Phyllophaga capillata is recognized for producing swarms with high population densities in a relatively short time of the year (~ 30 days), making it possible to collect from 3 to 7 thousand specimens in a single collection night with light traps (Oliveira 2019; Oliveira and Frizzas 2019). Modifications in the areas attacked by larvae between plots within the same property or between nearby properties over consecutive harvests are common for this pest, suggesting that the dispersal ability of adults is the determining factor for infestation patterns of the pest over time. This is the first study to evaluate the ability of P. capillata to displace in the field. Our main findings show that the species has a low recapture rate, with the largest number of adults recaptured at 50 m; however, we found that the species moves at least 250 m, that the distance of displacement increases over the swarming period, and that males have a greater capacity to move. We also observed that adults of this species showed a greater predisposition to fly on consecutive days.
In general, the recapture rates for Coleoptera in mark-release-recapture studies are not very high (Ranius and Hedin 2001; Arellano et al. 2008; Mansfield et al. 2016). The capture rates for P. capillata found in the present work ranged from 0.85–2.61%. In a study conducted in the United States, adult males of P. anxia (LeConte) were recaptured in traps containing the species' sex pheromone at rates ranging from 11–28% over the three years of the experiment (Robbins et al. 2008). It is possible that the differences in recapture rates may be related to the distances and type of trap used. For P. anxia, traps were positioned 20 m from the release point with 15 m between them, and we used longer distances (50, 150, and 250 m), which may have reduced the potential for insect recapture. Trap density (number of traps/area) decreases greatly with increasing distance (Lacey et al. 1995). In fact, higher recapture rates of P. capillata were observed where trap density was higher (at 50 m). Studies have also shown that pheromone traps are more efficient than light traps. The species P. brevidens (Bates) and P. lenis (Horn), for example, were attracted approximately 3.3 times more by sex pheromone traps than by light traps, and using both attractants in the same trap attracted approximately 17.5 times more adults than light alone (Zaragoza-Ortega et al. 2017). If P. capillata responds similarly to other congeneric species, the use of sex pheromones could help increase recapture rates for this species, once these pheromones are described and available for use in the field.
We observed that throughout the swarming period, the adults of P. capillata tended to increase the distance of displacement. In fact, in the first two collection dates, corresponding to the first half of the swarming period, the displacement was predominant up to 50 m (Fig. 2), suggesting that in this phase, the adults concentrate their activities in finding mating partners and show a tendency to move less. In the second half of the swarming period, recaptures at 150 m were similar to 50 m (third collection date), and 250 m became the predominant distance at the end of the swarm (fourth collection date) (Fig. 2), indicating that specimens may be more likely to disperse after copulation events.
Overall, males were more recaptured (66.7%) than females (33.3%), only this sex reached the greatest distance of displacement (250 m), and only males were collected on the second and fourth collection dates. These data suggest a greater ability of P. capillata males to move. Interestingly, on the second release date, although approximately 82% of females were released, only males were recaptured, confirming that males were more active on this date, perhaps in search of mating partners. Studies of P. crinita (Burmeister) have shown greater activity of males than females, which are less attracted to light traps and fly short distances to oviposits (Teetes et al. 1976; Stone 1986). We observed that females of P. capillata, however, are able to fly at least 150 m, a distance much greater than that reached by females of a congener, P. cuyabana, which in Brazil reached 20 m (Oliveira and Garcia 2003). In another study, P. bruneri was able to move for approximately 137 m (Wolfenbarger et al. 1976). The data obtained suggest that adults of P. capillata were able to move greater distances than other species of the genus Phyllophaga already studied (Wolfenbarger et al. 1976; Oliveira and Garcia 2003).
Studies have shown a pattern in the sex ratio of P. capillata throughout the reproductive period, with a predominance of males at the beginning and females at the end of the swarm and equal rates of males and females in the middle of the swarming period (Oliveira and Frizzas 2019). Here, we did not find this pattern in the insects collected to conduct the mark-release-recapture study on the four collection dates, with more males on the first and fourth collection dates, more females on the second date, and balance between the sexes on the third collection date. This result may have occurred because the present study was carried out in an atypical, drier year. The first rains in September and October of each year are the trigger for the resumption of adult insect activity in the Cerrado (Silva et al. 2011; Oliveira and Frizzas 2008; 2013; 2019). In the studies by Oliveira and Frizzas (2019), also in the Federal District, there was a rainfall of 201 mm, while in the present study, rainfall rates reached less than 50 mm in the same period. This factor may have affected the swarming pattern and consequently the sex ratio of P. capillata.
Regarding flight activity, we observed that most specimens of P. capillata (> 69% of males and > 52% of females) showed flight activity on consecutive nights. In contrast, adults of P. cuyabana showed flight activity on alternate nights, with this behavior being more marked for males (Oliveira and Garcia 2003). The results of the studies of flight activity under controlled conditions were confirmed in the field, since on the four collection dates, the percentage of adults that abandoned the release point ranged from 59.3–95.8%; on the dates when more males were released (first and fourth release), the percentages of insects that flew ranged from 86.3 to 95.8%; and on the dates with a predominance of females, this percentage ranged from 59.3–77.7%. These results, along with the recapture data, reinforce the hypothesis that males have a greater tendency to fly than females. We suggest that males move more during the entire swarming period in search of a mating partner and that females during some moments of the swarming, especially after copulation, may dedicate themselves more to oviposition, being less captured or becoming less attracted to the light trap.
During summer plantings in the Federal District, approximately 60% of the cultivated area is occupied with soybean crops, and the Federal District is responsible for the fourth highest productivity of the crop in Brazil (IBGE 2022; CONAB 2022). This wide crop coverage has as a consequence the spatial proximity between crops in the various soybean producing properties. Our results showed that adults of P. capillata showed good dispersal capacity (at least 250 m), with most adults flying on consecutive days. In addition, P. capillata presents a synchrony between the resumption of adult activity and the onset of rainfall in the Cerrado, a time when soybean planting occurs, allowing larvae of this species to find host plant availability (Oliveira and Frizzas 2019). This set of factors may explain, at least in part, the colonization of new plots or new properties by P. capillata, since adults can move for distances ranging from 50 to 250 m in just one night, facilitated by the proximity between the various soybean growing areas at the beginning of the rainy season. Several factors, however, influence the choice of new soybean areas during the swarming process by P. capillata adults. For example, P. capillata adults prefer to colonize areas with the presence of taller plants, which serve as points for sexual pheromone release by the female (Oliveira and Frizzas 2019; 2020). Thus, future studies that determine the main factors that influence the choice of adults for new areas for colonization, coupled with the knowledge of the dispersal capacity of this species, may allow design of preventive management strategies for P. capillata in Central Brazil.