An agroforestry system consists of at least of two components, the crop component and the woody component. A large number of
trees and shrubs are used as woody perennials with local crops in different agroforestry systems. There have been many studies on the insect pests of crops, but little or no information is available on the insect pests of the woody
perennials that are important components in most agroforestry systems. This may be because these plants were not considered economically important when they occurred only in their natural stands. In agroforestry systems, in which
woody plants and crops are grown together, the insect pests of one component can affect those of the other. It is, therefore, essential to know about the insect pests of each component plant in the assemblage. Because information
about insect pests is available for most of the crops used in agroforestry systems, this study concentrated on the insects associated with some commonly used multipurpose trees and shrubs.
The list of insects presented
in appendix 1 was compiled from a detailed literature search. The sources are indicated at the end of this section. The number of references used to compile these lists is large; therefore only the primary sources are listed. The
abstracts and titles in different databases and in the annotated lists did not always contain enough detail about the pests' taxonomy, nature of damage and status, etc. In such cases, the type of injury indicated in the appendix is
based on the injury normally caused by the species or genus on other hosts, or on the basis of characteristic family habits. The exact type of injury inflicted on any host under different ecogeographical conditions needs to be
confirmed for each insect species.
There have been many changes in the taxonomic status of various insects in the last 20 years—many new families have been raised and there have also been changes in generic and specific
nomenclature, besides reorganization of some species into different families or subfamilies. Thus an insect included in a particular family in the older literature may well be in another family in later works. The lists of insects
associated with multipurpose trees and shrubs presented in the appendix are based on a limited literature survey and are, therefore, not exhaustive.
4.2 Field observations
During the field visits to
different AFRENA sites and the ICRAF Machakos Research Station, a number of insect pests and their natural enemies were collected from woody plants (appendix 2). The worst hit plants were Cajanus cajan, Cassia siamea, Erythrina
spp and Sesbania sesban. Pigeonpea at Machakos suffered heavy insect attack, the most serious being that of the shoot scale, Coccus longulus. These scale insects were found to be attended by the ant,
Pheidole megacephaly, which protected them from natural enemies and also helped in their spread and establishment on uninfected plants. The affected plants dried up. Secondary infestation by termites on scale-affected plants
was observed. The incidence of termites was low on healthy plants but it was heavy on cocci-infested plants. If the ants were removed from the cocci-infested shoots, a parasitoid complex consisting of Metaphycus stanleyi,
Eupelmus sp, Cheiloneurus carinatus and Tremblaya minor
quickly developed on the cocci colonies. The development and spread of coccids could be curtailed by preventing ants from nursing the cocci colonies. A number of pod-borer insects were observed, but not all of these could be reared. Further investigations on the cocci-parasitoid complex could yield useful results.
In Cameroon, pigeonpea plants were seriously infested by the spittle bug, Ptyelus grossus,
which fed gregariously in a ring around the shoot, resulting in the entire shoot drying up, from the point of infestation upwards. Another serious insect pest on pigeonpea in Cameroon was a stem borer, which attacked the plants 10–30 cm from the ground. The infested plants dried up. We were unable to rear the borer larvae to the adult stage; therefore the pest remains unidentified. These two insect pests on pigeonpea in Cameroon need further study.
Calliandra calothyrsus
plants at Machakos were prone to attack by a number of insect pests. The most important of these was a cossid borer, which inflicted heavy injury on plants in agroforestry trials. Again, we were unable to rear it to the adult stage during the period of the fellowship; hence it remains unidentified. A severe attack of mealy bug,
Spilococcus sp, was observed in Machakos. The spread of this insect was very rapid. The leaves of affected plants dried up. The mealy bugs were attended by the ant Pheidole megacephaly. When the ants were removed from
the mealybug colonies, a predator–parasite complex quickly developed. Leucopsis sp and Nephus sp were recorded as predators on Spilococcus sp. The parasitoid complex consisted of Anagrys nigrescens,
Cheiloneurus carinatus, Pseudectroma sp, Aprostocetus sp and Pachyneuron
sp. There is a need to make further observations on the pest–parasite complex, as it carries enormous scope for biocontrol studies on this pest. Further studies are also required on the cossid borer.
