In July 2015, Nicolas Odaglia, Research Engineer at CIRAD, was responsible for assessing the impact of a biocontrol action done eight years before in Grande Comore to control whitefly populations affecting the island’s coconut trees. The conclusion of his survey, carried out in collaboration with an INRAPE team, confirms the opinion of Comorian specialists: the results are positive. Coconut groves of the Comoros, although still old, are healthier and more productive than ten years ago and two parasitoids useful for the fight against whiteflies are present all around the main island.
Aleurotrachelus atratus… Behind this pretty, little and seemingly harmless whitefly hides a piercing-sucking pest of the coconut tree. Whiteflies colonize leaves to feed on the sap of the infested plant. They also secrete honeydew, a sweet substance that promotes the development of sooty mould on the leaves, a black fungus which inhibits photosynthesis. Both actions weaken the coconut tree, its nuts production decreases, and it can sometimes wither away.
First survey on coconut trees
To assess the impact of the biocontrol action led in 2007 against A. atratus, Nicolas Odaglia had, as a first step, to estimate the population density of whiteflies and their two main enemies in Grande Comore. Therefore, he took samples on coconut trees in 23 locations around the island, covering nearly all the coastline and a few sites in the hinterland (Figure 1).
It appears from this ecological survey that the majority of the Comorian coconut trees are healthy. The average larval density of Aleurotrachelus atratus is 0.34 per cm2. The pest populations seem now maintained at an acceptable level in Grande Comore, notably thanks to parasitism. Indeed, two parasitoids of the whitefly, Eretmocerus cocois and Encarsia basicincta were found on the majority of the sampled sites.
We are far from the early 2000s when the coconut plantations in the Comoros islands were damaged due to A. atratus, leading to heavy losses in yields and even, in the most harmful situations, to the death of the trees. The Comorian Ministry of Agriculture reckoned at the time that this whitefly was the cause of a coconut production drop of 87% in Grande Comore, 61% in Anjouan and 40% in Moheli. This then represented a scourge for those islands covered with coconut trees that play an essential role within the Comorian society (food, traditional medicine, etc.).
It was this situation that had prompted INRAPE and CIRAD, in 2007, to conduct this first biocontrol action of against A. atratus, as part of the PRPV program. The parasitoid then introduced was E. cocois, collected in Reunion Island. This tiny wasp has the particularity of laying inside the whitefly larvae. The larvae of the parasitoid develop at the expense of their hosts and emerge as adults, leaving the whiteflies dead.
Nicolas Borowiec, former entomologist for CIRAD, had placed a coconut tree infested with whiteflies in a containment cage and had then released 300 females of E. cocois inside the cage. Once the parasitoid acclimatized to his new surroundings and its effectiveness confirmed by monitoring the whitefly population, the cage was removed to let it disperse naturally in the entire island.
These actions in 2007 were also the opportunity to carry on an ecological survey to know the distribution of whiteflies in the Comoros. It was during this investigation that another enemy of the whiteflies was discovered, probably resulting from an accidental introduction. This species has now been identified as Encarsia basicincta. This is the major coconut whitefly parasitoid in Grande Comore, with about 77% of parasitism today, against 23% for the one introduced by CIRAD.
Ecological survey on papaya trees
Alongside this assessment of the state of the Comorian coconut trees, Nicolas Odaglia’s mission had a second goal: to carry out an ecological survey focused on the papaya mealybug, Paracoccus marginatus. This insect, extremely polyphagous, feeds on more than 60 plant species, including some major crops such as cassava. The bug sucks plant sap by inserting its stylet into the epidermis of the leaf, stem and fruit and while feeding on the plant fluid, it injects its toxic saliva into the host which ultimately leads to the death of the plant. This time, the objective was to estimate the level of infestation of the mealybug on the papaya trees of Grande Comore and to establish whether or not this pest has enemies naturally present. This survey was essentially aimed to assess the need for a new biocontrol action.
The sampling took place on 19 of the 25 sites (Figure 1). "We will not have the parasitism rate because it is impossible to determine on colonies like these", tells us Nicolas Odaglia. "There really are thousands of larvae [editor’s note: on the papaya leaves] and it is impossible to count them, whereas on a coconut tree, you can easily count the parasitized larvae and the ones emerged as whiteflies, and thus determine a rate of parasitism."
He adds: "The papaya trees seem more infested than on Reunion Island, with large mealybug colonies on the lower level of the leaves. But, according to INRAPE officials, the trend is improving. The plants are generally less attacked and fruits are spared in most cases."
Because, and this is good news, parasitized mealybug larvae, also known as mummies, are present in almost every sample. They were isolated in test tubes in order to retrieve the parasitoids likely to emerge from them. "At least three types of parasitoids have been morphologically discriminated, two of which are particularly similar to two species found on Reunion Island: Marietta leopardina et Acerophagus nubilipennis" says the Research Engineer.
Biomolecular analysis will be carried out to confirm the identification of those individuals. They will be carried out through a process of DNA barcoding in the course of 2016 at the Plant Protection Platform (3P) of Saint-Pierre, Reunion Island. This process will establish the genetic characterization of the insects collected in the field thanks to one of their genes, and thus reveal their "true identities".
If these tests show that some of these mealybug parasitoids are indeed Acerophagus nubilipennis, their introduction in Grande Comore will be useless since there would be naturally present. "Rearing parasitoids in the INRAPE laboratories may be considered in order to release them in the most infested areas" concludes Nicolas Odaglia but an additional introduction of auxiliary insects will probably not be necessary.
These ecological surveys in Grande Comore and the barcoding process carried out at the 3P in Saint-Pierre were the subject of a reportage realized in the framework of the ePRPV project:
 Aleurotrachelus atratus Hempel., piercing-sucking Hemiptera of the Aleyrodidae family from neotropical origins, found on more than 50 species of palm trees, described for the first time in Brazil in 1978 on a coconut tree (Cocos nucifera), its main host.
 Eretmocerus cocois Delvare sp. n., Hymenoptera of the Chalcidoidea family.
 Encarsia basicincta Gahan 1927, Hymenoptera of the Aphelinidae family.
 Delvare G. et al., Description of Eretmocerus cocois sp. n. (Hymenoptera : Chalcidoidea), a parasitoid of Aleurotrachelus atratus (Hemiptera : Aleyrodidae) on the coconut palm, Zootaxa, 2008, n°1723, p. 47.
 Paracoccus marginatus Williams & Granara de Willink 1992, piercing-sucking Hemiptera of the Pseudococidae family, originating from Central America.
 Chellappan M. et al., College of Horticulture, Kerala Agricultural University, Thrissur; PPNMU, Kerala Agricultural University, Thrissur Host range and distribution pattern of papaya mealy bug, Paracoccus marginatus Williams and Granara de Willink (Hemiptera : Pseudococcidae) on selected Euphorbiaceae hosts, Kerala Journal of Tropical Agriculture, n°51, 2013 (1-2) p. 51.
 Acerophagus nubilipennis Dozier 1926, parasitoid of the Encyrtidae family.