Efficacy of Ricinus communis L., Cassia occidentalis L. and Bacillus thuringiensis against Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)

Helicoverpa armigera is one of the most destructive pests of field crops worldwide. The Study was designed to investigate through laboratory screening the insecticidal activity of Ricinus communis, Cassia occidentalis and Bacillus thuringiensis var. Kurstaki (Btk) and their combinations against 2 larval instar of H. armigera. Five concentrations (4%, 6%, 8%, 10% and 12%) of each plant extract and (0.62, 1.25, 2.5, 5, and 10 mg/ml) of Bt were used in this experiment. Mortality (%) was recorded after 24,48,72 and 96 hrs post treatment. The results showed that R. communis, C. occidentalis and Btk have insecticidal activity against 2 instar larvae of H. armigera. The highest concentration (12%) caused 80% and 70% larval mortality after 96 hrs for R. communis and C. occidental respectively. The statistical analysis revealed that there is a significant differences between all treatments and control. Additionally, all combinations (plant/plant or plant/Bt) increased the mortality %. The results also clearly demonstrate that the R. communis are significantly more toxic than C. occidental where the LC50 values were 6.4 % for R. communis and 8.1% for C. occidental. On the other hand LC50 value for Bt was 0.41 mg/ml. The binary mixture of R. communis and C. occidental have a potentiation effect after 48 hrs. Regarding the mixture of Bt and plant extracts the results revealed that R. communis and Bt mixture induced a potentiation effect whereas C. occidental and Bt mixture induced a an additive effects. DOI: 10.14302/issn.2474-7785.jarh-19-3015 JOURNAL OF AGRONOMY RESEARCH


Introduction
Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) is one of the most destructive pests of field crops worldwide. It is a highly polyphagous multivoltine, and economically important pest of cotton and other crops and has developed resistance against most of the modern classes of synthetic insecticides [1]. In semiarid tropics the annual losses caused by this pest estimated by US$ 2 billion, even though US$ 500 million worth pesticides are applied to control this pest [2].
African bollworm has been reported on 35 crops and 25 wild host plants in eastern and southern Africa.
In Sudan, it attack a wide range of host plants such as cotton, sunflower, french beans, dry beans, okra, peas, legumes, maize, sorghum, tobacco and tomato [3]. Also it was reported as the main insect pest on chickpea.
Economics losses ascribed to direct yield reduction and cost of chemical applications to control this pest are considerable [4].
The problem of this pest is magnified due to its direct attack on fruiting structures, voracious feeding habits, high mobility, fecundity and multivoltine overlapping generations. Besides, it has developed resistance to broad spectrum of insecticides due to exposure of successive generations while moving from one crop to another, which made this pest highly resistant to many insecticides such as cyclodiene, pyrethroids, organophosphates, carbamates etc [5].
Some insecticides such as carbaryl, karate, cypermethrin, dimethoate and monocrotophos have been used to control this pest [6], cyprofen 220 ul [3], and kung fu 5% ec and karate zeon 10% were also used in sudan. [7]. however because of the health and environmental risks of synthetic insecticides, recently the pesticidal effects of botanical extracts have been studied by many researchers worldwide [8][9][10].
Castor bean (Ricinus communis) has traditionally been used in agriculture. In fact it is a unique species of the genus ricinus in the family euphorbiaceae as its seeds contain 2.8-3% toxic substances such as ricin, a potent inhibitor of protein synthesis, and agglutinin-1 [11].
Cassia occidentalis are candidate plants and strongly suggests that they possess chemical compounds possibly oils with insecticidal properties [12].
Also the combined effects between botanicals and Bacillus thuringiensis (Bt) have been explored by many researchers [16][17][18]. The purpose of this study were to:

Bioassay Tests
Second larval instar was used in this study.
Fruits dipping method [21] was followed, small pieces of fresh okra fruits were dipped for 30 seconds in different concentrations and left to dry under room conditions for 10 minutes. Ten pre starved larvae (one hour) were used for each treatment and each treatment was replicated three times. Three replicates were also used as a control set. All treated larvae were kept in Petridishes 9 cm in diameter at temperature of 25±1 °c.

Statistical Analysis
The obtained data were statistically analyzed according to analysis of variance (ANOVA); Duncan's Multiple Range Test was used for means separation using genstat version 12.1 Also the data were subjected to Probit analysis using SPSS 16.0 software.

Effect of Pant Extracts
The results presented in (

Activity of Bacillus thuringiensis ( Bt)
The results shown in (Figure 1) proved that all Bt concentrations generated a significantly (p < .001) higher mortality percentage than the control throughout the experimental period. It should be noted that the percent mortality increases with the increase of both concentration and exposure period. LC 50 value of Bt was 0.41 mg/ml as shown in (Table 2).

Combinations (Joint Action) Activity
Paired mixtures of plant extracts and Bt were tested against 2 nd larval instar of H. armigera as described in material and method section. The results shown in (   In fact its highest concentration (12%) gave 80% mortality of tested larvae after 96 hrs of application.
Kodjo et al. [24] found that 5% oil emulsion of R. communis caused 89.58% mortality of the diamondback moth Plutella xylostella in ingestion toxicity test.
Parajapati et al. [25] also recorded that the seed extracts of R. communis showed better insecticidal activity than the leaf extracts against S. Frugiper due to the active compounds such as castor oil and ricinine. Annona squamosa L., Datura stramonium L., Eucalyptus globules Labile, Ipomea carnea Jacq., Lantana camara L., Nicotiana tabacum L., and Pongamia pinnata L. Showed a synergistic effect when mixed with Btk.

Conclusion
The obtained results clearly proved that the R. communis, C. occidentalis and Bt have insecticidal activity against 2 nd instar larvae of H. armigera.
Its observed form the study finding that the efficacy of R. communis and C. occidental can be enhanced by mixing them together or can be used in a combination of Bt which may reduce the amount required for application to control this pest as well as to reduce the environmental hazards.