Characteristically, Erythrina
spp are attacked by a number of insect-pest species. Leaf skeletonizing was very common, though the responsible insects were not collected during the visits. Gall insects were also observed. Many sucking insects were observed on leaves and shoots.
Erythrina poeppigiana
plants in pure stand suffered heavy damage from an unidentified shoot borer at Mashitshi in Burundi. The same plant species raised with banana in a nearby agroforestry field was free from borer attack. This was a clearcut case of agroforestry contributing to low insect-pest injury of component plants.
In all agroforestry trials in Southern and Eastern Africa AFRENA sites, Sesbania sesban was found to be attacked heavily by insect pests. Mesoplatys orchroptera, a chrysomelid beetle, defoliated plants
at all stages of growth. It is a major insect pest of sesbanias in Africa. A number of other defoliators also existed on this host. At Rwerere, in Rwanda, unidentified cicadellids caused discolouration of leaflets, reducing the
photosynthetic efficiency of the plants. In Butare, a complex of sucking insects was observed on sesbania plants. If sesbania plants are to find a successful place in agroforestry in Africa, effective plant-protection measures need
to be devised.
Termites are a major pest at some sites, but at other places damage from them is not very serious. In Karuzi, Burundi, alnus, casuarina and grevillea plants suffered heavy termite injury during the establishment
phase. Once they were established, the chance of mortality caused by termites was reduced. Termites tended to attack weakened plants; healthy plants often withstood moderate termite attack. Further studies are required to establish
the role of termites in agroforestry. In some places, as at Rubona in Rwanda, the termites contributed to decomposition of mulch material without affecting the hedgerows in the agroforestry field. Mulch also contributed to
population build-up of termites. If there is moisture stress and plants become weak, the termites may attack weakened plants. If there is no damage to component plants under stress conditions in an agroforestry system, but the
decomposition process continues, this would be a great advantage of agroforestry practices.
The insects listed in the following tables were collected during visits to AFRENA sites and to ICRAF's Machakos Research
Station. The collection was identified at the National Museums of Kenya, Nairobi, and the CAB International Institute of Entomology (IIE), London. The appendix lists only those insects that were collected and preserved during
visits to experimental sites. However, some of the important insect pests observed during the visits do not appear in these lists because they were not identified. Also, the list represents only those insects observed during a
single visit to most experimental sites. The whole insect complex of each multipurpose tree or shrub species at each site could be much larger. More frequent visits are required to record the occurrence of different insects at
different times of the year and at different stages of plant growth. A follow-up research effort along these lines could yield useful information.
4.3 Primary sources of information used to compile lists of insects
associated with multipurpose trees and shrubs
AGRICOLA database on CD Rom 1970–1978 Silverplatter. USDA.
AGRICOLA database on CD Rom 1979–1984 Silverplatter. USDA.
AGRICOLA database on CD Rom 1984–Oct 1991 Silverplatter. USDA.
AGRIS database on CD Rom 1986–1988 Silverplatter. FAO.
AGRIS database on CD Rom 1989–April 1991 Silverplatter. FAO.
CAB Abstract database on CD Rom, vol. 1, 1984–1986. CABI.
CAB Abstract database on CD Rom, vol. 2, 1987–1989. CABI.
CAB Abstract database on CD Rom, vol. 3, 1990. CABI.
Harwood C.E. 1989.
Grevillea robusta: an annotated bibliography. Nairobi: ICRAF.
LePelley R.H. 1959. Agricultural insects of East Africa. Nairobi: East Africa High Commission.
Karanja M.K. and Chege F.M. 1985.
Forest insects of Kenya: an annotated list. Nairobi: Kenya Agricultural Research Institute.
Leucaena Research Reports, vol. 1–11.
Nitrogen Fixing Tree Research Reports, vol. 1–8.
