Assessment of population development
Ovipositional preference
From the Table 1 it is evident that oviposition preference of
C. maculatus on different host grains was varied significantly. Mean number of eggs laid on different host grains ranged from 116.33 to 66.33 eggs/50 seeds of host grains. Significantly higher egg counts per 50 seeds were recorded on cowpea (116.33). Significantly lowest numbers of eggs per 50 seeds were recorded in red gram (66.33) which was on par with soybean (69.00). Though, chickpea grains were recorded 93.67 eggs per 50 seeds which was on par with black gram (87.67).
These above results reveal that the obvious effects of different host grains on the ovipositional preference of
C. maculaus. Studies by investigators demonstrated that the growth and development of
C. maculaus is greatly affected by nutritional value of the host grains (
Creadland, 1987;
Van Hius and Derooy, 1998), temperature and humidity (
Chandrakantha and Mathavans, 1986), host grain size
(El-Hafway et al., 1972, Sandhu et al., 1987) and competition
(Meyer et al., 1986).
The present findings are in conformity with
Girish et al., (1974) who concluded that ovi-positional preference of the bruchid might be guided by smoothness of seed coat and size of the grain. Similarly,
Bansod et al., (2006) reported that seed surface had a definite relation with the rate of oviposition by females of
C. maculatus. The females of
C. maculatus avoided rough seed coat for oviposition. However, there were no significant differences recorded among seed coat thickness, seed coat colour, seed length, seed density for oviposition by
C. maculatus. Thus, the present findings are of suggestive that nutritional content of the host grain plays a significant role in host grain preference.
Per cent survival
The per cent survival was highest on cowpea (91.12%) followed by green gram (90.03%) and chickpea (87.38%) which were on par with each other. This was followed by chickpea (87.38%), black gram (77.70%) and red gram (72.83%). The least per cent survival was recorded in soybean (59.74%) which was significantly different among the host grains (Table 1).
These results were in conformity with the findings of
Bhaduria and Jakhmola (2006) who reported that the number of adults emerged were directly proportional to average number of eggs. The average number of eggs laid on green gram, red gram and black gram after fifteen days of release was 109.3, 52.6 and 50.6 per fifty grains, respectively and the number of adults emerged subsequently were 73.6, 31.3 and 19.6, respectively.
Mean developmental period
One egg/seed
The least mean developmental period of 27.00 days was recorded in cowpea having one egg per grain followed by green gram (28.17), chickpea (29.96) which were on par with each other. Maximum mean developmental period of 35.84 days was recorded on soybean followed by red gram (33.72) and black gram (32.33) which was on par with each other and significantly different from the other earlier mentioned grains (Table 2).
Two eggs/seed
The least mean developmental period of 28.34 days was recorded in cowpea having two eggs per grain followed by green gram (30.07), chickpea (32.15) which were on par with each other. Maximum mean developmental period of 37.07 days was recorded on soybean followed by red gram (36.08) and black gram (33.66) which Was on par with each other and significantly different from the other earlier mentioned grains (Table 2).
Three eggs/seed
The least mean developmental period of 29.45 days was recorded in cowpea having three eggs per grain followed by green gram (31.63), chickpea (34.03) which were on par with each other. Maximum mean developmental period of 39.39 days was recorded on soybean followed by red gram (37.14) and black gram (34.60) which was on par with each other and significantly different from the other earlier mentioned grains (Table 2).
The results obtained under this experiment can be supported by the findings of
Ghosal and Senapati (2007) and
Sharma et al., (2016), who opined that the developmental period of
C. maculatus was shortest in cowpea (26 days) and the longest in pea (31 days). The developmental period in pea was very high may be because of physico-chemical properties of the host-grains. The mean developmental period of
C. maculatus in different pulses was ranged from 29.33 to 36.00 days. Cowpea, green gram, bengal gram and horse gram recorded significantly the lowest developmental period of 29.33, 29.67, 31.33 and 31.67days, respectively and they were on par with each other. Whereas, red gram (36 days) recorded maximum of developmental period followed by black gram (32.67) and field bean (32.33 days)
(Shivanna et al., 2011). Similarly
Sachdeva et al., (1986) found variation in the development period of
C.
maculatus on different varieties of cowpea. It was maximum (35.50 days) in CG-7 and minimum (29.00 days) in V-87 variety of cowpea. Legume seeds which had the highest mean egg counts and high per cent of adult emergence correspondingly had the shortest development period.
