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1. Introduction
Diploschema rotundicolle (Audinet-Serville, 1834) (Coleoptera: Cerambycidae) is a South American citrus pest distributed throughout the center-south regions of Brazil, Argentina and Uruguay1. The adults are elongated (25-40×8-10 mm) and characterized by a light-brown elytra with a continuous dark-brown border, dark-brown head, pronotum, antennae and legs2. The adults are nocturnal, females oviposit in the apex of branches upon young flush and leaf axils3. After egg eclosion, the larvae perforate the epidermis and once in woody tissue they dig longitudinal galleries heading to thicker branches, usually reaching the principal trunk4. By the end of the larval stage, the larvae prepare a pupal chamber with an exit opening for the adult to exit2.
In subtropical areas -Sao Paulo, Brazil- the larval stage was reported to be uninterrupted from 8-10 months, and the adults were observed from November to April2. In temperate regions -Santa Catarina, RS, Brazil- larval activity was recorded from January to October of the second year, reaching up to 20-22 months, with no activity in winter periods5. In these conditions, adults were reported active from November to January. In Pelotas, RS, Brazil, closer to Uruguay, adults were reported in February and March6. There are no records of biological aspects of this pest in Uruguayan climatological conditions.
Currently D. rotundicolle is considered a primary pest for citrus in Uruguay7. Over the last years, inefficient management of this pest has led to serious focalized population explosions, particularly in lemon orchards in the south (unpublished data). The deterioration caused by serious infestation levels is currently one of the limitations in an orchard’s service life and has a direct impact on its productivity and value. Larval feeding results in poor yields, tree weakening and indirect damages due to invaders of empty galleries1.
Woodborer control is extremely complicated since the larvae are protected inside the wood8. Insecticides are also much restricted in citrus crops destined to fresh fruit consumption. Therefore, the current management strategy for this insect relies almost exclusively on cultural control, which consists in the pruning of twigs with evidence of oviposition damage. This strategy has proven expensive and ineffective, partially because of imprecise timing of the interventions. No monitoring tools for adults are so far available.
The objective of this study was to determine the flight period of adults in the conditions prevalent in southern Uruguay and to evaluate a monitoring device for adults.
2. Materials and methods
Field surveys were performed in four citrus groves located in San José (Kiyú, 2 groves: 34°42′1″S 56°43′37″W; 34°25'59.8"S 57°40'56.3"W); Montevideo (El Espinillo, 34°48'58.6"S 56°22'53.6"W), and Canelones (Las Brujas, 34°37'13.1"S 56°21'18.5"W), all of them in southern Uruguay. Assays were performed from December to April, in four seasons (2015-2016, 2017-2018, 2018-2019 and 2019-2020). In Kiyú we worked in one grove, “Quinta 20”, in 2015-2016; the others seasons we worked in another orchard nearby (4,7 km apart), “Quinta 7”.
Homemade cross-vane traps with standard design (e. g., Alpha Scents Inc., West Linn, Oregon, USA; ChemTica Internacional SA, San Jose, Costa Rica) were used as trapping devices (black corrugated plastic or sheet iron; panel measures: 74×40 cm) and coupled in the bottom to plastic trap basins. These were partially filled with soapy water and salt to kill and preserve captured beetles. White fluorescent tubes (60 cm; 380-750 nm - Cold white, 6500 K) or LED light devices (50 cm per vane; LEDs 5730, 120 LEDs/m- cold white, 6000-6500 K) were coupled to the traps.
Populations were monitored with 1 to 3 traps/grove depending on the location and year. The traps were deployed within 1-ha citrus plots, 6 m apart from the plot border and with a separation of at least 20 m between traps. They were hung either from tree branches or a holder, with a height of 60 cm from the ground to the basin.
Mesh covers were set up around highly infested trees chosen from visual observation of abundant sawdust at their base. The trees were partially pruned and the mesh covered the whole tree structure, tied around the trunk at ground level. Throughout the seasons, a total of 25 trees were mesh-covered covering the four studied groves.
Traps and wrapped trees were checked weekly from early summer (mid-December) to early fall (April).
When present, Eucalyptus globulus windbreaks surrounding the citrus plots were checked for hidden D. rotundicolle adults. This was performed by extracting loosen barks and searching underneath for adults. Trees surrounding at least four citrus blocks were checked in each monitoring date.
Monthly visual observation of larval activity, recorded as presence of fresh frass in the base of the trees, was carried out throughout the whole year. Also, visual observation of oviposition damage was recorded. The damage is observed as characteristic wilting of the apex of branches in their first ca. 20 cm.
Weather recordings were obtained from Estación Experimental INIA Las Brujas, Canelones, Uruguay.
Statistical analysis: Male and female comparisons of caches in light traps, emergence on mesh-covered trees and extraction of windbreaks barks were all done with Chi-square tests.
Data included in the article were processed for clarity. Raw data are available upon request.
3. Results
Light traps showed good performance for monitoring D. rotundicolle adult flight. In all seasons and groves, adult catches were observed between late January and April. Peak of higher catches was observed around mid-February, except for the 2015-2016 season, in which higher catches were recorded later in February (Figures 1 and 2). Oviposition damage was observed shortly after the peak of higher catches: 2015-2016: February 16; 2017-2018: February 19; 2018-2019: February 20, and 2019-2020: March 27.
Results of the 2015-2016 season could have slight timing inaccuracies caused by monitoring dates.
