Franja Baca¹, Jovo Stojcic², Vojsilav Trkulja², Slavko Radanovic³, Dragisa Lopandic¹, Dragica Zivanovic³, Dusko Paravac^4
1 Maize Research Institute, Zemun Polje, Belgrade-Zemun, Yugoslavia
² Institute of Agriculture, Banja Luka, Republika Srpska; Bosnia Herzegovina
³ Agricultural Station, Bijeljina, Republika Srpska; Bosnia Herzegovina
⁴ Agricultural Station, Doboj, Republika Srpska; Bosnia Herzegovina

The abundance of imagoes of western corn root worm (WCR) and their territorial distribution over the regions of Republika Srpska were monitored by pheromone traps "Csalomon" and yellow sticky traps "Pherocom®". The survey encompassed all communities as in 1998, with a further spread towards west and south west all the way to the border with Croatia. Traps were set in 51 locations within 24 communities.

One pheromone and one yellow sticky trap were set in each observed location. Imago flights were monitored during two months, i.e. 61 to 64 days depending on location. Traps were set up in large areas under maize with the inter-trap distance of 100 m at about 50 m away from the road from July 14 to July 17. The first trap survey was done on July 30 (traps set on July 14 and 15) and August 2 (traps set on July 16 and 17). The second trap survey and their replacement were done on August 14 and 15. The succeeding two surveys were performed on August 30 and 31 and September 15, 16 and 17.

The '99 results correspond with the ones obtained in 1998. Out of 3749 registered WCR 3535 were found on pheromone traps (94.3%), while only 214 (or 5.7%) were found on yellow sticky traps. Adult insects were trapped in 31 locations (60.80/o), i.e. 25 (49.01/o) on pheromone, i.e. yellow sticky traps, respectively. It should be emphasized that yellow sticky traps failed in locations with the lowest abundance of WCR imagoes. Their utilisable value is well-known in cases of a great insect abundance and they are employed in determination of a threshold of economic importance. The insect abundance in 1999 on pheromone traps was 16.5 times as great as on yellow sticky traps, while the corresponding value in 1998 was 36.2 times. The decrease from 36.2 to 16.5 times points out to the efficiency increase with the growth of WCR abundance.

The obtained data on the number of registered insects on both traps indicate that the greatest abundance and mass migration of WCR insects were during July and August of 1999. In the course of 46 to 48 days a total number of 3246 (92.3%), i.e. 199 insects (93.9%) were registered on pheromone, i.e. yellow sticky traps, respectively. The corresponding numbers in the first survey, the second two weeks of July, were 1314 (37.2%) and 72 (33.6%), respectively. These numbers in the second survey (the first two weeks of August) were 848 (24.0%) and 78 (36.4%), respectively. Traps were replaced, for the first time, in the second part of August and a total number of 1102 (3 1.2%), i. e. 49 insects (33.6%) were registered on pheromone, i.e. yellow sticky traps, respectively, in the third survey. During September only 271 (7.7%) and 15 insects (7.0%), respectively, were registered.

According to the obtained data in 1999, the insect abundance increased three, i.e. two times on pheromone, i.e. yellow sticky traps, respectively. The total number of trapped insects per location over the investigation period varied from 1 to 569 on pheromone traps, i.e. 1 to 47 on yellow sticky traps. Based on data obtained for the number of insets trapped on pheromone traps, all localities can be classified into three groups:

1. Locations with the total number of over 300 trapped beetles, or a daily maximum of 9 insects, in 2000, the economic level of larvae if maize is continuously cropped would amount to: 569 in Amajlija, 489 in Brodac, 309 in Bijeljina probably 233 in Brezovo Polje and 227 in Karakaj.
2. Locations with the total number of 100 - 200 trapped insects with the economic level of beetles in 2000 of- 190 in Tmovi, 159 in DragIjevae, 158 in Tabus and 126 in Plazulja.
3. Locations with less than 100 trapped insects: 12 out of the total 22 locations are with 50 - 100 trapped insects, 6 locations with 25-50 insects and 4 locations with less than 50 insects or less than an insect per day.