NEEM AS AN ANTIFEEDANT
In 1959, a young German scientist working in Sudan during a desert locust plague noticed that neem was the only tree to remain green and healthy when all the other plants were defoliated. Later studies showed that locusts would not feed on crops treated with aqueous suspensions of neem seeds, and azadirachtin was identified as the component of neem responsible for this effect. Azadirachtin has since been labelled the most potent antifeedant known for the desert locust, Schistocerca gregaria.
Azadirachtin also prevents other insect species from feeding, but its potency varies between species. For example, another African locust species, Locusta migratoria, is far more willing to eat azadirachtin than S. gregaria, and grasshoppers in North America show no antifeedant response at all. They are susceptible to the growth regulatory effects, however.
Azadirachtin is non-volatile, so an insect must taste it, rather than smell it, in order to respond to it. A taste of azadirachtin stimulates at least one 'deterrent neurone' in insects which show an antifeedant response. The strength of 'deterrent neurone' responses has been correlated with the strength of antifeedant responses.
Experiments used to determine the antifeedant effect of azadirachtin
| Paired choice test (n=4) |
No-choice test (n=2) |
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| The antifeedant response of insects to azadirachtin is measured using both "paired choice" tests (above, left), and "no-choice" tests (above, right). In a paired choice test, each insect is given a choice between an azadirachtin treated leaf and an untreated leaf. In a no-choice test, half the insects are given azadirachtin treated leaves, and half are given untreated leaves. If azadirachtin acts as an antifeedant, then insects will eat more of the untreated leaves than the treated leaves. The difference between the amount of untreated and treated leaves consumed is used to calculate the feeding deterrence of the treatment. Usually insects are far more sensitive to azadirachtin in choice tests than in no choice tests. |
Although the antifeedant effect of azadirachtin may be useful for short-term crop protection, insects which are exposed repeatedly to azadirachtin become less sensitive to it. For example, S. gregaria is reported to adapt completely to azadirachtin after 7 or 8 days of exposure. This is an example of habituation, which is a common response of many animals to repeated stimulus.
Substances other than azadirachtin can influence the strength of the antifeedant effect. In some insects the stimulatory effect of a preferred food source can overcome the antifeedant effect of azadirachtin. One report showed that the sensitivity of L. migratoria to azadirachtin treated food decreased with the sugar content of the food. These results are shown graphically on the clickable chart to the right.
Some evidence suggests that non-azadirachtin components of neem may slow the ability of insects to habituate to azadirachtin. For example, Asian armyworm larvae, which normally habituate to low levels of purified azadirachtin after two days of exposure, did not habituate to a neem extract containing the same absolute amount of azadirachtin after three days of daily exposure. These results are shown graphically on the clickable chart to the left.
Michael Bomford
mbomford@sfu.ca
