Vanilla

Serge Quilici, Agathe Richard, and Kenny Le Roux

A few years after it was recorded on Reunion Island, a French island in the southwestern part of the Indian Ocean, the scale Conchaspis angraeci Cockerell, 1893 was of increasing concern for the vanilla producers on this island because of the damage it caused to this crop. By absorbing the sap of its host plants, this armored scale first leads to the formation of chlorotic spots on the leaves, then to a drying out of the leaf, and eventually, in extreme cases, to the death of the vine itself. As this pest has a wide geographical distribution over the recent years, we review its taxonomic position, its biology, ecology, natural enemies, and control.

Taxonomy and Morphology

The Angraecum scale or vanilla scale, C. angraeci, belongs to the family Conchaspididae (Hemiptera: Coccoidea) whose members closely resemble Diaspididae, with which they have long been confused. The family Conchaspididae is now regarded as a separate group within the Coccoidea. Female species in this family have a feature in common with members of the family Diaspididae—they build an armor that is independent from the rest of the insect body and their last abdominal segments are fused to form a pygidium, which is involved in the secretion of the armor. The genus Conchaspis contains 22 species (BenDov, 1981). One of the main morphological characteristics of C. angraeci female species is the aspect of the armor, circular in outline and conical in shape. The cone presents 6–8 ridges, radiating from the apex but ending without reaching the margin (Hamon, 1979). The armor is usually white, but may look grayish or yellowish in some cases.

Geographical Distribution

This species, common in tropical and subtropical areas, was first described from Jamaica (Cockerell, 1893). For a long time, it was mainly restricted to the Nearctic and Neotropical regions, whereas it is known in Mexico, the United States, Bermuda, Barbados, Cuba, Brazil, Ecuador, Nicaragua, Panama, Peru, Puerto-Rico, Surinam, Trinidad and Tobago, and Venezuela (BenDov, 1981). It was only in 1973 that it was also recorded in the Afrotropical region, in Angola and Cameroon, on Acalypha spp. (BenDov, 1974). It has since been found in Hawaii (Beardsley, 1993), in Australia (BenDov et al., 1985), in the Fiji Islands (Williams and Watson, 1990), in Malaysia (Takagi, 1992, 1997), and in French Polynesia (Quilici, 2002). In the southwestern Indian Ocean, it was first recorded in Reunion Island in 1997 (Richard et al., 2003), then in Mayotte, a French island belonging to the Comoros archipelago (Richard, 2003), and more recently in Mauritius in 2007 (S. Quilici and J.C. Streito, unpubl. data).

Host Plants

The species was described from material collected as Angraecum eburneum Bory var. virens (Orchidaceae) (Cockerell, 1893) but appears polyphagous and is recorded on numerous host plants belonging to various families among which the Euphorbiaceae and Orchidaceae are the most common (BenDov, 1981; Hamon, 1979; Mamet, 1954). In Florida, its host plant has a wide range and includes about 30 species of woody plants and orchids such as Plumeria sp. (Apocynaceae), Stephanotis sp. (Asclepiadaceae), Rhododendron sp. (Ericaceae), Acalypha wilkesiana Müll., Codiaeum variegatum (L.), Euphorbia lactea Haw. cv. “cristata” (Euphorbiaceae), Flacourtia indica (Burm. F.) Merrill (Flacourtiaceae), Hibiscus rosa-sinensis L. (Malvaceae), Ficus sagitata Vahl (Moraceae), Bougainvillea sp. (Nyctaginaceae), Angraecum superbum Thouars, Epidendrum secundum Jacq. (Orchidaceae), Pittosporum tobira Ait. (Pittosporaceae), Coccoloba uvifera (L.) (Polygonaceae) . . . (Hamon, 1979). In Reunion Island, apart from vanilla, it was also found on an exotic vine belonging to the Asclepiadaceae: Hoya bella Hooker (Richard et al., 2003), on Angraecum eburneum Bory and Bulbophyllum prismaticum Thouars (Orchidaceae). In Mauritius, very high populations were observed on Euphorbia lactea Haw. (S. Quilici, unpubl. data).

