Research has shown that neem extracts can influence nearly 200 species of insects. It is significant that some of these pests are resistant to pesticides, or are inherently difficult to control with conventional pesticides (floral thrips, diamond back moth and several leaf miners). Most neem products belong to the category of medium to broad spectrum pesticides, i.e. they are effective over a wide range of pests.
 

Biological Effects of Neem on Insects
It is important to understand that the action of neem products as pest control agents can be at different level and ways. This is very important since the farmer is used to the "knock-out" effect of chemical pesticides. Neem extracts do not exhibit this type of effect of pests but affect them in several other ways.

Insect Growth Regulation
It is a very interesting properly of neem products and unique in nature, since it works on juvenile hormone. The insect Iarva feeds and when it grows, it sheds the old skin and again starts growing. This particular shedding of old skin is the phenomenon of ecdysis or moulting and is governed by an enzyme ecdysone. When the neem component, especially azadirachtin enter into the body of larve, the activity of ecdysone is suppressed and the Iarva fails to moult, remains in the Iarva stage and ultimately dies, If the concentration of azadirachtin is not sufficient enough, the Iarva manages to enter the pupal stage but dies at this stage and if the concentration is still less, the adult emerging from the pupa is 100% malformed, absolutely sterile without any capacity for reproduction.

Feeding Deterrent
The most important properly of neem is feeding deterrence. When an insect Iarva sits on the leaf, the Iarva is hungry and it wants to feed on the leaf. This particular trigger of feeding is given through the maxillary glands. When these maxillary glands give a trigger, peristalsis in the alimentary canal is speeded up, the Iarva feels hungry and it starts feeding on the surface of the leaf. When the leaf is treated with neem product, because of the presence of azadirachtin, salanin and melandriol there is an anti-peristaltic wave in the alimentary canal and this produces something similar to a vomiting sensation in the insect. Because of this sensation the insect does not feed on the neem treated surface. Its ability to swallow is also blocked.

Oviposition Deterrent
Another way in which neem reduces pests is by not allowing the females to deposit eggs. This property is known as oviposition deterrence, and comes in very handy when the seeds in storage are coated with neem kernel powder and neem oil. The seeds or grains obtained from the market are already infested with some insects. Even these grains could be treated with neem seed kernel extract or neem oil; after this treatment the insects will not feed on them. There will be no further damage to the already damaged grains and at the same time when the female comes to the egg laying prevented.

There are also other modes of action known such as -

Use of neem products does not give immediate results like chemical insecticides. Some patience is required after application of neem products.

Effect of Neem on Different Insects
It is important to understand how neem products act on the insects.

Orthoptera (Grasshoppers, Crickets, Katydids, etc.)
Neem product act as antifeedants. Several species of these insects refuse to need on plants treated with neem for several days to several weeks. Recently it has also been discovered that neem products convert the gregarious swarms of locust to solitary forms.

Homoptera (Cicadas, Aphids, Scale insects, Leafhoppers, etc.)
These insects exhibit sensitivity to neem products to varying degrees. In leafhoppers and planthoppers neem product show considerable antifeedant and growth regulating effects. Scale insects are not much affected. In some cases, the host plant may influence the degree of control; this seems to apply to some whiteflies. When neem products are used to control whiteflies if exhibits control on some crops but not on others. The ability of certain homopterous insects to carry and transmit viruses are also influenced by them products. Low doses prevent the green rice leafhopper from infecting rice fields with tungro virus.

Thysanoptera (Thrips)
Neem products are very effective in controlling thrips larve which are found in the soil. Their effect is moderate when used on the thrips and related pests found on plants. Neem oil is more effective - this could be because the oil coat suffocates these tiny creatures.

Coleptera (Beetles, Weevils)
Larvae of all kinds of beetles refuse to feed on plants treated with neem. Their growth is retared and some soft skinned ones are killed on contact.

Lepidoptea (Moths, Skippers, Millers and Butterflies)
Neem products act as growth deterrents in the case of the larvae of most lepidopterous pests. They also act as antifeedant.

Diptera(Flies)
Insects like flies, face flies, bot flies, houseflies and horn flies are affected by neem products.

Hymenoptera (Bees, Wasps, Sawflies, Ants, etc.)
Bugs like rice bugs and vegetable bugs are affected by neem products in that they exhibit antifeedant and growth deterrent properties.

Mode of Action of Neem Products on Specific Pests
This section describes the manner of action of neem solitarization of the gregarious nymphs of locusts. Doses equal to 2.5 litres / hectare prevent the juveniles from forming massive moving plagues which are extremely devastating to the crops. Through they are not killed , they become solitary, lethargic, almost motionless and are highly susceptible to predators such as birds. Grasshopper nymphs are also affected in a similar fashion.

Cockroach
Neem kills the young cockroaches. Adults are inhibited from laying eggs. It has been observed that baits which were impregnated with commercial preparations of neem seed extract retarded growth of Oriental, Brown banded and German cockroaches. Growth of first instar nymphs of all there species was retarded and they died within 10 weeks. Last instar nymphs exhibited retarded growth and died within 9 weeks.

Brown Planthopper
Neem cake is extremely successful in controlling brown planthopper and other rice pests. Neem oil is also effective. The several way in which these could be used against brown plant hoppers is discussed in detail in the appendix.

