The beetles that attack the trees of the arid zones of the Old World are many and varied. The majority feed casually on leaves and flowers. Others attack the roots or have larvae which bore into trunks and branches. It is upon its seeds, however, that a tree depends for the continuation of the species. Beetles whose larvae devour seeds are enemies of the plant and threaten the survival of the species.
This group of beetles exerts a greater influence than any other on trees and shrubs of the family Leguminosae growing in the tropics. It is essential therefore for the protection of the Acacia species with which this project is concerned, that the beetles belonging to this group be recognised along with the damage they cause. In addition, the basic fundamentals of the life-cycle must be appreciated.
A bruchid beetle of the type found in acacia.
Actual size: 2.5 – 3.0 mm
The family is considered by most authorities to be closely related to the Chrysomelidae (Leaf Beetles) and the Cerambycidae (Longhorn Beetles). At one time they were placed next to the Curculionidae (true Weevils) and as a relic of this obsolete taxonomic position, a number of publications retain the misnomer “weevil” in the vernacular name which has given rise to a misconception of the true relationship of this group. For example, the common economic species Callosobruchus maculatus (F) has been referred to as the “Southern Cowpea Weevil” (Larson 1927) whilst the related species C. chinensis (L.) was, as recently as 1975, referred to as the “Bean Weevil” (Doria and Raros 1975).
The family Bruchidae is divided into some 56 genera, 10 of which are to be found in the Old World. The taxonomy of the Old World genera has remained reasonably static since their original description in the late 18th or early 19th centuries with most species being assigned to Bruchus. The addition of a new genus Bruchidius by Schilsky in 1905 and one or two other genera such as Tuberculobruchus and Conicobruchus are the only other changes that have taken place. The first and only cataloguing of the group was undertaken by Pic (1913) and since that time numerous new species have been described. Some attempt at reclassification was made by Bridwell in 1946 but his concern was mainly for New World genera.
Interest in this area has developed through the work of Bottimer, Kingsolver and Johnson. Any revisions which have been made in the Old World were concerned with genera and species associated with economic crops rather than regional taxonomic revisions irrespective of host. In consequence, large unwieldy genera have been formed containing numerous unrelated species.
Bruchidae are, for the most part, small insects, the largest of them being no more than 10 mm long with the majority of species in the 3–4 mm range. They are oval-shaped but appear slightly truncated at both ends. This is due to the habit the beetle has of holding its head at right-angles to the rest of the body and also from the position of the shield-shaped last dorsal abdominal segment (the Pygidium) which is placed in an almost vertical position. Most species have large compound eyes with a deep ‘U’-shaped cleft opening towards the front. The antennae arise from this cleft and in all but a few genera are thin and about half the length of the whole insect. The segments of the antennae are wedge-shaped (Fig. 1) for most of the length. The thorax of the Bruchidae varies from broadly transverse with a single tooth along each side, to the conical form of Callosobruchus (Fig. 2). The elytra are almost square, being in most genera only slightly longer than broad (see Plate 1). In the male the extreme apex is curved towards the front of the insect (Fig. 3a), whereas in the female it is straight (Fig. 3b). The legs have well developed claws at the tips of all the tarsi. The hind femora are broad, in some genera extremely so, and armed with spines on the ventral side near to the apex. This latter character is used as a diagnostic feature of the various genera and species (see Fig. 4).
Most bruchids have flattened scale-like hairs (the pubescence) covering the thorax and elytra and these consist of areas of varying shades of pale brown to black interspersed with small patches of white. They form patterns on the elytra which may be characteristic in some species. However, this pattern formation cannot be relied upon for certain identification and other diagnostic characters must therefore be used.
Fig. 1 - Antenna of a typical bruchid
Fig. 2 - Outline of thorax of some common
genera of Bruchidae
(a) Caryedon (b) Bruchus (c) Bruchidius (d) Callosobruchus (e) Acanthoscelides (f) Spermophagus
Fig. 3 - Side view of the abdomen of a (a) male and (b) female bruchid showing differences in the shape of the pygidium
Little biological study has been made on the majority of the bruchid species other than those associated with grain legume crops, therefore few details are recorded regarding the life-cycle. Any serious attempt to control the bruchid species associated with arid-zone leguminous trees is hampered by lack of data. We need to know, for example, the time of oviposition on a particular host, life-cycle, adult host range, etc. This information is also lacking for some of the common economic species under field conditions. It is well known that Callosobruchus maculatus (F.), completes a life-cycle of egg to adult under laboratory conditions of 27°C 70% R.H. in 28 days but the time taken for the same insect to complete a cycle under field conditions has never been adequately recorded.
Plate 2 - Bruchid life-cycle chart
Bruchids can be divided into two groups, albeit somewhat artificial ones. The first group attacks the seeds while the young pods are developing, and the larvae on hatching enter the seeds and grow to maturity within the developing seeds and pod, the adult beetles emergence being timed for seed maturity or soon afterwards. The second group can also commence infestation of the host in the field, but at a later stage of pod development, i.e. when the pods commence to ripen and seed growth has almost finished. The adults of this group emerge soon after harvest and continue to reinfest the same seeds. It is not essential for them to seek out a newly matured pod on which to oviposit.
