| dc.description.abstract | In a study designed to investigate the insecticidal potential of Melia
volkensii (Gurke), various tests for biological activity in different alcoholic extracts
were carried out on l.ocusta migratoria niigratoriodes (R&F). Attempts were also
made to improve extraction methods. Several extracts of M. volkensii were tested
for antifcedant and toxic effects on L migratoria nymphs and adults and their
mechanism of action tentatively established. The presence of active ssbstances was
routinely monitored using the Aedes aegypti 2,11 instar larvae bioassay.
Various concentrations of ethanol and methanol were tested for their
efficiency ill extraction or biologically active substances Irom III. volkensii fruit
The most efficient were found to be 60% ethanol and 80% methanol. A
comparative study of fruits from Embu and Tsavo areas of Kenya revealed no
difference in the content of insecticidal substances.
Various parts of M. volkensii were screened for biological activity. In the
A. aegypt] 2nd instar larvae ,bioassay, seed coat and root bark extracts had LC50's of
100 and 120 ppm respectively while stem wood had an LC5o of 1700 ppm in 48
hours, In locust antifeedant tests, root wood, root bark, endocarp, fruit coat and
testa extracts showed high activities while stem wood, stern bark and leaves
showed low activities In fruits, more than 90% of the active material was
recovered from the fruit coat and endocarp. Ripe and unripe fruits had equal
quantities of active material.
In storage stability tests, the' extracts and M. volkensii fruit powder were
found to be highly stable, with extracts stored for two years at room temperature
retaining insecticidal activity
A procedure developed in this investigation demostrated that biologically
'active material was recoverable by precipitation from concentrated 80% methanol
extract solutions by cooling to 0oC.
Antifeedant effects were 'induced 111 locusts through topical treatment,
injection or food treatment. A drastic reduction in food intake resulted in locusts
treated directly on the labial and maxillary palps This effect was found irreversible
with time, The rate of peristalsis of the alimentary canal in locusts was however,
not affected by the extracts
Acute toxicity tests on locusts revealed high co~tact poison activity in one
M. volkeusii fruit fraction named B This activity was enhanced by addition of
olive oil to the extract solution. All crude and purified fractions showed stomach
poison act ivity on locusts when administered as bait at concentrations above O. 1%.
This WaS not enhanced by purification of the crude extracts. Locusts injected with
doses above I00pg or ex tIact per gralll or body weight went into paralysis in a
few hours In this state, peristalsis of the alimentary canal, malpighian, tubule
activity and heart pulsations were not impaired. Astate of stupor could also be
induced in locusts in 24 hours through topical treatment with closes above
IOOOpg/g of body weight.
The onset of paralysis in locusts treated with M. volkensii fruit extracts was
enhanced by high post -t reatment temperatures. A post -treatment temperature rise
from 15 to 40 oC increased the toxicity of the extracts on locusts by a factor of ten.
Compared to azadirachtin, M. volkeusii fruit extracts revealed low growth
inhibitory effects on l.. migratoria 5th instar nymphs.
The combined insecticidal and physical properties of M. volkeusii extracts,
i.e. relative antifeedant, stomach poison, contact poison, good thermostability,
long shelf-life and positive temperature coefficient of paralytic action would make
the tree an attractive source of botanical locusticide for large-scale application
especially in the tropics. | |
