African bollwormHelicoverpaarmigera is a worldwide-spread species that shows a high degree of polyphagia and it is considered as one of the main agricultural pests in the world. Its presence has been reported in diverse continents, such as Africa, Europe, Asia and Oceania 1. in addition to feeding on high value crops it is an extremely dangerous pest because its reproduction rate is extremely high and it can migrate over a long distance 2. It has been reported on 67 host families, including Asteraceae, Fabaceae, Malvaceae, Poaceae and Solanaceae and this pest has caused losses to economically important crops such as cotton, legumes, sorghum, maize, tomato, soybean, ornamental plants, and fruit trees 3. It develops 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 pesticides such as cyclodiene, pyrethroids, organophosphates, carbamates etc 4.

Because of the hazards of synthetic insecticides, recently the pesticidal effects of botanical extracts have been investigated by several researchers worldwide 57.

Nut-grass C. rotundus is growing commonly and considered as noxious weed but at the same time it contains tremendous important medicinal as well as pharmacological properties. The most effective parts of this perennial herb are rhizomes and tubers. C. rotundus contains many secondary metabolites such as sesquiterpenes, quinones, flavonoids, saponins, alkaloids, phenolic acids, coumarins and steroids (steroidal glycoside, sitosteryl-(6'-hentriacontanoyl)-β-dgalactopyranoside 8.

Jimsonweed Datura stramonium is a cosmopolitan weed of cultivated fields. It is characterized by its narcotic, hallucinogenic, and medicinal properties, as well as its effects in human poisonings 9. The phytochemical screening revealed that it contains phenols, flavonoids, tannins, saponins, alkaloids, steroids and glycosides. All parts of the plant are toxic, but the ripe seeds contained the highest amount of alkaloids 7 .

Insecticide synergists have been used not only to monitor the insecticide resistance mechanisms but also as an admixture in these insecticides for the control of many insects. They contribute significantly to the improvement of insecticides efficacy, particularly when problems of resistance need to be addressed. Piperonyl butoxide which is isolated from sesame oil has been used as a synergist with many organophosphates and pyrethroid insecticides to control various pests 10.

In the last four decades many botanical formulations have proven to be potent and effective as many as conventional synthetic pesticides even at low concentrations. In fact, botanical insecticides have drawn great attention as major control agents in organic farming. Higher plants are extremely abundant with biologically active secondary metabolites. Over 80% of all known alkaloids, phenols and other secondary metabolites were produced by higher plants11. Stoll 12 demonstrated that the use of plant extracts to control destructive insects is not new, rotenone, nicotine and pyrethrin have been used for a considerable time in small scale subsistence and also commercial agriculture. The objectives of this study were to evaluate the lethal effect of C. rotundusand D. stramonium on the H. amigeraand to investigate the synergistic effect of sesame oil and tested extracts.

The experiments were conducted in the Research Laboratory, College of Agricultural Studies (Shambat), Sudan University of Science and Technology (SUST), during February-March, 2021, where the temperature was between 25-32°C.

Larval instars of H. armigerawere collected from unsprayed tomato plants grown in Gamouaia Agricultural irrigated scheme Southern Khartoum and brought to the laboratory for rearing. Early instar were reared in groups of 100 larvae in plastic cages 19 cm in diameter covered with muslin cloth and fed on okra fruits, whereas 4^th_ instars were reared separately in plastic cubs 5 cm in diameter and 7 cm in height to avoid cannibalism the bottom of each cubs was filled with sand for pupations. Upon emergence the adults were transferred to plastic cages 31x20x19 cm covered with muslin cloth and fed on 10% sugar solution13, cotton stripes were hung on the margins of the cages for eggs laying and were replaced daily with new stripes while newly hatching larvae were transferred to the larval rearing cages. The rearing process continued until a sufficient number of homogenous population of larvae was collected for the experiments.

Tubers of C. rotunduswere collected from Arashkool scheme White Nile State and seeds of D. stramonium were collected from river bank, Omdurman area and brought to the laboratory where they were shade-dried. After complete dryness the plant samples were crushed separately by an electronic blender, 120g of prepared seeds powder were extracted with absolute ethanol using soxhlet apparatus, extraction continued for six hours, and the ethanol solvent was removed off the crude extract by rotary evaporator10.

