Weevils of the genus Ceutorhynchus Germ associated with oilseed rape in northern Serbia

Ivan L. Sivčev1*, Lazar I. Sivčev1, Snežana Pešić2, Draga Graora3, Vladimir Tomić4 and Boris Dudić4 1Institute for Plant Protection and Environment, Teodora Drajzera 9, Belgrade, Serbia 2Faculty of Science, University of Kragujevac, Radoja Domanovića 12, Kragujevac, Serbia 3Faculty of Agriculture, University of Belgrade, Nemanjina 6, Zemun, Serbia 4Faculty of Biology, University of Belgrade, Studentski trg 16, Serbia (*ivansivcev2011@gmail.com) Received: October 1, 2015 Accepted: October 16, 2015

Ongoing climate changes can influence the phenology and abundance of weevils. Adaptation of different species to climate changes may vary from their becoming rare to excessive abundance (Putten et al., 2010). Thus, climate change may also lead to a change in insect abundance and distribution. The region of northern Serbia has been predicted to sustain significant climate change in the future, and records about these insects' present status are important. The aim of this study was to research the genus Ceuthorynchus in OSR fields in northern Serbia, where its production is concentrated, although not extensive at present.

MATERIAL ANd METHOdS
Experimental site: The trial was set up in northern Serbia (Stari Žednik, Subotica, GPS coordinates: N45 57.280 E19 37.554). The soil is calcic chernozem, loamy type texture. The climate is mild continental with mean annual temperature of 11.2°C and annual precipitation of about 530 l/m 2 . Global climate change has already severely affected this area and the most prominent impact is expected to occur by the end of the 21 st century (Rubel & Kottek, 2010). E x pe r i m e nta l f iel d s: T hree com mercia l experimental fields of 1 ha each were drilled with OSR cv. Excalibur on September 17, 2010, and with winter wheat (WW) in the following growing season, on 20 October 2011. The fields had never been planted with oilseed rape before. The OSR fields were managed in accordance with conventional, integrated and organic production requirements. No herbicides, foliar fungicides or molluscides were applied over the OSR growing period. On March 25, 2011, the conventionally managed OSR field was sprayed with chlorpyrifos + cypermethrin (500 g/ha a.i. + 50 g/ ha) and the field with integrated management with cypermethrin (40 g/ha a.i.). The organic field was sprayed with spinosad (96 g/ha a.i) on April 6, 2011. A standard tractor sprayer with 300 l water/ha was used. The fallow period lasted from the OSR harvest on June 21 until October 20, 2011 when WW was sown. The succeeding WW crop was managed without any insecticide treatments and was the same in three experimental fields.
Sampling and identification: All sampling of Ceuthorynchus specimens was performed at 8 points set up 50 m apart in the middle of each field. The Ceuthorynchus weevil specimens were sampled using yellow water traps (YWTs) and emergence traps, while the presence of larvae was assessed by plant dissection. The collected weevils were identified according to Hoffmann (1954), Smreczyński (1974) and Freude et al. (1983).
Yellow water traps: Yellow water traps were used to monitor the autumn and spring immigration of adult Ceutorhynchus weevils into the OSR fields. From October 2, 2010 (BBCH 12-13) three YWTs were set up in each field and checked 4 times until the end of November . The sampling was discontinued in December. From January 2011, four traps were placed in each field and the sampling period was extended to 2-4 week intervals. In the spring, sampling was conducted again at 7-days intervals until May 15, 2011. The traps were mounted on a holder and continuously raised up to be always above crop level. Emergence traps: The emergence of a new generation of Ceutorhynchus imagines was monitored using emergence traps (www.ecotech-bonn.de) with a pitfall trap inside each. The sampling with emergence traps continued from the end of OSR flowering (BBCH 68) until harvest on June 21, 2011, then during the fallow period and the growing period of WW (October 20, 2011-June 21, 2012). The emergence traps were checked every 7 to 14 days.  1795). Based on a total of 3386 specimens checked in the laboratory, we concluded that their abundance varied greatly. The most numerous was C. napi that accounted for 81% of all collected specimens (2712 insects). The second most numerous was C. pallidactylus with 18% share (609 insects). The remaining 6 species were detected sporadically and their frequency was 1% of all collected specimens. Among them, 14 insects belonged to the species C. obstrictus, 14 were C. picitarsis, 6 were C. erysimi, 11 were C. minutus, 3 were C. sulcicolis and 17 insects were C. typhae (Figure 1). The largest number of specimens (55%) was collected in the field with organic management, while the counts of weevil specimens in the other two management systems were similar -23.2% in the conventional and 21.7% in the integral management system.  Records of Ceutorhynchus species trapped in YWTs: The two most numerous weevil species have similar immigrating flight patterns. Rape stem weevil (C. napi) and cabbage stem weevil (C. pallidactylus) began to immigrate into the oilseed rape fields in autumn, i.e. during November (BBCH 16-17). The number of adults caught in YWTs during autumn was small. In our experimental fields, we recorded the mean number of up to 1 cabbage stem weevil and up to 2 rape stem weevils per YWT. In total, during the autumn immigration there were 11 C. pallidactillus and 5 C. napi imagines. Over the following period until February 12, 2011 (BBCH 19-20) up to 15.8 cabbage stem weevils and 22.8 rape stem weevils were found per YWT/week. Massive flight of both species started during the later half of March and was at its maximum on March 23 (BBCH 22-25). The maximum mean number of cabbage stem weevils was 72.3 per YWT/ week while the mean number of rape stem weevils was 148.8 per YWT/week. By April 15 (BBCH 55-57), the flight of C. pallidactilus was over, whereas a few more individuals of C. napi were caught until April 27. During the whole sampling period from January 30 until April 7, 2011, the ratio of male/female specimens of C. napi collected in YWT was 1.3:1, while C. pallidactylus had a respective ratio of 0.8:1. C. obstrictus insects immigrating into the OSR fields were found in YWTs in the spring of 2011. A total number of 14 specimens (11♂♂/3♀♀ ) was caught in YWTs traps in 3 fields on April 7, 2011 (BBCH 55-57). C. picitarsis was recorded in YWTs during weevil immigration into the OSR fields in the spring of 2011. On March 23 and March 30, 2011, five specimens (2♂♂/3♀♀) and 9 specimens (5♂/4♀), respectively, were trapped in 12 YWTs. C. typhae was also found in the YWTs, 12 specimens were captured on March 30 (6♂/6♀) and 5 on April 7, 2011 (5♂). Three C. erysimi males were found in a YWT on March 23, 2011, and another 3♂ on May 26, 2011. C. minutus was represented by 11♀ on March 23, 2011, while C. sulcicollis (3♀) was recorded on March 30, 2011.

