Amblycerus crassipunctatus, RIBEIRO- COSTA, RIBEIRO- COSTA, 1999

Johnson, Clarence Dan, Romero, Jesús & Raimúndez-Urrutia, Elena, 2001, Ecology Of Amblycerus Crassipunctatus Ribeiro-Costa (Coleoptera: Bruchidae) In Seeds Of Humiriaceae, A New Host Family For Bruchids, With An Ecological Comparision To Other Species Of Amblycerus, The Coleopterists Bulletin 55 (1), pp. 37-48 : 38-41

Results

The hostplant. The family Humiriaceae and the 14 to 16 species of Vantanea are primarily from South America but also occur in Panama and Costa Rica ( Mabberley 1997). Species of Vantanea and the family Humiriaceae are shrubs or trees. In the Amazon , bats disperse fruits of species of Vantanea and Humiria but we know of no reports of bats feeding on fruits of V. minor . The size of the plant is dependent upon the area in which they grow. In the study area (see Host Records below), on sandy soil, V. minor is a shrub that may reach a height of 2 m and foliage diameter of 3 m. The average size is 1.5 m in height and 2 m in foliage diameter. In areas with more rainfall and thus moist soils, V. minor may become large trees. The mature fruit of V. minor ( Figs. 1 , 6 ) varies from 2.5 to 3.3 centimeters in diameter. The fruits persist on the plant for long periods of time. The fruits are almost impenetrable so they are most efficiently opened with a saw. The seeds are not as hard as the fruits, and have a high lipid content. Apparently, it is difficult for insects to enter and exit from these fruits because Amblycerus crassipunctatus is the only beetle that feeds in its seeds. (A species of Curculionidae , however, was found feeding in seeds of another species of Humiriaceae , Humiria balsamifera (Aubl.) St. Hil. , by Raimúndez­Urrutia.)

Host Records for A. crassipunctatus : Vantanea minor Bentham : Venezuela: Bolivar: Arbustal Riworiwo, via Kavanayen , Gran Sabana , VIII­1993, E. Raimúndez, collector (2 specimens.); Shrub land near the Pemon (native people of the Gran Sabana ) community of Iworiwo (also known as Liworiwo ), 5°36.88'N – 61°29.66'W, 1,208 m a. s. l., V­15­1998, E. Raimúndez, collector (7 specimens) .

The insect. One of the problems with studying bruchid beetles is that they spend the major portion of their lives inside host seeds and fruits. Thus it is difficult to follow larval behavior and development. Special instruments and techniques must be used for these studies that are also very time consuming. An advantage of studying bruchid beetles is that it is relatively easy to associate the insects with their hosts if one has sufficient diligence to collect seeds and plants and to have the plants identified accurately. The insects must be carefully reared from the seeds and identified.

A sample of 21 fruits was collected in Venezuela and brought to the laboratory. We used ambient laboratory temperature for rearing the insects and the following results were obtained. More than half of the fruits (52.38%) had bruchid eggs glued to them ( Table 1 ). The most eggs laid on a fruit was eight, and the most adults that emerged from one fruit was two. The eggs were oviposited on the surface of fruits, which places them in Guild A of Johnson (1981).

We counted the number of eggs laid on each fruit and then counted the number of adults that emerged from each fruit. The average number of adults that emerged per egg laid was 21.87% ( Table 1 ). We considered that this average was probably low when compared with results that would be obtained under more controlled conditions and probably in nature. Examination of the eggs showed no evidence of mechanical damage or parasitism. We did find differences between hatched eggs and those not hatched. The eggs that hatched had dust that came from the hole drilled beneath the egg to provide entrance into the fruit. The eggs that did not hatch were transparent, and there was no sign of drilling beneath them, so these eggs may be infertile and could have been the reason for the low emergence. Another possibility was that the temperature and humidity were not ideal for them to thrive.

In this study of 21 fruits, six fruits (28.5%) had bruchid exit holes in them, so 28.5% of the fruits were damaged by A. crassipunctatus. In a separate study using 41 fruits, only 16 fruits (39%) contained seeds that had been fed upon by A. crassipunctatus.

Eggs of A. crassipunctatus are ovoid, 1.2 to 1.3 mm in length and 0.60 to 0.72 in width ( Fig. 3 ). A flange surrounds eggs with glue on the periphery ( Fig. 4 ). This is not unique to eggs of species of Amblycerus ( Johnson and Kingsolver 1975), but may be common within the genus. The glue on the periphery of the flange that the female uses to attach the egg to the fruit is the only portion of the egg that is attached to the substrate. This makes eggs appear larger than they actually are. The size of the egg and flange together is 1.47 to 2.50 mm in length and 1.25 to 2.12 in width. The fringe area between the glue and the egg delimit an area that is not attached to the substrate. Eggs are therefore suspended above the surface of the substrate (see Johnson and Kingsolver 1975, Fig. 10). This kind of attachment may prevent the egg from becoming detached from the fruit as it matures and during eclosion of the first instar larva or protect eggs against mechanical injuries. Although eggs are most often laid singly, they are occasionally oviposited in clumps of two or three and occasionally overlap each other.

The first instar larva hatches, drills through the bottom of the egg, then drills into the fruit and then into a seed. The entry hole in the fruit is about 0.325 mm in diameter. Once inside the fruit, the larva feeds on seeds, leaving frass packed inside the cavities where the seeds developed ( Fig. 2 ). If these larvae are similar to other bruchid larvae, they then develop through several instars, feeding as they grow. The last larval instar builds a thin, translucent cocoon inside the fruit and also makes a round, almost complete exit hole in the fruit wall leaving only a thin window of tissue in the wall. The larva then pupates inside its pupal chamber ( Fig. 5 ). When an adult emerges from the fruit it pushes on the round window and exits. The exit hole is 2.6 to 3.5 mm in diameter ( Fig. 6 ). The complete cycle is concluded in about two months.

Figures

Fig. 1. Fruits of Vantanea minor with eggs (E) and exit holes of Amblycerus crassipunctatus.

Fig. 6. Fruit of Vantanea minor with exit hole (E) of Amblycerus crassipunctatus.

Fig. 3. Fruit of Vantanea minor with eggs (E) and adult (A) in exit hole of Amblycerus crassipunctatus.

Fig. 4. Egg of Amblycerus crassipunctatus showing egg (E), flange (F) and peripheral glue (G).

Fig. 2. Cross section through fruit of Vantanea minor showing seed cavities filled with frass (F) produced by Amblycerus crassipunctatus.

Fig. 5. Longitudinal section showing two halves of the same fruit of Vantanea minor with hard endocarpal tissue (EN); pupal chamber (P) lined with a thin, translucent membrane; frass­filled seed cavities (F); and exit hole (E) of Amblycerus crassipunctatus.