Population fluctuation and infestation of Drosophila suzukii in berry crops in Southern Brazil

Drosophila suzukii is an insect pest with high capacity to cause damage to soft-skinned fruit whose behavior on host crops in Brazil is still unknown. This study aims to determine the population fluctuation of D. suzukii on different berry crops from 2014 to 2017 and infestation. In each orchard, two traps containing attractant (biological yeast + sugar + water) were installed each week; and sorting was done in laboratory after two days hold-up period. At the same time, fruits were harvested to assess the presence of D. suzukii. The emergence of D. suzukii was observed in December (late spring), population increased from January to March (summer) and terminated in June (late fall). In the winter, there were no adults in the traps. The crop of blackberry was highly preferred in first two years, followed by raspberry and strawberry. In the third year, the preference was for raspberry. The blueberry is less attacked due to the asynchronization between the fruiting time and occurrence of the pest.


INTRODUCTION
Drosophila suzukii (Matsumura, 1931) (Diptera, Drosophilidae) is a polyphagous quarantine pest (Cha et al., 2014) with elevated dispersion capacity, which cause damage in developing fruits.The registered species in 2008 was in several places in the United States, Canada and EU; they bring about elevated damage to various fruit crops (Hauser, 2011).In Brazil, the first registered occurrence was made at the Vacaria City Region, at Rio Grande do Sul State, (In Southern Brazil), with 30% of damaged fruits of strawberry (Santos, 2014).Worldwide, fruits and its sclerotized serrated ovipositor is capable of puncturing the skin of straight fruits for oviposition (Lee et al., 2012;Wallingford et al., 2015).Burrack et al. (2017) reported that in USA, the insect pest has a pattern of preference for some crops and the synchrony between fruiting and the presence of the insect pest is a key factor for the management of D. suzukii.As the pest was identified in Brazil at only four years, there is a lack of information about the population aspects on host crops as well as the resulting damage.These aspects limit the preparation of control and management practices of this species in Brazil.Thus, the objective of this study is to determine the population fluctuation of D. suzukii on the crops of blackberry, raspberry, strawberry and blueberry to correlate the occurrence with fruit infestation with eggs.

MATERIALS AND METHODS
The study was carried out in a commercial orchard with diverse berries production, located at the Vacaria city region, Rio Grande do Sul State,Brazil (28°28'41.95"S and 50°58'11.74"W)between August of 2014 and December of 2017.The assessed crops were blackberry (variety "Tupy" and "Thompson"), and raspberry (variety "Heritage") both crops grown in the soil, under plastic high tunnel cover; strawberry (variety "Albion") grown into greenhouse and cropped on substrate, and blueberry (variety "O"Neal" and "Duke") grown under hail nets (Figure 1).For the study, each crop was considered as an experimental unit.
In each crop, two traps were installed for monitoring.It was made from colorless Coca Cola™ plastic bottles (250-ml) previously washed with tap water and dried which contained five holes of 0.5 cm diameter; they were filled with a yeast based Sacharomyces spp., sugar and water as suggested by Santos (2016), for monitoring adults of D. suzukii (Figure 2).
Traps were changed at weekly interval and placed at the intermediate position of the plant"s canopy (blackberry, raspberry and blueberry) and above the plants on strawberry, keeping for two days hold up period.They were gathered for assessment at the Embrapa Uva and Vinho Entomologic Lab facilities, using a stereomicroscope with a 2-fold enhancement for insect identification and counting.At the time of trap collection, a sample of 15 fruits from each experimental unit was picked up randomly for the assessment of D. suzukii oviposition.
The data were tabulated and the total average of adults trapped per day in each trap was calculated (symbolized in Brazil by MAD) from each monitoring trap.A pest population curve for each fruit crop was prepared throughout the experiment.The sex ratio was assessed using the total number of D. suzukii trapped during the study, in each crop, and subjected to the chi-square (χ 2 ) test at 5% of probability.MAD and the percentage of eggs found in the fruits were used to determine the difference among crops and years.The data were subjected to analysis of variance and the means were sorted by the Tukey test at 5% of probability.