Species composition and abundance:
Weevil larvae in OSR stems, leaf petioles and pods: Our autumn assessment of stem weevil larvae by dissection of OSR stems and leaf petioles, performed on November 10 (BBCH 17-18) and November 30, 2010 (BBCH 18-19), showed that weevil larvae were not present. Weevil larvae were found in OSR plants in the following spring, from April 12  to May 3, 2011 (BBCH 55-71). Dissection of OSR pods (BBCH 71-73) revealed the presence of a single cabbage pod weevil larva. The mean number of Dasyneura brassicae Winn. larvae in the dissected pods ranged from 3.6 up to 26.3 larvae per pod. This indicates that C. obstrictus is of no relevance to the infestation of OSR by brassica pod midge (D. brassicae).
Records of new generation weevils in emergence cages: The emergence of a new generation of C. pallidactylus weevils occurred from May 26 until June 5, 2011, when OSR was at the ripening stage (BBCH 79-80), and ended by June 21 when OSR was at the final growth stage (BBCH 87-88). A great majority of new generation C. pallidactullus weevils emerged from the OSR field in June. Later in the season, only one imago of C. pallidactilus was found on September 9, 2011 and one more on November 14, 2011 in the same field, which was by that time sown with winter wheat. On March 26, 2012 a single weevil was again found in an emergence cage.
In the spring of 2012, a new generation of C. napi weevils emerged in the subsequent winter wheat crop. The emergence traps revealed weevil emergence from March 5 until April 2, 2012. Two weevils emerged until May 7, 2012. The peak of emergence in the sampling period occurred from March 12 to March 19, 2012.

dISCUSSION
The weevil species assemblage that we found in our experimental OSR fields is typical for OSR crop stands throughout Europe (Alford et al., 2003;Grantiņa et al., 2011;Milovac et al., 2010;Toshova et al., 2009;Vaitelytė et al., 2013;Williams & Cook, 2010). However, the abundance of weevils and their consequential economic significance are quite different. Our results show that two stem weevils are the most numerous and therefore most significant weevil pests of oilseed rape in the region of northern Serbia. The most frequent species was C. napi, which accounted for 81% of the total number of recorded weevils, and it was followed by C. pallidactylus with 18%. These two species comprise 99% of the total 3386 weevil specimens collected and checked in the laboratory. Other weevils that are widespread throughout Europe, such as C. obstrictus, were not abundant in our OSR fields and hence of no economic importance since only 14 specimens were found. Dissection of the sampled pods showed a low percentage of infestation with C. obstrictus as only one larva was found. The low level of infested pods is consistent with a very small number of C. obstrictus weevils found in YWTs. Compared with 11.6% of pods infested by Dasyneura brassicae Winn. such a small number of C. obstrictus is not supporting the synergism between these two species (Graora et al., 2015). This indicates that infestation with C. obstrictus does not necessarily accompany OSR infestation with brassica pod midge.
Early spring insecticide treatments of OSR primarily target pollen beetles (Meligethes spp.) and Ceutorhynchusstem weevils. Usually one treatment is sufficient for these most important pests (Williams, 2010). However, the timing of insecticide treatment greatly depends on which one of the two stem weevil species is dominant. Although C. pallidactylus and C. napi share a similar phenology, their control can differ regarding the timing of insecticide application (Büchs, 1998). C. napi is more destructive, so that simultaneous and highly abundant migration of males and females call for immediate insecticide treatments. On the other hand, C. pallidactillus is characterized by protandrous migration and does not require immediate insecticide treatment. There is a need therefore for clarification of stem weevil predominance in any region of interest. If no YWT monitoring is practiced, a delayed insecticide application may miss a significant part of C. napi population and thus increase dramatically the infestation of OSR plants. It is therefore necessary to monitor the immigration of stem weevils into OSR fields in springtime very carefully in order to secure proper timing of insecticide treatments when population density has exceeded action threshold.
Our results also showed that C. picitarsis and C. typhae were present in low numbers, while other Ceutorhynchusspecies were even more sporadic. The two species are associated mainly with weeds typical for OSR crop, such as Capsella bursa pastoris (L.) Medik. and Papaver rhoeas L. Concerning some other Ceutorhynchinispecies, Ethelcus denticulatus (Schrank 1781), Stenocarus cardui (Herbst 1784), S. ruficornis (Stephens 1831), and Neoglocianus albovittatus (Germar 1824) were found as single specimens and were associated with P. rhoeas. Of the other weevil species relevant for oilseed rape, we recorded a sporadic presence of Aulacobaris coerulescens (Scopoli 1763) (Curculionidae, Baridinae).