One of the aims of the forester is to ensure a good supply of sound viable seed. From the biological data given elsewhere in this publication it will be seen that some families, notably the Bruchidae, develop as larvae wholly within the seeds. As previously mentioned, the seeds and larvae of most bruchids reach maturity together and no further damage will occur after this point. Until the emergence of the beetles there is little or no evidence to suggest the seed contains a live insect. Attempts to control the infestation at this stage would be a pointless exercise as very few tree-infesting species have the capability of producing further generations on mature seeds. Seeds affected in this way will germinate very badly or not at all. If seed is collected from ‘wild’ trees, then it must be subjected to rigorous screening. Trees which are grown specifically for the purpose of providing seed and over which the forester has reasonable control, can be treated with preventive measures against insect attack in the early stages of pod formation.
The trees selected for seed production can have individual branches covered with sleeves or pollination bags made from woven glass fibre material to exclude the pest species. Acacia species with long thorns are not easy to cover in this way but the method was successfully used by Conway (1975) when studying the emergence periods of bruchids from hosts in the Gambia. Sleeves made from this material are extremely durable being resistant to tearing and extreme climatic conditions. Timing the covering of selected branches will be dependant on the biology of the insect. Some indications of the choice and timing of oviposition sites of bruchid species attacking Acacia tortilis are given by Lamprey et al. (1974). In the Lake Manyara National Park, Tanzania, oviposition took place between 30 and 100 days after pod initiation on A. tortilis ssp. spirocarpa.
If sleeving of branches proves to be impractical, then the possibility of treating small areas of the tree with insecticidal sprays should be considered. No research has been carried out on the treatment of trees with insecticides for the prevention of insect attack on seed pods. The chemicals Tetrachlorvinphos, Pirimiphosmethyl and Endosulfan have all been used successfully against bruchids on grain legumes as have DDT/BHC mixtures but the latter should be avoided if possible as they give problems with long-term residues, and after prolonged use resistance to the insecticide is induced.
Seeds infested by bruchids, but without adult emergence holes, may appear, to the untrained eye, to be normal viable seed. Bruchid beetles, in their developmental phase, as already shown, are hidden inside the seeds, the entry holes of the first stage larvae being almost invisible. The insects' development continues unhindered even though the seeds are removed from the pods and put into store. If seeds are sent to a central laboratory with adequate facilities, then the use of X-rays for batch examination, as detailed by Milner et al. (1952) and Swaminathan and Kamra (1961) can be employed. Details of any damage can be clearly seen and in most instances the stage of the insect can be detected (Osuji 1982).
Small quantities of seed can be examined with a binocular microscope for larval entry holes but these are difficult to locate. The alternative approach to these non-destructive methods is to adopt that used by Karshon (1975). Here samples of seed were softened by soaking in water which had been brought to boiling point (prior to immersion) or sulfuric acid and then subjected to cutting tests. The results of this examination were expressed as percentage damage of the sample. Storage of seeds at the temperatures recommended for seed banks will also inhibit development of any hidden infestation, but prevention is the only way to ensure a high percentage of viable seed.
Material grown in one area and subsequently distributed to another, must be covered by a phytosanitary certificate. To ensure that hidden infestation is controlled, the seed must be subjected to fumigation with methyl bromide or similar chemicals. The dosage of fumigant required to ensure 100% kill of all insects may, unfortunately, have adverse effect on the germination factor of the seed. To avoid this hazard X-rayed samples are the only way of ensuring that undamaged seed is despatched.