RESULTS AND DISCUSSION
The total number of D. suzukii trapped in all sites was 8553 (4304 males and 4249 females) with no significant difference on the sexual proportion (χ2= 0.354; DF = 1; p = 0.5593), during the 177 evaluations.
In general, female D. suzukii adults was seen constantly at late spring (November 2014), males being detected from December of that year.Trapping continued until June 2015.During the winter the pest was not detected; a new incidence occurred only during the following late spring (December 2015), following the same pattern as previous year, with trapping stopped in late October.The same behaviour was found in the subsequent years of the study (Figure 3).On the blackberry crop, throughout the experiment, 2724 individuals were entrapped (1346 males and 1378 females) without any significant difference between males and females (χ 2 = 0.376; DF = 1; p = 0.5525).The presence of D. suzukii was first observed on September 22 nd , 2014 when an insect was trapped at the experimental unit without the presence of developing fruits; but this did not happen in the subsequent years.5, Appendix Table 2).
The trapping of D. suzukii on the crop started in December 2014 although with decreased number of adults trapped, compared to blackberry and strawberry.The greatest records occurred in March 16 th , 2015 when 4.25 males and 3.50 females (MAD) were trapped.The end occurred in April 2015.In the following years the fluctuations of D. suzukii followed the same pattern, but with elevation of the population in the year 2017 (Figure 6, Appendix Table 3).
On the blueberry crop, 416 individuals were trapped (115 males and 301 females) with significant difference on the sexual proportion (χ 2 = 37,59; DF = 1; p = 0.0001).The population of D. suzukii remained below the level found for the other crops studied.The trapping started in December 2014 and ended in January 2015; it resumed in December 2015, when there was greater occurrence in the crop with 15.75 MAD of females in December 28 th , 2015.In 2016 and 2017, the pattern of occurrence was     Table 1).3).  4).
On the assessed crops, blackberry, raspberry and strawberry, a similar trend in trap catches of D. suzukii start in early spring; was observed with higher numbers trapped throughout the summer, and then declined during winter, except blueberry, which started and ended during summer.The period of D. suzukii occurrence is in agreement with the records obtained from Europe and United Stated (Harris et al., 2014;Kinjo et al., 2014;Mazzetto et al., 2015;Aldea, 2015); it demonstrates a relation with temperature and availability of any food resource at the region.D. suzukii is a pest adapted to environments with mild temperatures at a range between 13.4 and 28.1ºC (Tochen et al., 2014).At the study"s region, there is suitable temperature for the species as pointed out by Benito et al. (2016) as a critical area for the establishment of the pest in Brazil, confirmed by this study.Burrack et al. (2017) stated that the D. suzukii infestation risk is also associated to seasonality of fruit production and the preference of the pest for the host plants.In fact, the occurrence of adults in the traps was significantly greater on the raspberry crop in relation to strawberry and blueberry crops in the first and second years, and all crops in the third year of crop assessment (Table 1).On raspberry, it is possible to observe significant elevation on the occurrence of the pest in the third year of crop assessment (Table 1).Thus, the absence of D. suzukii during the winter may be related to the lack of host fruit crops apart from low temperatures.It is possible that during this period the insect pest could be in diapause, as related to other countries (Dalton et al., 2011;Walsh et al., 2011).
The fruit infestation with D. suzukii eggs tailed the adult"s population fluctuation in all crops, and varied between 6 and 100% in blackberry, raspberry and strawberry.Blackberry presented the greatest number of sampling occasions (trap"s gathering) with 100% of attacked fruits (16.7, 16.7 and 26.7%) on the first, second and third cropping seasons, respectively.On raspberry and strawberry, the totality of infested fruits with eggs was only seen once in the third year of study.
The mean infestation percentage in blackberry varied between 37.1 and 55.3% being statistically superior to the other crops in the first two years, and similar to what was found in raspberry and strawberry crops in the third year (Table 2).On raspberry, the percentage varied from 28.1 to 37.1% with significant elevation in the third year of study (Table 2).The same behaviour of significant elevation of fruit infestation was verified with the strawberry crop, where the percentages varied from 15.3 to 28.7%.On blueberry crop, the infestation was low and statistically similar in the three years of study (Table 2).
The period of crop"s fruiting of the assessed crops at the study"s region: blackberry (December to March),  raspberry (December to May), blueberry (December to January) and strawberry (December to May) are linked to the period of incidence of the pest in the orchards.The staggered host fruit supply and the insect"s preference guided the population dynamics at the studied area, where there is a clear dispersion among crops.Based on the collected populations and the eggs infestations, the pest preference for blackberry and raspberry in Southern Brazil is outstanding.
Although reports from overseas point out the preference of D. suzukii to raspberry crop, Aldea (2015) in Spain observed greater population and damage caused by the pest in raspberry, and little damage in strawberry.In the United States, Burrack et al. (2013) host preference tests were performed in laboratory with raspberry, blackberry, strawberry and blueberry where greater oviposition was found in raspberry and little oviposition in blueberry.
In the present study, strawberry seemed not to be the preferential host crop for D. suzukii when other host crops are available around the cropping site, although in places with exclusive strawberry production the losses might be significant, as reported by Santos (2014) in Vacaria, RS.
The lower infestation level at the blueberry crop in this study is related to the variety of fruit species grown closely in the same area, which results in low infestation of D. suzukii.The variety O"Neal (southern highbush) is considered as precocious at the study"s region, and the ripening of fruits occurring until December, but for the variety Duke (highbush) classified as late-bearing variety produces until January (Pagot, 2006).The fact is its" fruiting period and harvesting happens when D. suzukii is with reduced populations and beginning of infestation, making these varieties to bypass the attack from the study"s region.For other crops and places this aspect has been related; for example, strawberry in the United States reportedly has low D. suzukii damage due to asynchrony between fruit production and the presence of the pest (Burrack et al., 2017).