Stored Product Insect
Use of neem for controlling pests of stored products is one of the oldest traditional uses of neem in Asia. Neem products in this context basically seem to act as repellants. After treatment of jute sacks with neem oil or extracts, weevils (Sitophilus sp.) and flour beetles (Tribolium sp.) do not penetrate for several months. Stored grains are kept mostly away from sunlight and hence the degradation problem of neem products is also less. Neem oil is also very effective against pests of stored beans, cow peas and other legumes. Treatment with neem produces does not affect the germinating capacity in anyway. Neem is used to protect stored roots and tubers against potato moth. It is shown that small amount of neem extends the storage life of potatoes by three months.

Leafminers
Neem products are used very effectively against leaf miners. When the neem are added to the soil through the roots they reach the crop’s leaves and start acting on leaf miners feasting on leaves. The moulting hormones of leaf miners their own juvenile skins.

Mosquitoes
The larvae of different species of mosquitoes are affected by neem. Crushed neem seeds when thrown into pools prevented mosquitoes from breeding.

Fruit Flies
Fruit Flies are serious horticultural pests. their presence in the tropics affects dozens of delicious fruits. it has been shown that spraying dilute neem solution under fruit trees gave a 100% control of these pests. More interestingly the neem products were also compatible with the biocontrol organisms used to control fruit flies.

Nematodes
Thread worms or nematodes are highly devastating agricultural pests and also the most difficult to control. Neem products affect several types of nematodes. Aqueous extracts of dried and powdered neem have shown up to 100% mortality of nematodes from 12 hours onwards in laboratory experiments. Aqueous extracts of seed kernel and seed coat are also nematicidal. Hatching from egg masses of nematodes was reduced when they were treated with seed kernel and coat extracts. Aqueous neem cake extracts inhibit hatching of second stage juveniles. In Aligarh, India, amending soil with neem cake helped to bring down the root-knot index to zero in tomatoes. this is very important because tomatoes are highly sensitive to nematodes. Cardamom growers of South India have taken to the use of neem cake for nematode control. They incorporate 100 to 259 kg per hectare of neem for their cardamom fields every year. Nearly 3000 tonnes of neem cake are now used in the cardamom hills of India annually. It is transported and sold by pesticide dealers 250 to 300 kms away.

Methods of application: Neem cake can be applied as a soil amendment, either alone or in combination with wheat straw and inorganic fertilizers. Soil drenching with aqueous extracts of seed kernel and seed coat at lower does but for more number of times (one at the time of sowing and others at monthly intervals) also reduces the number of galls in chickpea. All organic amendments including neem products are effective against plant parasitic nematodes only after a certain period of decomposition. Maximum reduction in plant parasitic nematode population is found only after 75 days of application of neem cake, though the reduction in noticed after 45 days. Bare roots of plants like tomato can be dipped in neem extracts for different periods of time. Seeds can also be soaked in aqueous extracts. When seeds are treated in this way there is reduction in the penetration of second stage juveniles. Seeds can also be coated with cake, kernel or seed coat. This is also effective against root-knot nematodes in a number of crops like chickpea and groundnut.

Snails
Neem extracts also kill snails. Aqueous solution of neem fruit results in a 100% kill of the snail, Melania scabra. These snails are vectors of lung flukes, a parasitic flatworm that encysts in the lungs of livestock, wildlife and human.

Plant Viruses
Beside the insecticidal and nematicidal properly, neem is also a promising agent for control of plant disease. Yellow vein mosaic of okra, yellow mosaic of grain legumes and leaf curl of chilli cause enormous loss. Neem oil in combination with paraffin oil greatly reduces disease incidences. Rice fields sprayed with neem oil have had significantly lower incidence of ragged-stunt virus which affects rice and is transmitted by the brown planthopper. Neem oil and custard apple oil interfere with the transmission of tungro virus, another rice pest. Neem leaf extracts have been shown to reduce transmission of tobacco mosaic treated with neem cake were significantly free of rice tungro virus.

Fungi
Neem has been demonstrated to possess antifungal activity. Fungi are constantly evolving enemies of farmers and some can reach epidemic proportions. A few of these have no cures and some can make certain crops impossible to grow. Neem oil has been shown to protect chickpea seeds against fungal diseases. Neem seeds extracted are also beneficial against fungal disease. Neem seeds extract are also beneficial leaf fungi. It prevents powdery mildew disease when sprayed before the outbreak of the disease.
Neem leaf extracts do not kill fungi. However when treated with neem leaf extracts the fungus Aspergillus flavus does not produce aflatoxin. The extracts halt the formation of substances called polyketides. It is these substances that the fungi convert the formation of aflatoxin in cotton balls.

Effect on Non - Target Species
One of the problems with the use of chemical pesticides has been their impact on "non-target" species. Often they have proved harmful to various other species in the ecosystem that could be beneficial. This section briefly reviews the information available about the effect on non-target species. This also helps us to assess how well the use of neem may be used in conjunction with other methods such as biocontrol using predators like spiders.

Earthworms
Neem leaves and seed kernels when incorporated into potting soil, which contained earthworms increased the earthworm population by 25%.

Effect on other Beneficial Insects
Neem products have proved to be remarkably benign to spiders and also other insects such as bees that pollinate crops and trees, ladybug beetles that consume aphids and also wasps which act as parasites on various crop pests. Neem products have to be ingested to be effective. Those insects which feed on plant tissues, therefore, easily succumb. However natural enemies like spiders feed only on other insects and bees feed on nectar. Hence they rarely come in contact with significant concentration of neem products.