Proterops basalis Walker, 1874

Achterberg, C. van & Chenon, R. Desmier de, 2009, The first report of the biology of Proterops borneoensis Szépligeti (Hymenoptera: Braconidae: Ichneutinae), with the description of a new species from China, Journal of Natural History 43 (11 - 12), pp. 619-633 : 630-632

Proterops basalis Walker, 1874

Proterops basalis Walker, 1874, p. 308 ; Shenefelt, 1973, p. 811.

Microplitis theretrae Watanabe, 1937, p. 108 . Syn. nov.

Snellenius therethrae ; Nixon, 1965, p. 270–271; Shenefelt, 1973, p. 783.

Notes

Belongs to the subfamily Microgastrinae and the tribe Microplitini because it has the hind coxa comparatively short, not reaching the level of the apex of the first tergite and vein r-m of the forewing present. The temple is very finely rugulose and the first metasomal tergite is 2.1 times as long as its apical width. Runs to the genus Snellenius Westwood, 1882 , and runs in the key by Nixon (1965) to the only known species from Japan: Snellenius therethrae ( Watanabe, 1937) . Its sculpture is more developed than indicated by Nixon (1965), but the sculpture seems not to be a reliable character to separate species of this genus in southeast Asia and, therefore, S. therethrae is synonymized (syn. nov.).

Material

Holotype female, ‘‘Type’’, ‘‘B.M. Type Hym., 3.c.783’’, ‘‘B.M. Type Hym. Proterops basalis Walker, 1874 ’’, ‘‘ Proterops basalis ’’, ‘‘242’’, ‘‘262’’, ‘‘ Japan, Coll. F. Walker, 1914-71’’, ‘‘Not in Gen. insect., G. Nixon, 7.ii.[19]58’’ (BMNH).

Biology of Proterops borneoensis and Cibdela janthina

For the first time P. borneoensis Szépligeti was reared in large numbers from the sawfly C. janthina (Klug, 1834) (Argidae) feeding on R. alceifolius Poir. at the Marihat Experimental Research Station, Indonesian Oil Palm Research Institute at Pematang Siantar, Sumatra. The parasitoid lays its eggs in the second to fourth larval instars of the argid larva. The braconid adult emerges nearly always from the precocious prepupa of its host; with one layer of silk present or without any silk, depending on the stage of the spinning of the cocoon by the host. The host plant, R. alceifolius is a woody plant species invasive in the Western Indian Ocean; e.g. in Réunion it is one of the most abundant invasive species ( Kueffer and Lavergne 2004). It invades all open habitats that are sufficiently humid and is especially abundant in forest openings (trails, gaps, deforested areas), along rivers and along the edges of humid forests. A biological control research programme was launched in 1996 by the Conseil régional de la Réunion ( Bourgeois et al. 2009). A total of 38 insect species were collected in southeast Asia, of which the four more promising ones progressed through biological studies and host specificity tests. From these, the sawfly C. janthina has been selected as a potential biological control agent. Before releasing C. janthina in Réunion, studies are focusing on complementary tests in quarantine on Réunion ( Kueffer and Lavergne 2004) and care should be taken to avoid the introduction of its parasitoid P. borneoensis .

Food preferences

For comparison, four different Rubus species were included in the same experiment: R. alceifolius Poir. , R. moluccanus L., R. pyrifolius Sm. and R. elongatus Sm. Once released into the plots of the latter three, the larvae of C. janthina were feeding on these plants and have been very well established for many generations over several years. Feeding preference was observed on R. alceifolius and R. moluccanus , and apart from some attempts at consumption, there was no development on the other Rubus species. When larvae of C. janthina were placed with both R. alceifolius and R. moluccanus , the leaves of R. moluccanus were almost uneaten.

Biology of C. janthina

Immediately after the emergence of the female, mating takes place on the R. alceifolius leaves in full sunshine, but the males were flying 2–3 days prior to the emergence of the females. After 3 days the eggs are laid on the lower part of the leaves with each egg spaced some distance from the others and inserted into the main veins of the leaves. The female inserts the eggs only in the upper three newly opened young leaves ( Figure 2 A–C), with the maximum number of eggs in the second leaf. After hatching the larvae are gregarious. The first and second instars skeletonize the lower epidermis in spots ( Figure 2D ), then the following instars feed on the border of the leaf, hanging in an ‘‘S’’ position, attached to the plant just by their legs, consuming the entire leaf except the veins ( Figure 2 E–G). This results in a characteristic defoliation aspect of the Rubus plant where only the stem and veins remain ( Figure 2G ). The host plant recovers but after several defoliations, the plant withers. When full grown, the larvae fall onto the soil and pupation occurs on the soil under the host plant. They form cocoons with two layers of silk on which particles of soil and dry leaves are stuck. The life cycle is 52–70 days: egg 10–11 days, larval instars 25–30 days, pupation 10–15 days and adult 7–14 days.