The two groups are separated partly by host specificity, those of the second group being associated with low growing herbaceous crops of the Fabaceae such as cowpeas, mung beans, etc., whereas the majority of the first group are associated with trees and shrubs of the Caesalpineaceae and Mimosaceae.
Fig. 4 - Hind femora of some common genera of Bruchidae
(a) Bruchus (b) Acanthoscelides (c) Caryedon
(d) Bruchidius (e) Spermophagus (f) Callosobruchus
Bruchid eggs are of two main types - cylindrical and hemi-ovoid. In the first, the larva emerges from one end of the egg and wanders on the pod surface before entering through a suitable crack or crevice. In the second, the egg is firmly fixed to the substrate by a secretion which covers it and forms an explanate rim to seal the egg to the substrate. This performs two functions - it protects the egg from desiccation and ensures that it remains firmly fixed to the host and, more importantly, assists the larva to enter the seed pods by acting as a fulcrum to enable it to bite its way into the pod.
The majority of bruchid species infesting the trees of Acacia, with which this project is concerned, lay eggs of the first type on the surface of the developing pods. The most common oviposition site is the suture but in some species, particularly those with lomentaceous pods, the eggs are usually deposited in the constricted areas between the seeds. This may afford a little more protection to the egg than if it was laid in a more exposed position on the pod. Most bruchids in the field lay single eggs at widely spaced intervals. One or two species have developed a multi-oviposition technique in which groups of 6 to 8 eggs that are hemi-ovoid in shape are deposited with ⅓ or more of the lower surface overlapping the egg immediately below. This was first recorded and illustrated by Teran (1962) of the South American bruchid Pseudopachymerina spinnipes (Erich.), a species introduced and now widespread in parts of the Mediterranean, the Middle East and parts of Africa, on its two hosts Acacia farnesiana and A. caven. Mass oviposition has the advantage as Prevett (1967) pointed out, that only the upper egg layers are exposed to parasitoid attack or other forms of damage.
Whilst the majority of species oviposit on young developing pods, a few species select the fully matured pods of trees. The site chosen by the female is usually the point of dehisance along the suture.
As a general rule egg development takes 5–10 days from oviposition. The larva's first priority on emergence is to gain access to the seed. For those larvae entering a pod wall first, and then the seed, the process can be a long and precarious one taking several hours.
It is stated in the literature that the larvae ingest the material removed to penetrate the seed and gain access to the cotyledons. The testa of some species of legumes contain toxins and failure of the larvae to survive this penetration has been attributed to the ingestion of toxic materials. The conclusion was reached that the toxicity of the testa provides an effective barrier to protect the seed.
It has been demonstrated recently (Southgate 1983) that the larvae of Callosobruchus sp. do not ingest the material removed (in order to penetrate the testa) but rasp it away and discard it without passing it through the gut. Although only one genus has been looked at, there is no reason to doubt that the same would apply to all Bruchids.
Bruchid larvae first form a tunnel within the seed and then enlarge this to make a cell. During the period of growth, which may take 3–4 weeks or as many months, dependant on prevailing climatic conditions, the larvae moult 4 times and then pupate. Before this happens preparation is made for the emergence of the adults. The area of the cell nearest to the outside is cleared and enlarged and only a thin layer of testa left which forms a circular ‘window’ of semi-translucent material. On emergence the adults bite their way out leaving circular holes.
In one Old World genus at least the larvae leave the seeds when fully grown and pupate in papery cocoons formed either in the pod or attached to the outside. One Caryedon species (Skaife 1926) occurring in Southern Africa, has larvae which enter the soil to pupate. The length of time spent in the pupal cells or cocoons is variable. Whilst the greater proportion of adults emerge as soon as they are fully developed, others may stay within the cell or cocoon for several months as in Caryedon serratus spp. palaestinicus Southgate (Donahaye, Navarro and Calderon 1966). This adult diapause is designed to carry the species over to the next season when pods are available.
There is little information recorded on the dispersion and flight of adult bruchids, even for the species which attack economic crops. Mathwig (1971) working on bruchids associated with Gledetsia sp. (Honey Locusts) in Texas, discovered that flight for oviposition took place at night. Whilst this may be applicable for this species, it is not true for all. The threshold temperature above which bruchids become sufficiently active for flight is normally 25°C. In semi-arid zones, with sparse vegetation, temperatures fall well below this at night. Further work is required on this aspect of flight and on the feeding habits of the adults. By analogy with evidence derived from laboratory experiments it seems that it is not essential for beetles to feed. Adults are attracted to flowers of Acacia and related trees and also to extra-floral nectaries and congregation at these sites brings the sexes together for mating. The feeding stimulus may also trigger off release of sex pheromone. The intake of pollen or nectar is not essential for the maturation of the ovaries but such feeding by the female would undoubtedly increase fecundity and possibly extend the life of the insect.