On vanilla, it is recorded on Vanilla planifolia (G. Jackson) in Fiji (Williams and Watson, 1990), Reunion Island (Richard et al., 2003), Mauritius (S. Quilici, unpubl. data), and Mayotte (Richard, 2003) and also attacks Vanilla tahitensis J.W. Moore in French Polynesia and Mauritius (M. Grisoni, pers. comm.). In the island of Mayotte, it has also been observed on Vanilla humblotii Rchb. F. (R. Gigant, pers. comm.).

Symptoms and Damage

On vanilla crops, on which the scale causes damage, it is eurymerous and may develop on leaves, stems, aerial roots, or pods (Richard et al., 2003). With its sucking mouthparts, the scale probably injects a toxin through leaves or stems, which produce chlorotic spots evolving into necrosis, and leading to the weakening of the vine and eventually its death (Figure 9.1). In Florida, damage leading to plant weakening, has been observed to be severe on Hibiscus, Pittosporum, orchids, and seagrapes (Hamon, 1979).

FIGURE 9.1 (See color insert following page 136.) Damage caused by the Angraecum scale: (a) on a vanilla plant; (b) detail of damage on a vanilla leaf.

In Reunion Island, though no precise economic study has been conducted to evaluate the damage caused by this scale, many producers claim that they observed a decrease in their production over the years following the arrival of the scale (Richard et al., 2003). A survey conducted in 2002 showed heavy infestations in the vanilla plots of the eastern area of the island and in 2003, the scale extended its distribution area and damage up to the southeastern part of the island.

In the Comoros archipelago, the scale was detected in 1999 on the French island of Mayotte, where it rapidly spread; a survey conducted in 2007, showed high infestations, with up to 50% of the vines attacked on particular plots, while the scale was not detected in Grande Comore and Moheli, belonging to the Islamic Republic of Comoros (Grisoni and Abdoul-Karime, 2007). In Mauritius, limited damage was observed in 2007 in commercial shade houses in the north of the island (Grisoni, 2007). In French Polynesia, the damage observed in 2002 was apparently limited to localized outbreaks in a few plantations in Moorea and Tahaa (M. Grisoni, pers. comm.). Although extensive surveys were conducted since 1998 in the major vanilla-growing areas of Madagascar, C. angraeci has not been reported in this country (Alabouvette and Grisoni, 2009).

Life-History Traits and Ecology

Few data are available on the biology of this pest species. Takagi (1992) showed that the females in the genus Conchaspis have only three instars (L1, L2, and adult), while males have five instars (L1 mobile then fixed, L2, prepupa, pupa, and adult). In C. angraeci, males are unknown and this species probably reproduces mostly by parthenogenesis. The female lays a maximum of 80 eggs under its armor (Figure 9.2). After hatching, the mobile first instar larva (L1), which is the only mobile instar (crawler), quickly gets out of the armor. After its mobile phase, the L1 then settles and starts secreting its own armor. After some growth, it transforms into an L2, which changes into an adult female, beginning to lay eggs at the end of its pre-oviposition period. During rearing trials on young vanilla vines, the duration of the preimaginal cycle appeared to be long (90 days at T = 25°C; RH = 70%; photoperiod: L12:D12).

FIGURE 9.2 Different instars of Conchaspis angraeci: (a) adult female; (b) female with armor removed, showing eggs; (c) first instar (L1), second instar (L2), and adult female.

In Reunion Island, while vanilla crops are localized in the eastern, the southeastern and the southern part of the island, surveys carried out in 2001–2003 showed that the species was more prevalent in the eastern and south-eastern areas, which are also the areas with a higher rainfall. A study conducted in four vanilla plots in 2002– 2003 showed that larval stages (L1 and L2) were more abundant during the warm season (December to April). The adult stage dominated compared to the other stages, whatever the cultural mode (intensive crop, intercropped with sugarcane, under trees, shade house) or the season.