Conclusion
The occurrence of D. suzukii in Southern Brazil is related to the availability of yielding host fruits which start at late spring and end on late fall, without winter.
The fruit infestation with D. suzukii eggs tails the adults" population fluctuation on the host crops, and might reach 100%.
In the presence of many host crops cultivated closely in an area, the preferred crops by D. suzukii are blackberry and raspberry, followed by strawberry.Among the Blackberry and raspberry crops there was inversion of adults occurrence along the years, higher for blackberry in 2014/15 and 2015/16, and raspberry in 2016/17.
The attack on the blueberry crop is little in early and mid-bearing varieties due to lack of synchrony between the occurrence of the pest and fruit production.

Figure 2 .
Figure 2. (A) Picture of the trap, (B) Detail of hole.
The constant occurrence of the pest and elevated MAD at the site were registered from November 24

Table 1 .
Mean adult insects (MAD) trapped (± standard error) of Drosophila suzukii trapped on the crops: blackberry, raspberry, strawberry and blueberry from Vacaria, RS, Brazil in 2014 to 2017.

Table 2 .
Mean percentage of ovipositions (± standard error) of Drosophila suzukii in fruits of the crops: blackberry, raspberry, strawberry and blueberry from Vacaria, RS, Brazil in 2014 to 2017.
Ab*Upper case letters compared to cropping seasons in each crop (line), and lower case letters compared to crops in each year (column) by the Tukey test at 5% of probability.

Table 1 .
Mean adult insects (male and female) trapped per day (MAD) e percentage of eggs infestation (%) of Drosophila suzukii on the blackberry from Vacaria, RS, Brazil in 2014 to 2017.

Table 2 .
Mean adult insects (male and female) trapped per day (MAD) e percentage of eggs infestation (%) of Drosophila suzukii on the raspberry from Vacaria, RS, Brazil in 2014 to 2017.

Table 3 .
Mean adult insects (male and female) trapped per day (MAD) e percentage of eggs infestation (%) of Drosophila suzukii on the strawberry from Vacaria, RS, Brazil in 2014 to 2017.

Table 4 .
Mean adult insects (male and female) trapped per day (MAD) e percentage of eggs infestation (%) of Drosophila suzukii on the blueberry from Vacaria, RS, Brazil in 2014 to 2017.