Natural Crop Protection in the Tropics: Vegetable oils

The protection of stored products such as rice, wheat, maize or beans with vegetable oils is another simple, convenient and cost-effective method. Tested and successfully used in practice are the oils of peanut, coconut, safflower, mustard, castor bean, cottonseed, soybean and maize. Sunflower seed oil has not proved very effective.

How do vegetable oils work?

According to LIENARD et al., the effect of vegetable oil on insect pests of stored products involves four different and complementary mechanisms:

1. Toxicity to the eggs and first instar larvae is the consequence of the occlusion of a short funnel at the posterior end of the egg.

2. The oil coat leads to a reduction of egg adherence on the treated seed which prevents the first instar larva from penetrating the seed.

3. Some oil constituents have a direct toxic effect.

4. Various fatty acids contained in the oil have a toxic effect.

As time passes after a treatment, two factors change:

1. Adult females increasingly avoid laying eggs on seeds which have been treated with oil more than 7 days previously. Very few eggs indeed are laid on seeds which were treated 60 days before being newly infested with beetles.

2. The toxic effect on eggs and young larvae decreases over time. In practice this means that the protective coating of oil must be renewed periodically. However, it may be that the microclimate of the storage facility may influence the duration of the protective properties of the oil.

The germination capacity of oil-treated seeds is judged differently. PANDEY et al. state that mung beans suffer no damage in germination. On the other hand VUN-TAI-QI notes a diminishing germination capability in wheat as the percentage of oil used in the treatment increases. At 1 ml oil per kg of wheat, 82–90% of the wheat remained capable of germination, but this dropped to between 32.5 and 42.4% at 5 ml per kg and to between 27.5 and 52.5% at 10 ml per kg. Therefore he recommends that wheat which is intended for seed should be treated in some other way and that vegetable oils should only be used to protect grains which are intended for food. Germination appears to be most inhibited by cottonseed and soybean oil.

Experience with different oil treatments

_{Neem oil}

Several experiments made with different oils in Madagascar have shown that neem oil is more effective than other oils and less costly in the regions where neem trees grow. Depending on types of beans, the ambient temperature and the duration of the storage, 2–5 ml of neem oil mixed with 1 litre of beans are sufficient, whereas for other oils no less than 10 ml are required for one litre of beans. When well mixed this amount should be sufficient to coat the entire surface of the beans with oil. A 50 kg sack requires 150 ml of oil and the protection lasts for about 6 months.

Adults of weevils which lay eggs on seeds and which are in permanent contact with the oil are particularly well controlled by neem oil. Generally, oil kills the eggs that are or will be laid on the seeds but has no effect on larvae that are developing inside the seeds.

This treatment is particularly recommended for the protection of seed material. Unless beans meant for consumption are duly cleaned after storage, a slightly bitter taste might remain, but this is not hazardous to health. This method does not require money and is easy to apply.

Target insects

_{Storage insects}

Bean bruchids Acanthoscelides obtectus

Cowpea weevil Callosobruchus spp.

Rice moth Corcyra cephalonica

Overview: Vegetable oils in the control of storage pests

The printed version contains the description of the following oils:

Coconut (Cocos nucifera), Cottonseed (Gossypium spp.), Dennettia (Dennettia tripelata), Neem (Azadirachta indica), Peanut (Arachis hypogaea), Rice husk (Oryza spp.), Sesame (Sesamum indicum), Shea butter (Butyrospermum parkii)