FUMIGATION OF SEED FOR INSECT AND FUNGUS CONTROL

Seed must be free of insects and fungi. Preferably, insects and fungi, which can seriously harm seeds and reduce germination, should be completely destroyed before storage. A review of seed insects of Prosopis species has been prepared within the framework of the present FAO/IBPGR project (Johnston 1983). However, a short review of pests and diseases and their control is given below.

Insects

By far the most numerous (in both species and individuals), most specialized, and best known insects to affect Prosopis fruit are members of the bruchid family. Nine genera of Bruchidae are known to infest Prosopis fruit and seed, with three of these genera obligately restricted to Prosopis. Species of bruchid insects found in both North and South America and their Prosopis hosts are listed in Table 4.

A generalized life cycle of a bruchid insect on a fruit of Prosopis is illustrated in Figure 3.

Insecticidal dusts can be used for treating Prosopis seed. These dusts may be active chemicals, or they may be inert chemicals, such as finely ground feldspar, alumina, silica, dolomite, and anhydrite.

Carbon bisulfide is a commonly used fumigant to rid Prosopis seed of insects. Three other fumigants, i.e., methylbromide, paradicholorobenzene, and naphthalene, also produce good results.

To fumigate, seed and the insecticide are placed together in a tightly sealed container for a period of time not to exceed 24 hours. Seed should only be fumigated when moisture content is below 12 percent and the temperature is less than 30°C. The seed should be aerated immediately after fumigation. As many fumigants are dangerous to use, the directions given by the manufacturer should be carefully followed.

Fumigation of Prosopis fruit before extraction of the seed can also be an effective control measure in many instances. If the pods are fumigated promptly after collection, such as with methylbromide or phostoxin, bruchid insects are killed; afterwhich, healthy seed can be extracted. Also, freezing the pods for several weeks at -20°C can be effective in reducing the number of bruchid emergence holes.

Use of Orthene¹ sprayed three weeks after flowering and at subsequent three-week intervals, is helpful in preventing bruchid insects from entering unripe Prosopis fruit still on the tree.

¹ The use of a trade name of a chemical does not imply a favouring of one brand or make over another; it is only an indication of the type of chemical recommended.

Fungi

Most damaging fungi develop and propagate under humid conditions. Therefore, serious problems with fungi are relatively minimal in arid and semi-arid regions. Seed that has been dried to a moisture content of 10 percent or less, and then properly sealed in air-tight containers, is usually safe from fungal attack.

Table 4

Bruchid Insect Species Associated with Prosopis

Source: Kingsolver et al. 1977.

Figure 3. A generalized life cycle of a bruchid insect on a Prosopis fruit: A. eggs glued to the surface of a pod or inserted into old emergence holes of a previous bruchid generation (large circular hole); B. penetration holes of larvae that have bored into the pod (pod with holes and egg about 2x, enlarged first instar larva 15x); C. cross-section of a fruit showing the tunnel into the seed bored by larva (magnified about 5 times natural size); D. larva developing inside hollow seeds (4.5x); E. pupa occupying larval feeding chamber (the hole in the seed coat near the head of the pupa is part of the tunnel to the endocarp of the pod cut by the larva before pupation, 4.5x); and F. adult emerging through the precut tunnel (5x).

Source: Pfaffenberger and Johnson 1976; Kingsolver et al. 1977 (Redrawn).

If problems do occur, chemicals with a sulfur base are commonly used against seed-destroying fungi. Formaldehyde solutions are also successfully used to destroy fungi that have already attacked the seed.