5. INTERACTION BETWEEN TREES AND CROPS

5.6 Pests and diseases

Mixtures of trees and crops make a more diverse environment than monocropping. A diverse environment enables a greater variety of species of all kinds of organisms — both desirable and less desirable — to thrive. With increased diversity, the risks of pests and diseases may also increase. On the other hand, a greater diversity of species also allows for better regulatory mechanisms which may reduce the seriousness of the pests and diseases.

Pests and diseases can be attributed to the agroforestry technology in use only if an organism has been promoted by one component in the system and is harmful to the other component or components. The most obvious situation would be if trees planted in or near a farmer's field introduced a pest that wiped out or reduced his crop yields. Birds being attracted to nest in certain trees is another such example of relevance for Kenya. The build-up of nematode populations under Sesbania trees has been mentioned as a potential threat to intercropping Sesbania in fields, but so far there is little evidence that this is a significant problem.

Beside these situations where both trees and crops are involved, there are of course numerous pests and diseases that affect only one component in the system. The occurrence of such pests has nothing to do with the agroforestry technology concerned, but since there are two important pests that have recently occurred in Kenya those two will be briefly discussed here. These are the cypress aphid and the Leucaena psyllid.

A third situation which is of interest when discussing pest and disease problems associated with agroforestry is when an agroforestry species turns into a weed itself. In the Kenyan context Acacia mearnsii, Leucaena, Prosopis and guava (Psidium guajava) have shown such potential.

Birds

Trees attract various kinds of birds as they are sources of food (fruit, nectar, insects), shade, shelter and nesting sites, and birds can be beneficial in many ways. In an agroforestry system insect-eating birds may help control insect populations and birds of prey help control rodent populations.

Some birds, however, feed on grain and have long been a problem for cultivation of certain crops. Traditionally, both children and adults would spend much time guarding crops, but nowadays such labour is unavailable. A destructive bird species in this respect is the red-billed quelea, Quelea quelea, which may sometimes invade cultivated areas in very large numbers. Little is known about the effects of trees on the severity of damage caused by Quelea.

Nematode build-up under Sesbania

Populations of nematodes, particularly the root-knot nematode, have been reported to infest Sesbania, reducing their growth. Nematodes cause irregular swellings called root galls that are quite distinct from the nitrogen-fixing root nodules that arise from the root surface (Evans and Macklin, 1990). Nematodes have a reputation for being harmful to certain crops, notably Irish potatoes and bananas, but so far there is little evidence that growing Sesbania would create any significant problems in this regard in Kenya.

The cypress aphid

The cypress aphid is an exotic aphid that first appeared in southern Africa around 1986. From there it has gradually spread northwards and appeared in Kenya in 1990, where it turned out to be a very serious pest on Cupressus lusitanica. Damage has also been reported on other conifers, e.g. Widdringtonia nodiflora (Mlanje cedar) in Malawi, and on Juniperus procera in Kenya. This pest is of significant economic importance for forestry, and since cypress has been widely used for fencing in Kenya it has also affected many small-scale farmers.

Some cypress trees appear to be more resistant than others, and if this can be confirmed it should form a basis for potential breeding from such resistant trees to obtain a cypress population which will be more resistant to the aphid. Efforts have been made to find natural enemies, so far with no practical results. The aphids are active mainly during the dry season and application of pesticides kills them. This method is not relevant for small-scale farmers, however, and may also be unacceptable from an environmental point of view.

The Leucaena psyllid

Leucaena leucocephala, which is widely grown throughout the tropics, was suddenly attacked in Hawaii in 1984 by a sap-sucking insect, Heteropsylla cubana. The insect, which seems to have originated from the same area as Leucaena, has spread from the Pacific to Southeast Asia, Australia and India and appeared on the Kenya coast and in Embu in 1992 (ICRAF, pers. comm., 1992). Worldwide this pest may cause losses amounting to millions of dollars.

Two different strategies are being tried to control the pest: plant resistance and biological control through the use of natural enemies. Initial results suggest that both strategies seem to be working well, although it is too early to predict what the final solution will be (Murphy, 1990).

Recent findings from the ILCA research station at Mtwapa indicate that Leucaena diversifolia is resistant to attack by the psyllid. Two different accessions of L. diversifolia have been planted there. One is slightly attacked and the other appears not to be attacked at all.

Trees turning into weeds

There are several examples of exotic tree and shrub species in Kenya which are now spreading on their own. In this sense such species can be regarded as weeds, but on the other hand they may also be useful in some circumstances and therefore it is not always true to say that their spread is a nuisance. Uncontrolled spread of a tree or shrub species is, nevertheless, a warning sign that the species has the potential to become a nuisance. It is always difficult to predict the possible end result in the early stages of such processes.

Acacia mearnsii is known as a weed in many parts of Africa, including Kenya. Seeds of A. mearnsii remain viable for decades, and after fire they may germinate over large areas. This is useful if regeneration is desired, but it can also lead to an additional weeding burden. A. mearnsii is a useful tree, but it competes aggressively with other plants and therefore its natural spread may not always be desirable.

Leucaena definitely qualifies as a weed in certain situations. In warm areas with sufficient rainfall, Leucaena spreads very effectively if it is allowed to produce seeds, and thus it adds to the weed flora in fields. Along the Kenya coast the spread of Leucaena has been very conspicuous.

Prosopis is spreading in irrigation schemes and along rivers in dry and hot areas. So far, the positive and negative effects of this spread have not been quantified.

Guava is also spreading on its own in some areas. So far there has been no significant negative effect, however.