HLB vector psyllid dispersal mapping

Diaphorina citri is the psyllid vector of HLB, a highly devastating disease in citrus orchards. Mapping the behavior and dispersion of the insect is essential for adopting appropriate management strategies.

08.03.2016 | 20:59 (UTC -3)

Huanglongbing (HLB) is the most devastating citrus disease in the world. In Brazil, according to data from the Agricultural Defense Coordination (CDA), the disease was responsible for the eradication of approximately 35 million orange trees since 2004. Furthermore, it reached 64,1% of plots in São Paulo, according to Fundecitrus withdrawals. The pronounced increase in the spread of bacteria associated with the disease is closely related to the displacement of the psyllid vector diaphorina citri in orchards, as the production of citrus seedlings is regulated in Brazil. Therefore, knowledge of the insect's displacement capacity and the factors involved in its flight are extremely important for the development of effective management tactics to combat the disease.

In this sense, with the objective of studying the distance and speed of flight together with the identification of the main factors acting in this process, a study was carried out at the “Luiz de Queiroz” School of Agriculture, University of São Paulo (Esalq-USP) , with financial support from Fundecitrus and CNPq, which investigated the effect of citrus plant sprouts and the feasibility of using bait plants as barriers to psyllid dispersal.

The study is part of the project entitled “Biotic and abiotic factors influencing the displacement of diaphorina citri Kuwayama (Hemiptera: Liviidae)", by doctoral student Arthur Fernando Tomaseto, from the Department of Entomology and Acarology at Esalq/Usp, with the assistance of undergraduate student in Agricultural Engineering Gabriel Margutti Passos, and its main goal is to understand the flight process of the insect and assist in the development of control measures to contain the spread of citrus HLB. Currently, the initial results are being prepared for publication in an international scientific journal.

The work used the methodology of marking, releasing and recapturing psyllids using four different post-fluorescent colors. The releases took place in an experimental area located at Fazenda Areão, in Piracicaba, São Paulo, after planting around 600 Hamlin orange seedlings. The seedlings were distributed in four adjacent experimental areas, formed by concentric circles located 18m, 24m and 30m from the release point. Insects marked with the same color were released into the same circular area. After release, the insects were recaptured in 200 yellow sticky traps hung on the branches of citrus seedlings at a density of one trap every 9 m.

Non-host plants

In this study, the influence of non-host vegetation covers [corn (zea mays) and signal grass (Brachiaria decumbens)] in the dispersion of D. citri. These plants were sown between the release center and the first citrus row (located 18m away) in each experimental area, being interspersed in the four sectors of the circles.

Four groups of a thousand insects were released per experimental area. Each group was marked with a distinct color of fluorescent powder. Insects that crossed the plant barriers were recaptured by the traps and analyzed to check their color. During release, signal grass had an average height of 50cm and corn, 2m.

According to the results, regardless of the presence of corn or signal grass, the number of insects recaptured was the same, that is, the use of a physical barrier 2m high did not interfere with the insects' flight. This can be explained by the insect having the ability to fly at altitudes greater than 2m and/or by having the ability to move very quickly, making it possible to bypass the physical barrier.

In this study, it was observed that when the orchard had sprouts, the insects tended to accumulate in the first row of citrus plants (located 18m from the release center), which encouraged the verification of the effect of sprouts on insect dispersal.

Effect of sprouts

Assuming that females of D. citri are dependent on shoots to lay eggs, it would be very likely that the movement of the insect would be influenced by this factor. Thus, two releases of four thousand insects each (one thousand in each experimental area) were carried out in two different conditions in the orchard. In the first of them, all orange plants were pruned to eliminate new vegetation, creating the condition of no shoots. And in the second, the insects were released when the orchard had shoots measuring 15cm-30cm. The traps were changed six hours, one, three and five days after release.

The results showed how quickly dispersion can occur in the field, especially when the orchard did not have sprouts. When the insects were released in the absence of sprouts condition, six hours after release were enough for some to fly at a distance of 140m from the release center, compared to just 45m in the presence of sprouts condition. The same difference was observed one day after release, when insects were recaptured at 200m and 60m, under conditions of absence and presence of sprouts, respectively. When evaluating the proportions of insects at each monitoring distance, there were no differences in psyllid recaptures at 18m, 24m, 30m and more than 30m in the no-sprout condition. However, the proportions differed when the insects were released under the presence of sprouts, with 85% of the individuals accumulating at distances of 18m and 24m from the release center. These results show that the phenological condition of the host plant can have a great influence on the movement of the insect in the field and, consequently, on the spread of the disease. These data suggest that the use of bait plants can be efficient as a tactic for controlling HLB in citrus.

Bait plants

After verifying the influence of shoots on the psyllid's flight, the feasibility of using bait plants as barriers or reducers of insect movement in the field was tested. In this experiment, the effect of myrtle seedlings (Murraya paniculata), citrus with buds and citrus without buds in the dispersion of D. citri. Approximately 50 pots of each test plant were placed between the release center and the first row of citrus plants in the experimental orchard (located 18 m from the release center). In one of the experimental areas, pots of host plants were not placed, characterizing the control treatment.

Four thousand marked insects were released (one thousand in each experimental area), evaluating the number of psyllids recaptured in yellow sticky traps hanging from plants located in concentric circles 18m, 24m and 30m from the release center. The assessments took place six hours, one and three days after release. Therefore, only those insects that managed to pass through the host plant barrier and reach the orchard plants would be recaptured.

According to the results, the number of insects recaptured per trap in the control was always higher than in the other treatments, suggesting that the use of a host plant for the insect can reduce its movement in the field. These data can help to understand the edge effect of citrus HLB, so characteristic in Brazilian orchards, that is, when populations of the insect arrive at a certain property, coming from external areas and making horizontal flights, they tend to stop in the first row of plants. hostesses they meet.

When comparing the different treatments, it was observed that in the circle containing myrtle (0,02 insect/trap), the number of insects recaptured was statistically lower than the treatment of citrus with sprouts (0,28 insect/trap) and control (0,80 insect/trap), suggesting great potential for this species as a bait plant.

Conclusions

Studies of dispersion and the factors involved in this process are extremely important for the management of plant diseases associated with vectors. Understanding the flight capacity of an insect vector helps to understand the cycle of pathogen-host relationships, which, in turn, constitutes one of the pillars for effective management of the problem.

The use of bait plants, with sprouts, as barriers to the movement of D. citri should be incorporated as another tool for the management of HLB in citrus, which currently consists of the use of healthy seedlings, eradication of symptomatic plants and regional control of the insect vector. These plants can be placed on the edge of plots that are constantly subject to psyllid infestations from external inoculum sources. Immigrant insects will accumulate on these plants and chemical control can be directed there, increasing mortality efficiency and reducing the spread of bacteria associated with citrus HLB.

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