Index of susceptibility
The index of susceptibility of the host-grain was higher in cowpea (8.68) which was on par with green gram (8.15) followed by chickpea (7.32) and black gram (6.76) which were on par with each other and followed by red gram (5.94). The least index of susceptibility was recorded in soybean (5.47). The index of susceptibility had positive correlation with the mean number of adults emerged and resulted in heavier infestation (
Adam and Baidoo, 2008) (Table 2).
Assessment of damage
Per cent grain damage
The damage of
C. maculatus in different host-grains in terms of per cent grain damage has increased significantly with increase in storage period. The grain damage caused by
C. maculatus in terms of per cent grain damage not only significantly different among the host-grain selected but also significantly increased from 30 days after insect release (DAIR) to 120 days after insect release.
In the present investigation it was found that per cent grain damage after 30 DAIR was highest in cowpea (17.70) followed in decreasing order by green gram (16.02) and chickpea (14.04) black gram (12.39), red gram (10.41) and least in soybean (9.12). The per cent grain damage was further increased after 60 DAIR in the same order of preference as of cowpea (38.46%), green gram (35.95%), chickpea (35.62%), black gram (30.19%), red gram (21.55%) and soybean (14.27%). After 90 DAIR per cent grain damage was further increased due to increased exposure and pest population number that recorded 72.72 % in cowpea followed by green gram (70.75%), chickpea (66.99%), black gram (54.60%), red gram (29.33%) and soybean (18.26%). The per cent of grains damaged was even increased after 120 DAIR in cowpea (92.25%), green gram (90.19%), chickpea (86.70%), black gram (81.71%), red gram (65.92%) and soybean (27.47%), but with preferential insect infestation (Table 3).
The above findings are in support of earlier studies conducted by
Rawat and Srivastava (2011) indicating that the per cent seed damage by
C. chinensis was maximum in cowpea (96.15%) followed by green gram (88.44%) and minimum seed damage was observed in mothbean (74.22%). Simailarly,
Caswell (1981) stated that the bruchid,
C. maculatus (F.) can cause 100% damage during storage in five months while up to 50% damage was recorded during storage in 3 to 4 months.
Mphuru (1981) stated that the inability of
C. maculatus to develop on soybean can be attributed mainly to the high protein-carbohydrate ratio of the seed and in part to its saponin content
(Applebaum et al., 1969). Also, this bruchid is known for not being capable of attacking seeds with a high fat content like soybean.
Per cent weight loss of grains
The per cent weight loss of different host-grains was increased significantly with increase in storage period. The per cent weight loss caused by
C. maculatus has not only significantly different among the host-grain selected but also significantly increased from 30 days after insect release (DAIR) to 120 days after insect release.
In the present investigation it was found that per cent weight loss after 30 DAIR was highest in cowpea (11.09), green gram (10.21) and chickpea (8.97) and least in soybean (3.90), red gram (5.63) and black gram (7.54). The per cent weight loss was further increased after 60 DAIR in cowpea (18.02), green gram (16.79), chickpea (14.04), black gram (13.82), red gram (13.13) and soybean (10.36). After 90 DAIR per cent weight loss in cowpea was recorded to be 50.50% followed by green gram (47.40%), chickpea (40.55%), black gram (27.52%), red gram (24.19%) and soybean (16.56%). The per cent weight loss has increased with increase in storage period and after 120 DAIR the weight loss in cowpea was observed to be 76.27% followed by green gram (75.92%), chickpea (69.48%), black gram (48.59%), red gram (42.04%) and soybean (20.02%) (Table 4).
The above results are in support of earlier studies conducted by
Doharey et al., (1987) indicating that gradual increase in weight loss in green gram due to
C. maculatus with progressive increase in storage period causing 1.15, 36.36, 48.84 and 51.91 per cent weight loss after 30, 60, 90 and 120 days after storage period, respectively. Similarly,
Malaikozhundan and Raj (2012) reported that the cowpea CoCp-7 recorded significantly the highest oviposition percentage, adult emergence, the shortest developmental period, the highest susceptibility indices and the highest weight loss.