Limiting factors of C. janthina

Each female of C. janthina lays between 43 and 74 eggs (average 56). The fecundity is sufficient to completely defoliate the plots after two or three generations if the larvae are not collected to avoid the loss of the plants. However, there are limiting factors. Regarding predators, birds do not feed on the larvae, but some Salticidae spiders prey on the adults. Another spider, Oxyopes quadridentatus Thorell (Oxyopidae) , causes more damage by feeding on the young larvae and the adults, but because of the huge population of C. janthina its impact is negligible. The parasitoids are a greater limiting factor: a species of Eulophidae (Chalcidoidea) destroys a large quantity of eggs once a year during the period when there is less rain. The braconid P. borneoensis attacks the larvae and is able to reduce the population by more than 90%. In Hainan the cocoons of Cibdela chinensis were heavily infected during our investigations by a Conura sp. near xanthostigma (Dalman) ( Chalcididae ), but this parasitoid has not been found on C. janthina in Sumatra.

Mode of infestation of P. borneoensis

This parasitoid was so common in the three plots where C. janthina was reared in the open, that at the peak of the parasitoid population, one person was able to collect on average 36 Proterops from 0800 to 1000 h, 161 from 1000 to 1200 h and only 11 from 1400 to 1600 h. To protect the population of C. janthina , one person needed over 10 days to use an electric racket to kill on average 200 Proterops per day. In total, about 2000 specimens were collected and killed during this period. Each year a similar infestation was observed. The number of P. borneoensis collected per hour and per person in the three plots varied according to the weather. For instance, on 9 and 10 July 2005 in full sunshine 61 and 97 specimens per hour were killed, respectively. During cloudy weather on 12 and 14 July 2005 22 and 10– 27 specimens per hour were killed, respectively. Obviously, P. borneoensis flies mostly in full sunshine at higher temperatures, mainly from 0900 to 1100 h in the morning. In the afternoon, even with sunshine and a relative high temperature, the activity is sharply reduced. Also in the afternoon, the feeding activity of C. janthina stops; the larvae merely hang from the leaves. There is perhaps a correlation between the feeding activity of the larvae (starting the release of volatile substances) and the ability of the braconid to find its host. The braconid adults have never been observed feeding on the Rubus flowers. The adults in flight or on the Rubus leaves appear similar to adults of C. janthina , except for the antennae.

The adults of Proterops fly around the young second to fourth instar larvae to lay eggs in the larvae. Among the numerous old larvae collected towards the end of the last instar for pupation to be dispatched to Réunion Island, or kept in the laboratory to check for the emergence of adults, we have never obtained Proterops . This supports the fact that the parasitoids lay eggs on the young larvae and complete their development before the larvae reach the last (eighth) instar. Then the larvae with the parasitoid inside fall to the soil for premature pupation. The emergence of P. borneoensis always occurs after the emergence of the adults of C. janthina when the young larvae are starting to feed and most adult C. janthina are dead.

In the case of the Conura sp. the parasitoid emerges from the cocoon of Cibdela chinensis obtained from late instar larvae taken from the field. The life strategy of Proterops is, by comparison, entirely different. Once a year at least, generally in July–November after the drier months and at the beginning of the rainy season, there is a build-up of the Proterops population. If left uncollected this parasitoid is able to reduce almost completely the population of C. janthina , resulting in a very limited number of Cibdela individuals in the following generations. The build-up takes 6–8 months before a dense population of the sawfly is obtained.

Figures

Figure 2. (A–G) Cibdela janthina (Klug), Indonesia, Sumatra, Pematang Siantar. (A) Adult female of C. janthina searching for oviposition. (B) Ibid, but ovipositing in leaf vein. (C) Eggs of C. janthina in leaf veins. (D) Second instar larvae. (E) Fourth instar larvae. (F) Fourth instar larvae in typical feeding position. (G) Defoliation damage by larvae of C. janthina. (H) Proterops borneoensis Szépligeti, ibid. (H) Adult female of P. borneoensis.