CHAPTER FIVE DISCUSSION
5.2 Laboratory studies
Mean duration for starvation to death of H. variegata was 17.4 days (range between 7 and 25 days). Both mean and range suggest that this beetle has a good chance for surviving adverse conditions which adds to it’s role as a good biocontrol agent.
Cannibalizationtestsindicatedthatpercent of eggs cannibalized by 4th. Istar larva and adult female of H. variegata Goeze was 93.6%
and 94% respectively. These results show that the species can successfully survive adverse food conditions by shifting to it’s own eggs, thus avoiding extinction. This phenomenon (cannibalism) has been reported in earlier studies on coccinellid predators. It has been argued that it adds to the efficiency of predators in their habitat.
Bashir (1968) showed that cannibalism occurred under scarcity of food or overcrowding. Takahashi (1987) studied cannibalism among the larvae of Coccinella septempunctata L. and concluded that when no aphids were provided, most of the eggs were eaten by larvae of all instars. Above percentages of cannibalism by larvae and females may not be considered as a better indication of voracity of these two stages.
Female H. variegata was given the choice between A.
craccivora Koch., A. gossypii Glover., S. graminum Rond. and M.
sacchari Zhnt. Results obtained did not show any clear preference for any of the 4 species of aphids. However, the beetle killed slightly higher number of A. gossypii. These results agree with Belikova et al., (1985) who concluded that Adonia variegata (= H. variegata) prefered cotton aphid over A. craccivora in a cotton- lucerne rotation in Ashkhabad and Maryisk regions of the Turkmen SSR, USSR.
Hippodamia variegata in the present study killed higher number of A.
craccivora in no-choice test (Table 25) indicating that the species was
offered. That H. variegata Goeze fed more on A. gossypii Glover than the other three aphid species in choice experiments seems to disagree with expectation that aphids feeding on graminaceous plants may be more acceptable to beetles than those aphids feeding on cotton plants (cotton contain a poisonous chemical substance known as gossypol).
In experiments involving A. nerii (Boy.), A. gossypii and M.
sacchari Zhnt. as food for H. variegata, the beetle was clearly found to less feed on A. nerii than the other two aphid species in both choice and no-choice tests. These results agree with Iperti (1960) who mentioned that A. nerii (Boy.) feeding on Nerium oleander L. was poisonous to most coccinellids except Adonia variegata (=H.
variegata). Feeding of the beetle on A. nerii may be random in this case as the coccinellid is bound to come into contact with this unpalatable aphid in the mixture. The beetle would have only learned and differentiated between aphid species after eating the first few aphids from each species of prey. The rejection of A. nerii can also be possible through it's high colouration and/or bad odour. This passive response has also been found by El-Fahal (1986) when studied the preference of Scymnus levailanti L.
The beetle (H. variegata) killed more M. sacchari than A.
craccivora Koch. in both choice and no-choice tests. These results may be comparable with similar results obtained by Ibrahim (1988) who also concluded that C. vicina Muls. ate more M. sacchari than
A. craccivora Koch. in choice tests. However, the predatory species used in the two studies were different and the reasons for differential choice may be due to other factors e.g. crop variety.
In another preference test, H. variegata Goeze ate more S.
graminum Rond. than Aphis craccivora in both choice and no-choice experiments. This may, again, be due to crop variety.
In a feeding efficiency involving M. sacchari Zhnt. and A.
gossypii Glover., food units of aphids consumed by adult H. variegata were compared. There were no significant differences in the units eaten of the two aphid species, though the beetle ate slightly more units of M. sacchari than A. gossypii. This finding seems to be acceptable as predators may be expected to prefer feeding on aphids on plants belonging to family Graminae than those aphids on cotton.
The effectiveness of a predatory beetle in aphid control can be enhanced by it's ability to attack all stages, and also by partial consumption of captured preys which in turn increases mortality of it's prey. This would mean a higher numerical response although in reality the total number of aphid consumed is low if judged by the food unit up-take. Generally, the number of aphids consumed varies with the aphid species, it's stage and density and hunger level of the predator.
Rates of aphids consumed by both 4th instar larvae and female H. variegata were compared. There was no significant difference in the number of aphids killed by the two beetle stages. However, 4th.
instar larva killed slightly more aphids than did adult female. These results agree with the views that coccinellid larvae are more voracious than their adults. However, the results disagree with Ofuya (1986) who found that 4th. instar larva and adult female of C. vicina Muls.
consumed 53.8 ±5.4 and 63.7 ± 4.1 whole A. craccivora Koch.
respectively. It is probable that these variations are due to difference in the species used in the two studies or may be to some other factors.