Natural Enemies

Records of natural enemies of this scale are recent. The first parasitoid was recorded in Hawaii by Beardsley and Tsuda (1990) who mention that, some ten years after its arrival in 1980, the scale was heavily parasitized by the Aphelinidae Marietta pulchella Howard, whose larvae develop in the same way as Aphytis spp. do on Diaspididae. Though the species in the genus Marietta are most frequently hyperparasitoids, the authors showed that M. pulchella does develop as a primary parasitoid in this case, which is the first record for a species in this genus. In Reunion Island, three parasitoid species have been found associated with the populations of the scale: one endoparasitoid, Encarsia lounsbury Berlese and Paoli (Aphelinidae), and two ectoparasitoid: Aphytis africanus Quednau (Aphelinidae) and Cephaleta sp. (Pteromalidae). While the first two species attack L1 and L2, the third one parasitizes the female adults (Richard et al., 2003). Globally, the parasitism rate remained very low, among the studied biotopes and the recorded parasi-toids are probably not specific to the vanilla scale.

In Reunion Island, a few predatory species, mostly thrips and mites, are also associated with high levels of population of the scale. Among the five thrips species found, three are known as scale predators: Aleurodothrips fasciapennis (Franklin), Karnyothrips flavipes (Jones), and Karnyothrips melaleucus (Bagnall), belonging to the Phlaeothripinae. Among the mites, two species have been collected, which are known to feed on eggs or mobile young instars of scale insects: Saniosulus nudus Summers (Eupalopsellidae) and Bdella distincta Baker and Balock (Bdellidae). Other species have also been collected, which are known to have predaceous habits: Cheletogenes sp. and Hemicheyletia wellsi (Baker) (Cheyletidae), Bdellodes sp. and Spinibedella depressa (Ewing) (Bdellidae) and Asca sp. (Ascidae). In Hawaii, the predatory mite Cheletogenes ornatus (Canestrini and Fanzago) has also been found preying on C. angraeci on the host Hibiscus (Goff and Conant, 1985). In Reunion Island, an entomopathogenous fungus, Fusarium coccidicola P. Henn, has been recorded, which is one of the four known species of Fusarium that is pathogenic on scale insects (Richard et al., 2003). Globally, the predation rate appeared low (2.5–6.5%), with a higher rate observed in plots situated under the cover of a forest.

Control Methods

In Hawaii, biological control solved the problems linked with the heavy infestations of C. angraeci: soon after its discovery in 1989, the Aphelinidae Marietta pulchella exerted a complete control on the scale, which is now considered to be rare (Beardsley, 1993).

Very few data are available on the results of chemical control trials against this scale. Experiments conducted in 2004 by the Plant Protection Service of Reunion Island showed a high effectiveness of imidacloprid (C.P.: Confidor; Bayer CropScience) and a good effectiveness, though delayed and inferior to that of imidacloprid, is of mineral oil (C.P.: Ovipron; Cerexagri) (SPV Réunion, 2004). Further experiments using mineral oils are required, with applications at different periods of the year, to determine its effectiveness in controlling the scale. As mobile instars are more sensitive to chemical control, such treatments, if not phytotoxic, should preferentially be applied during summer when the first instar is more abundant and active.

The vanilla scale has demonstrated, at least in certain areas, its ability to cause significant damage to vanilla production. Despite a few studies devoted to this scale, our knowledge is limited about the bio-ecology of this pest. Additional studies would be helpful to improve our knowledge on its natural enemies and their impact, as well as on possible control methods.

Acknowledgments

The authors express their sincere thanks to the taxonomists who identified the para-sitoids (G. Delvare, CIRAD, Montpellier, France), the predatory mites (E.A. Ueckermann, PPRI, Pretoria, South Africa), and the thrips (B. Michel, CIRAD, Montpellier, France).

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