Spain: New pests force an update of the Integrated Management program in citrus

The unexpected, sudden and widespread appearance of Scirtothrips aurantii in four regions of Alicante and Valencia requires a review of the Integrated Pest Management program for citrus. 

It is a crop that has been suffering for years from the continuous introduction of new invasive species. 

The Guide published by the ministry was updated in 2022, with the inclusion of new pests, which have been appearing in recent years, such as the new scale insects Delottococcus aberiae and Pulvinaria polygonata, the tetranychid mites Eutetranychus banksi and Eutetranychus orientalis, and the thrips species Chaetanaphothrips orchidii and Scirtothrips dorsalis. 

These new detections, together with the change in population dynamics, both of the various phytophagous insects that attack citrus fruits and their natural enemies, require an update of the pest management program.

Alberto Urbaneja, an entomologist at the Plant Protection and Biotechnology Centre of the Valencian Institute of Agricultural Research, explains that, in addition to the increase in the diversity of pests present in citrus areas, there are changes in the biological cycles and in their incidence, as well as in that of their natural enemies: the number of generations per year of many species has varied in recent times due to climatic conditions. Thus, the Valencian research centre is already collecting data on the seasonal dynamics and population structure of the main pests, and their predators and parasitoids.

Following the recent emergence of S. aurantii in four regions of Valencia and Alicante, the IVIA and the Plant Health Service are going to launch a new research project on this polyphagous species, which, for the moment, has also affected pomegranates, persimmons, and table grapes, but could become established in other crops, such as avocado, and on Scirtothrips dorsalis, also present in Alicante.

This project could last four years, which is necessary to complete a thorough and detailed analysis of the host plants of thrips and the implementation of sustainable solutions. It will combine knowledge of entomology, botany, ecology and agronomy. It includes, among other factors, the study of the biology and ecology of Scirtothrips sp.; the development of a sampling protocol and the calculation of treatment thresholds; the study of host plants; the search for natural enemies; insecticide efficacy tests; the establishment of best cultural practices; and its inclusion in the Integrated Pest Management program.

At a conference held at Finca Sinyent of the Valencian Farmers Association (AVA-ASAJA), Urbaneja called for the ″active participation″ of the agricultural sector and all the actors involved, farmers, technicians and scientists to ensure that the results of this project ″are relevant and applicable in the field″. The researcher warned that one of the main risks of S. aurantii is its rapid ″acquisition of resistance″ to insecticides.

José Catalán, a researcher at IVIA, explained that 25 species of thrips are associated with citrus, but the majority do not cause damage. In the case of the so-called South African citrus thrips, the damage is aesthetic: it does not affect the organoleptic quality of the fruit, but it depreciates its commercial value.

In persimmon and pomegranate, the damage is similar to that observed in citrus. On leaves, it causes silver spots and scars due to the feeding of larvae and adults, which scrape off the epidermal cells. It produces silver spots, scars and deformations on fruits that affect commercial quality. In young shoots, it can cause necrosis and premature leaf fall if the infestation is high.

S. aurantii thrives in warm climates with moderate to high temperatures. Regions with mild winters and hot summers are particularly vulnerable. The availability of suitable host crops, such as citrus and other fruits, influences its ability to establish in new areas.

Vicente Dalmau, head of the Plant Health Service, maintains that the new pest has only recently arrived in the Valencian Community; otherwise, it would have already been captured by the numerous chromotropic traps spread throughout the citrus-growing regions. ″It is a mystery how quickly it has spread. It will surely appear in new plots as the surveys progress,″ predicted Dalmau, who warned that, as it is a quarantine pest, it could affect the export conditions of the affected crops.

Faced with this new danger to Valencian agriculture, the Plant Health Department has quickly drawn up recommendations for its control. In citrus trees, tasks such as pruning to improve the ventilation and lighting inside the tree canopy are recommended. These seem to make it difficult for insect populations to establish themselves; however, they must be balanced to minimize the appearance of new shoots or suckers due to their effect of attracting the pest and being an ideal niche for its multiplication. Pruning will facilitate adequate wetting of the tree and thus improve the effectiveness of phytosanitary applications.

The natural spread of this pest is relatively limited, so it is essential to properly organize pruning, harvesting, and transport work and apply hygiene measures between plots to avoid possible involuntary spread to new areas. This would include cleaning harvesting boxes and baskets, workers' clothing, and vehicles.

It is also advisable to avoid the presence of vegetation that may host this pest; in the case of having plants or plantations of host crops in the vicinity of the plot, it is advisable to be extremely vigilant. Likewise, the placement of yellow chromotropic adhesive traps can facilitate the detection of thrips before it feeds on the recently set fruit and damages it.

The Plant Health Department reminds us of the importance of respecting the auxiliary fauna present in the crop. ″Although it is possible that predatory insects (Franklinothrips megalops, Orius spp., Chrysoperla spp.) or phytoseiids do not control the pest on their own, they contribute to its control or even prevent its establishment in the plot,″ it reported. The introduction of mites or predatory insects or other biological control organisms can be a complementary control tool, while the rational addition of organic fertilizer (manure) can be beneficial to increase the populations of soil predatory mites that exert control over the phases of the pest developed in leaf litter and soil.

Regarding chemical control, it is crucial to intensify surveillance and prospecting of crops to detect their possible entry as soon as possible and, if their presence is confirmed, to plan the most appropriate phytosanitary interventions. It is essential to refrain from carrying out repeated and indiscriminate phytosanitary applications (especially of pyrethrins or pyrethroids) due to the impact on auxiliary fauna and the predisposition to the proliferation of phytophagous mites. In addition, the residue level (MRL) of the active ingredients used must be taken into account and active ingredients must be applied with the same mode of action, twice in a row to minimize the risk of resistance developing.