African Journal of Biotechnology
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African Journal of Biotechnology Vol. 2 (11), pp. 425-428, November 2003 ISSN 1684-5315 © 2003 Academic Journals
Comparative efficacy of microbial and chemical insecticides on four major lepidopterous pests of cotton and their (insect) natural enemies
T. A. Fadare* and N. A. Amusa
Institute of Agricultural Research and Training, Obafemi Awolowo University, P.M.B. 5029, Moor Plantation, Ibadan, Nigeria.
*Corresponding author. E-mail: naamusa@softhome.net.
Accepted 23 October 2003
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Three microbial (biotrol, dipel and thuricide) and three chemical insecticides (monocrotophos, endosulfan and carbaryl) were compared for efficacy on four major lepidopterans and their natural enemies in replicated field trials at Moor Plantation, Ibadan. Thuricide was evaluated at different combinations with monocrotophos in a second trial. The results showed that the microbials caused the mortalities of destructive bollworms and leafroller but allowed the survival of their natural enemies. The chemicals on the other hand caused mortalities of both destructive and useful species. Both groups of insecticides enhanced seed cotton yields. Application of thuricide followed by monocrotophos was better than other combinations evaluated.
Key words: microbial insecticides, Bacillus thuringiensis, cotton.
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The cotton bollworm (Helicoverpa armigera Hubn), the spiny bollworms (Earias insulana Boisd and E. biplaga Wlk.), and the leafroller (Sylepta derogata F.) are major lepidopterous pests of cotton in Southwestern Nigeria. These insect pests are currently being controlled by the application of broad spectrum insecticides such as monocrotophos, endosulfan or carbaryl four times at weekly intervals during the growing season.
However, these broad spectrum materials are highly toxic to insect natural enemies (Hamilton and Attia, 1976). On the other hand, Bacillus thuringiensis Berl, is active against many lepidopterous species and has no adverse effects on natural enemies of target pests (Fadare and Osisanya, 1998). The lepidopterous pests natural enemies include parasites (syrphids, tachnids, braconids) and predators (coccinelids, forficulids, pentatomids and reduviids).
A control programme based on selective materials, which would allow survival of beneficial species and cause the mortality of destructive ones is desirable. The efficacy of B. thuringiensis could be increased by the addition of sublethal doses of pesticides and could be used in such a programme. Here we report the comparative efficacy of three microbial insecticides, dipel, biotrol, thuricide and three chemical insecticides, monocrotophos, endosuslfan and carbaryl on cotton bollworm, spiny bollworms, the leafroller and their natural enemies.
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The treatments comprised three microbials, dipel at 0.52kg/ha, biotrol, 0.56, thuricide, 0.50 and three chemical insecticides, monocrotophos at 0.68kg. a.i./ha, endosulfan, 0.75, carbaryl, 1.50, each in 225 litres of water/ha, and different combinations of one microbial (thuricide) and one chemical insecticide (monocrotophos). The treatments were arranged in a randomized complete block design experiment with four replicates. The cotton (Samaru ’77) plots of 10 m X 5 m each were established as per standard agronomic practices for cotton production in South-Western Nigeria. The experiment was conducted over two years.
The treatments were applied with a 9-liter pressurized Falcon sprayer to the plants when one plant per plot was infested by any of the target pests. Post spray counts of Sylepta were taken from 5 plants per plot while Helicoverpa and Earias damaged bolls were counted and removed from 10 randomly selected plants of each plot. Pre and post spray samplings of populations of natural enemies (parasites and predators) were carried out with an aerial net. Parasitised larvae and pupae were taken to the laboratory for emergence of parasites. The inner two rows of each 10 m X 5 m plot were used for the estimates of seed cotton yield. Data collected were subjected to statistical analysis. Efficacy of treatment was based on plot means of live leafroller, percentage bollworm damages, seed cotton yield and live leafroller enemies recovered from the sprayed and unsprayed plots.
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Experiment I
The post spray application mean Sylepta (leafroller) counts ranged from 2.65 to 3.17 per plant for the microbial insecticides and from 1.57 to 2.15 per plant for the chemicals. Both were however better than the 6.57 live leafroller per plant from the unsprayed control plots (P = 0.05) (Table 1). Percentage bollworm damages ranged from 12.22 to 13.18 per plant for plots sprayed with the microbials and were significantly higher (P = 0.05) than the range of 5 – 7 per plant for plots sprayed with the chemical insecticides. The percentage bollworm damage from the unsprayedcontrol treatment was 20.00 and was significantly higher from those of microbial and chemical insecticide treated plots (P = 0.05). Corresponding percentage bollworm control ranged from 35 – 36 for the microbials and 65 – 75 for the chemical insecticides (Table 1).
Table 1. Comparative effectiveness of microbial and chemical insecticides on cotton lepidopterans in the field.
No significant different between means with same letters at
5% level. *Each dispersed in 225 litres of water. **Computed as C – T/C x 100, where T is % damage in sprayed
treatments and C is % damage in unsprayed plot (control).
Mean seed cotton yields ranged from 980 to 1080kg/ha for the microbials and 900 – 1108 for the chemicals, and were not significantly different. However, the 388kg/ha seed cotton yield from the control plots was significantly lower than those from the sprayed treatment (P = 0.05). Corresponding percentage yield increases of sprayed plots over the control plots ranged from 153 – 178 for the microbials and 132 – 186 for the chemical insecticides (Table 1). The mean numbers of parasites and predators recovered from plots sprayed with microbial – and chemical insecticides were low and similar for both and not significantly different from those of the unsprayed control plots (Table 2). The numbers of braconids recovered from each plot were higher than the numbers recovered for other parasites (Table 2). Also, numbers of parasites and predators generally increased after spraying with the microbials, but stayed the same or reduced with chemical insecticides.
Table 2. Mean number of parasites and predators recorded in plots treated with microbial and chemical insecticides.
Experiment II
The results of the different combinations of microbial and chemical insecticides (thuricide/monocrotophos) are presented in Table 3. All sprayed treatments were better than the unsprayed control treatment. Corresponding percentage yield increases of 132.35 over the control was highest for T3 treated plots and lowest for T5, 91.41%.
Table 3. Effect of different combinations of thuricide and monocrotophos on cotton lepidopterans.
* No significant different between means with same letters
(5%). **Computed as C – T/C x 100, where T = Percent damage in
sprayed treatments and C = Percent damage in unsprayed treatment. Legend: T1
Thuricide alone
= 4 Applications T2
Thuricide and Monocrotophos
= 4 Applications
(Simultaneous) T3
Thuricide followed by Monocrotophos
= 2 Applications
each T4 Monocrotophos followed by Thuricide = 2 Applications each &nb | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||