When female H. variegata Goeze was fed on A. craccivora, A.
gossypii Glover, S. graminum Rond. and M. sacchari Zhnt., no single aphid species was found to significantly improve fecundity of the beetle, though slightly more eggs were produced when the beetles fed on M. sacchari followed by S. graminum A. craccivora and A.
gossypii the least.
This finding was in agreement with Ibrahim (1988) who showed that M. sacchari was the most suitable food for egg production followed by A. craccivora, A. gossypii and A. nerii (Boy.) the least.
Egg laying for the beetle (H. variegata) continued through life and fecundity and longevity were higher in the first generation than in the second one. These findings are in agreement with previous works.
Fore example. Sharaf El-Din (1963) reported that fecundity was affected by pray density in H. variegata up to 30 aphids after which any increase in aphids had no effect on number of eggs laid. This is
because, at high prey density, the predator may spend more time in host selection (Hagen, 1965).
No single aphid species (of the four species mentioned earlier) had clear influence on duration of development of the beetle. But mortality was different with the different aphid species, being 20%
when the larvae were fed on A. gossypii Glover and 10% when fed on both A. craccivora Koch. and M. sacchari Zhnt., while it was 0.0%
when the beetle fed on S. graminum Rond.. These results agree with Ibrahim (1988) who showed that larvae fed on Rhopalosiphun maidis (Fitch.) and M. sacchari developed rapidly well without showing any mortality and completed their life cycles in an optimal time. Those reared on A. craccivora developed with a slight increase in the mortality rate (5.88%), whereas those reared on A. gossypii had a longer period of development with mortality reaching 20%, but those larvae reared on A. nerii (Boy.) showed the longest developmental period and the highest mortality rate (75%). It follows from these results that not all accepted food must necessarily be essential and equally enabling optimum development of larvae and ripening of ovaries. Hodek (1966) added that alternative food is often accepted which does not permit development but serves only as a source of energy. He further stated that the causes of the unsuitability may be due to substances contained in aphids (usually derived from plant poisons) or that the aphids are deficient in the essential nutritive
For the assessment of the biotic potentialities of H. variegate Goeze, the life table data of the two generations reared under laboratory conditions were compiled as in Appendix Table 8 and 9.
The computations of these data were made as summarized in Table (35). Survival rates of the two generations indicated that egg hatchability was generally high for the species and that it was higher in the first generation than in the second one. The variation between the population growth of the two generations was insignificant although there was a slight increase in the innate capacity for increase (intrinsic rate of increase)(rm) in the second generation (rm being 0.136 and 0.137 for the first and the second generation respectively) (Table 35). These rates correspond with 14.6 and 14.5 days as period from egg to adult emergence for the first and the second generation respectively. These results agree with Ibrahim (1988) who explained that the short developmental period of the species is compensated by high (rm) value, i.e. if the period of development is longer, the (rm) of that species would be low, but if the growth period is shorter, (rm) would be greater. In this study, when the developmental period was 14.6 days, (rm) was 0.136, but when the period was 14.5 days (rm) was 0.147. Generally, (rm) is the statistic meant to indicate the biotic potentiality of the species. For example, Leslie and Park (1949) in their work on the flour beetle, Tribolium castaneum (Herbst), commented that the (rm) value summarizes in one figure such population attributes as fertility and fecundity. Messenger (1964) in
his studies of three parasites of the aphid, Therioaphis maculata (Buckton) used the intrinsic rate of natural increase (innate capacity for increase) as a bioclimatic index to examine the population growth of this aphid and it’s parasites under different temperatures. Bashir (1973)(cited by Adam, 1979) in his work on the predatory coccinellid, Olla abdominalis (Say) used the (rm) values as a dietetic index to evaluate the suitability of different types of diets for rearing this species. In this study, the calculated (rm) values would reflect the productivity of this predator (H. variegata Goeze). The (rm) values obtained under laboratory conditions cannot be expected to be the same to those under natural conditions. The laboratory conditions exclude many natural influences such as food variety, quantity, quality and seasonal physical environmental conditions and seasonal fluctuations, competitors and natural enemies.
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