Five concentrations (4%, 6%, 8%, 10% and 12%) were prepared by dilution, water plus 00.01% soap were used to make emulsion.

Obtained  data in (table 1) shows that all concentrations of the ethanolic extract of C. rotundusand D. stramonium scored a significantly higher mortality percentage than the control after 24hrs of exposure. The mortality percent increased with the increase of both concentration and exposure period. Tubers ethanolic extract of C. rotundus at 4% and 12% concentrations caused 46.7% and 90% mortality respectively after 72 hrs of application, whereas 12% concentration of seeds ethanolic extract D. stramoniuminduced  only 70% mortality after 72 hrs of exposure.

The  results exhibited in (table 1 & table 2) showed that each concentration of the tubers ethanolic extract of C. rotundus and seeds ethanolic extract of D. stramonium mixed with sesame oil gave significantly higher mortality percentage than its counterpart alone after 24hrs of exposure. Meanwhile after 48 hrs of application the lowest concentration of C. rotundus mixed with sesame oil caused 63.3% mortality which were not significantly different from that caused by the highest concentration (12%) of its counterpart of D. stramonium that cause 73.3% mortality. It should be noted that there is no significant differences among the highest concentrations (8%, 10% and 12%) of both plant extract after 96 hrs of exposure as indicated in (table 2).

The results exhibited in (table 3) clearly demonstrated that tubers ethanolic extract of C. rotundus are significantly more toxic than the seeds ethanolic extract of D. stramonium (LC₅₀ were 4.2% for C. rotundus and 7% for D. stramonium)

The obtained results revealed that all concentrations of the tubers ethanolic extract of C. rotundus generated significantly higher mortality percent than control throughout the experimental period. This clearly demonstrates that the tubers ethanolic extract of C. rotundus has a lethal effect against the H. armigera. Similar results were obtained by Imam, et al. 5 who found that the rhizomes of C. rotundus exhibited larvicidal activity against Aedes aegypti larvae and the mortality were in a dose dependant manner. Sharma and Gupta 15 noted that methanolic extract of C. rotundus tubers strongly inhibit the activity of acetylcholinesterases (ache). Another finding reveal that the acetone leaves extracts of C. rotundus at 50% exhibited significant mortality percentage of 46.6% and 51.6% repellency against Rice grains weevils Sitophilus oryzae16.

The present data also showed that the seeds ethanolic extract of D. stramonium scored a significantly higher mortality percentage than the control after 24 hrs of exposure and the mortality were dose and time dependent. Similar results were obtained by Karimzadeh and Rabiei 17. They indicated that flower, seed, and root extracts of D. stramonium were highly toxic against diamondback moth Plutellaxylostella larvae. Also, Abbasipour et al. 18 found that the D. stramonium extract had strong contact toxicity against Callosobruchus maculatus adults and the mortality increases with increase in the concentration and exposure period.

When sesame oil was added to each concentration of ethanolic extract of C. rotundus and D. stramonium it exhibited a synergistic effect.In fact, the lowest concentration (4%) of seeds ethanolic extract of D. stramonium gave only 40 % mortality after 96 hrs of exposure; however, when mixed with sesame oil it increased significantly to 63.3%. This may indicate that the detoxification mechanism in this insect involves mixed function oxidases which are known to be inhibited by sesame oil19. Similar result were obtained by Elnour 10 who indicated that the sesame oil have synergistic effect when mixed with Cassia occidentalis and Conocarpuslancifolius when tested against African melon lady bird beetle Henosepilachnaelaterii (Rossi). Another finding demonstrate that the sesame oil was a synergist with cypermethrin that played more or less the same role as Piperonyl butoxide (PB) in monooxygenase inhibition against diamondback moth Plutellaxylostella20.

This study clearly demonstrates that both tested plants have a lethal effect on the larvae of the African bollworm. However, tubers ethanolic extract of C. rotundus are significantly more toxic than the seeds ethanolic extract of D. stramonium. In addition the study revealed that sesame oil has a synergistic effect when added to the these plant extracts.

The authors are pleased to thank Mr. Elsadig Eltayep Eltom Elshukry, Agricultural Research Corporation, for his assistance in statistical analysis of experiments.