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Abstract
This is a study of an insect-plant interaction between the plant Asclepias sinaica and its herbivore Spilostethus pandurus in St Katherine Protectorate. The current study showed that:
Ecological study:
1- Spilostethus pandurus became active during summer and declined during winter.
2- The insect had a bimodal activity pattern during the summer day, with peaks in late morning and late afternoon. During winter the insect its activities to one peak in the late morning. Feeding activity was concentrated in the late afternoon when the weather was favourable.
3- The bugs preferred feeding on Asclepias seeds rather than on the sunflower or nut seeds; this may be helpful in using Asclepias as a trap plant.
4- Both adults and nymphs preferred seeds and fruits for feeding rather than leaves or stems. Both adults and nymphs were abundant during the seed ripening stages.
5- Life cycle, incubation, and pre-oviposition periods were short in summer and long in winter.
6- The insects had low flight capacity and short intervals between successive clutches, limiting their ability to move to nearby patches.
7- The duration of mating behaviour in the laboratory increased significantly from April until August with a peak in June. A high frequency of mating may be necessary for maximum egg fertility.
8- Spilostethus pandurus were found to be highly gregarious during all life stages (nymphs and adults), forming large overwintering aggregations, and also during the night at other times of the year.
Chemical study:
Investigation of the insect-plant interaction based on isolating chemical components from both plants and insects showed that:
1- Isolating components from Asclepias indicated that the major plant cardenolides from EtOAc eluant were calotropagenin, 7,8-dehydrocalotropine and calotropin.
2- The presence of detectable amounts of cardenolides was confirmed in the insect Spilostethus pandurus feeding on Asclepias sinaica
3- Quantitative fractionation of these compounds indicated the plant latex contained the highest percentages of cardenolides (calotropagenin, 7,8-dehydrocalotropin, and calotropin), followed by the plant extract, and the insect contained the lowest quantity of the cardenolides. This suggested that the plant stores cardenolides mainly in the phloem, and the insect feeds on small amounts of these toxic compounds, just enough to defend it but not enough to be harmful.
4- The insect contained a higher ratio of the polar components of the toxins than that found in the plant extract.
Pharmacological study:
To answer the question if there is any biological effect of these cardiac glycosides isolated from both the plant and its herbivore, the plant and insect extracts were applied to nicotinic and glutamate receptors. The results were as follows:
1- Plant and insect extracts were found to block nAChR in a reversible, non-competitive and voltage dependent and independent manner.
2- The plant and the insect extract blocked the glutamate receptor in a voltage-independent manner
3- The plant and the insect extract caused rise in the intracellular calcium ions concentration inside the cells, causing death.