in suppressing the aphid population considerably (Table 7).
Table 7
Correlation between aphid population and parasitization on bell pepper in summer and autumn crop
Population level at indicated infestation levels (aphids/plant) | Summer | Autumn |
1 | -0.6932 | -0.8531* |
2 | -0.8214** | -0.9283* |
4 | -0.8198** | -0.9116* |
8 | -0.3028 | -0.9440* |
Mean population | -0.7275 | -0.9495* |
*: Significant at p = 0.05%; **: Significant at p = 0.01% |