For about five years, citrus plantations in Galicia and northern Portugal have been home to one of the biggest threats to Mediterranean citrus farming: the African psyllid Trioza erytreae. This insect is the main vector of greening disease, or Huanglongbing (HLB), an incurable bacterial disease whose only control method is the eradication and containment of its vectors.
"We must stop the expansion of the psyllid to prevent it from reaching the main Spanish and Portuguese citrus growing areas, because it is a matter of time before the bacteria is detected," says researcher Alberto Urbaneja, coordinator of the Center for Plant Protection and Biotechnology of the Valencian Institute of Agricultural Research, IVIA. After the detection of the vector in Spain and Portugal, the ministries of agriculture of both countries launched programs to eradicate and contain the psyllid vector, but this has overcome all barriers.
Although the disease has not been detected in Spain or Portugal, IVIA researchers have been working for years in containing the spread of the psyllid. One of the most promoting lines of research currently underway is based on the introduction of a wasp of sub-Saharan origin that is a parasite to the psyllid vector: the Tamarixia dryi.
At the end of 2017, the Spanish Ministry of Agriculture, Fisheries and Food approved the import of this insect in order to study its potential before releasing it in the fields. Its breeding is carried out at the facilities of the Canary Islands Institute of Agricultural Research (ICIA) with funding from a national project financed by INIA, led by the IVIA and in which other national research centers involved in citrus research are taking part.
"Since the establishment of its breeding in 2018, the wasp has shown great effectiveness in controlling the psyllid in the Canary Islands," said ICIA researcher Estrella Hernández, who has monitored the spread of the wasp on the islands. Vector populations have decreased dramatically in the Canary Islands since the release of the wasp, to the point that it is currently difficult to find them.
The Ministry of Agriculture, Fisheries and Food has taken every precaution before authorizing its release in Spain. Although no scientific reference showed that these wasps could transmit bacterial diseases to plants, all the insects that were imported from South Africa, as well as their first offspring, were analyzed to confirm that they were free of this disease.
In view of these results, the Directorate General of Health of the Agricultural Production of the Ministry of Agriculture, Fisheries and Food of Spain authorized the experimental release of the wasp in Galicia, with the first ones being released at the end of October 2019. Within the framework of the collaboration in this area between Portugal and Spain, the Portuguese government has also released the wasp in its citrus plantations this fall.