In general, 2019-2020 season showed lower catches in comparison with previous seasons (Figure 1), which may be explained by a dry summer season with low precipitations (Figure 1 in Supplementary material). In 2018-2019, three groves were monitored and the results show slight differences in the onset of adult flights in the different groves (Figure 2).
Figure 1: Diploschema rotundicolle flight period in Kiyú groves, throughout the four surveyed seasons (2015-2020). Arrows indicate oviposition damage observation dates. Total catches are not comparable since devices (meshes and light traps) evaluated per grove per season were not equivalent. Cumulative data is shown for better observation of the peak
Figure 2: Cumulative Diploschema rotundicolle adult catches per trap in three groves in the 2018-2019 season
Significantly higher amounts of females were trapped in light traps in all seasons (5.2 females per male; P < 0,00001) (Figure 3A). Further, emergence upon mesh-covered trees also showed higher female emergence (1.8 females per male; P < 0,0001) (Figure 3B). A media of 6.8 ± 5.4 adults emerged per tree.
During the flight season, adults were found underneath loosen barks of Eucalyptus globulus windbreaks surrounding the citrus plots, particularly in seasons and groves with high population levels. Adults where either found as individual males or as one single male-female couple, suggesting that daytime shelter occurs with no gregarious behavior. Overall, adults under E. globulus windbreaks barks were more males than females (Figure 4).
Figure 3: Comparison of females and males captured in light traps (A) and emergence from mesh-covered trees (B). Data are shown as boxplots and correspond to cumulative catches in all groves and seasons. Asterisks indicate significant differences (Chi-square test, P < 0,05)
Figure 4: Extraction of D. rotundicolle adults from underneath loosen barks of Eucalyptus globulus windbreaks surrounding the citrus plots. Asterisks indicate significant differences (Chi-square test, P < 0,05)
Visual observation of larval activity was recorded throughout the whole year. Damage was not evenly distributed in the orchard but rather focused in certain parcels.
4. Discussion
Within the framework of integrated pest management, monitoring of insect populations in space and time is remarkably important in order to make informed decisions on control measures9. The results of this study showed that light cross-vane traps proved an effective tool for monitoring D. rotundicolle adults. Flight was consistently observed between January and April in southern Uruguay conditions, with population peaks between mid and late February. Emergence from mesh-covered trees was observed in the same period as adult catches in light traps. This may be an alternative method to detect the adult emergence period, although it is a more disruptive and laborious methodology.
Oviposition damage was generally observed close or shortly after the peak of higher catches in light traps, between mid and late February. However, in the 2019-2020 season, oviposition damage was observed approximately one month after the usual recorded period. This season, also characterized by low trap catches, was particularly dry, which may have caused increased eggs desiccation or delayed oviposition. Slightly lower relative humidity and higher temperatures were also observed during this season compared to 2017-2018 and 2018-2019 summers (supplementary material Figures 1 and 2).
Some variability was observed across seasons and groves. Particularly remarkable are the results obtained in season 2019-2020 in the three surveyed groves within Kiyú and Las Brujas, less than 50 km apart. In this season, flight onset was observed with a gap of about a month between groves. This flight period gap may be extremely relevant for an appropriate timing of cultural control interventions, and suggests that each grove should be monitored independently to maximize trimming effectiveness. Indeed, our observations indicate that trimming may be significantly optimized if it is planned according to adult monitoring information rather by the visual observation of wilted twigs, when the larvae may have already migrated proximally within the branch.
In this study we worked with 1-3 traps per citrus plot, a number of traps that proved to be sufficient in years and plots of higher population levels; when problematic situations arise and there is a need for stronger interventions. It should be mentioned, however, that poorer results were obtained in seasons of low populations, in which more conclusive results might have been obtained with a higher number of traps per plot. However, 1-3 traps were sufficient to generate general knowledge of the flight period, enough for management decision making at the grove scale. Further studies may be needed to define the monitoring strategy in situations of lower populations densities.
The fact that more females are trapped in light traps may be due to an unbalanced sex ratio, as observed in the emergence from mesh-covered trees. Another hypothesis is that females are the more mobile sex, and are trapped in their search for oviposition sites. Indeed, the trapping of more males underneath the refugee of loosen barks of E. globulus windbreaks, very close to citrus trees, is in line with this last hypothesis. It is possible that courtship and mating may take place within refugee sites, after which females take off for oviposition on citrus trees.
Interestingly, our confirmation that D. rotundicolle adults hide in eucalyptus trees enables another strategy of cultural control, that is, the manual removal of barks and beetles. This strategy may also be defined based on monitoring adult flight. Despite the fact that this measure requires crew labor and it might be as expensive as twig pruning, it may be performed intensively in more affected areas and seasons. Further, when possible, it would be desirable to remove this species of eucalyptus as windbreakers in groves and to avoid them in new orchards.
Finally, non-interrupted larval activity was observed throughout the year, as reported by Faria and others2; further, attacked trees within the groves were observed in focalized spots. The later observation highlights the need for an effort to monitor adult flight in different areas of the grove, to maximize not only the temporal but also the special benefits of following the dynamics of adult populations.
5. Conclusions
This study provides evidence that cross-vane light traps are an effective tool for monitoring D. rotundicolle adult flight period. One trap per ha would be enough to define interventions, especially when populations are high. In southern Uruguay, the adult flight period spans between January and April, with a peak around mid-February. Oviposition damage is generally observed around mid-late February. Eucalyptus globulus windbreaks surrounding citrus groves should be avoided since they provide suitable shelter for D. rotundicolle.
