Agricultural and Food Science in Finland, Vol. 10 (2001): 277–284 277 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 277–284. © Agricultural and Food Science in Finland Manuscript received August 2001 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 277–284. Blue sticky traps are more efficient for the monitoring of Lygus rugulipennis (Heteroptera, Miridae) than yellow sticky traps Jarmo K. Holopainen MTT Agrifood Research Finland, Plant Production Research, Plant Protection, FIN-31600 Jokioinen, Finland. Current address: Department of Ecology and Environmental Science, University of Kuopio, PO Box 1627, FIN-70211 Kuopio, Finland, e-mail: jarmo.holopainen@uku.fi Sakari Raiskio MTT Agrifood Research Finland, Plant Production Research, Plant Protection, FIN-31600 Jokioinen, Finland Anu Wulff Department of Ecology and Environmental Science, University of Kuopio, PO Box 1627, FIN-70211 Kuopio, Finland Kari Tiilikkala MTT Agrifood Research Finland, Plant Production Research, Plant Protection, FIN-31600 Jokioinen, Finland Lygus bugs (Heteroptera, Miridae) are important pests of several crop plants. For the development of an efficient monitoring and forecasting system for Lygus densities, low catches of bugs with yellow sticky traps have been a problem. We determined the spectral reflectance of yellow and blue traps and compared their catches of Lygus spp. during five consecutive growing seasons in carrot fields, with additional data collected from other crop plants. Blue sticky traps caught significantly more omnivorous Lygus rugulipennis (Popp.) than yellow sticky traps in carrot fields. Lygus kalmi (L.) that is a typical species in carrot fields, was equally caught by yellow and blue traps. Our results suggest that blue sticky traps are a convenient method for monitoring of L. rugulipennis especially during the seedling stage of cultivated carrot plants when sweep netting cannot be used. Factors affecting the efficiency of blue sticky traps to capture omnivorous bugs and thrips are discussed. Key words: Lygus rugulipennis, Lygus kalmi, European tarnished plant bug, omnivory, insect orien- tation, visual stimuli, trap type, integrated control Introduction In host plant detection by insects visual stimuli play an important role, especially among gener- alist insects which have host plants with varia- ble morphological characteristics. The physical stimulus perceived by the insect is affected by brightness (intensity of reflected light), hue (dominant wavelength of reflected light) and mailto: jarmo.holopainen@uku.fi 278 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Holopainen, J.K. et al. Lygus spp. monitoring with blue sticky traps saturation (spectral purity of reflected light) (Prokopy and Owen 1983). Relative to other colours, the yellow colour is known to attract many herbivorous insects (e.g. Johnson and Mueller 1988, Kostal and Finch 1996) and there- fore yellow water pans and sticky traps are usu- ally used for insect monitoring. In cabbage root flies the preference for yellow traps compared to blue and violet trap was innate even in young flies (Kostal and Finch 1996). Several species in the genus Lygus have hun- dreds of host plant species and they are signifi- cant pests of many important crop plants (Ho- lopainen and Varis 1991, Young 1986). The most important species in Europe is L. rugulipennis, which is a pest on crop plants as well as forest trees in nurseries (Holopainen 1986) like other Lygus species in Northern America (Schowalter and Stein 1987). The movement of Lygus bugs to the crops from their hibernation sites (Varis 1995) or from other cultivated plants (Fleisher et al. 1988) is easily monitored with sweep net sampling (Varis 1995). However, sweep netting is time-consuming, may harm emerging seed- lings and does not catch samples properly from small seedlings, which are often most severely damaged by Lygus spp. (Fye 1984, Holopainen 1986). For the development of efficient monitoring and forecasting systems of Lygus populations, the low catches of bugs with yellow sticky traps have been a problem. Especially, in organic farm- ing, monitoring of L. rugulipennis is needed for timing different plant protection measures. The use of virgin Lygus female (Slymaker and Tug- well 1984, Holopainen and Rikala 1991) as a bait in yellow sticky traps has increased catching ef- ficiency of yellow sticky traps significantly, but this method is too laborious for routine use. Therefore, we tested attractivity of blue sticky traps for monitoring purposes of L. rugulipen- nis and other Lygus spp. damaging especially small carrot seedlings. Material and methods Traps The traps were disposable 190 mm by 290 mm cardboards with sticky upper surface, yellow or blue in colour. The blue sticky traps we used are especially aimed to attract thrips species on Saintpaulia and Frankliniella occidentalis, while the yellow colour is expected to attract all other flying insects. Also the sticky glue of the traps lures certain insects (Catch-itTM, Silva miljö AB, Knäred, Sweden, http://www.silvandersson.se). Traps were formed into a vertically posi- tioned cylinder, fixed in a wooden stick at the level of carrot foliage top, being about 10 cm to 30 cm from soil surface depending on the height of foliage. Blue and yellow traps were placed in a row in alternating order, 10 m from each oth- er. Traps, replaced with new ones at one-week intervals, were collected, covered with transpar- ent plastic film and stored in a cold room before insect identification and counting. Reflectance of traps Spectrum of the sun and reflectance of sunlight from the traps were measured with a Macam 9910 spectrophotometer (Macam Photometrics Ltd, Livingston, Scotland) on a sunny morning between 9.30 a.m. and 10.00 a.m. in late April. Trap surfaces were at right-angle to the sun dur- ing measurements. Fields Research was carried out in one carrot and one grassland field in 1997, six carrot, five swede, five cabbage, three potato, two spinach, and one red beet field in 1998. In 1999 traps were tested in two carrot fields and a cabbage field, and in 2000 in two and in 2001 one carrot field, respec- tively. The size of the fields varied between 0.5 and 5 ha. Traps were arranged in one row, ex- 279 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 277–284. cept in two rows per field in 2000 and two rows per plot in 2001. The distance of the traps in row, parallel to the field edge, was 10 meters in each plot. The trap row was placed on the side of field, which was on the potential entering direction of the bugs. Usually traps were on the side that was closest to the forest. In 1997 two blue and two yellow traps per field per week were used in carrot and one blue and one yellow trap on grassland for four weeks. In 1998 several crop plants were studied using three blue and four yellow traps per field for eight weeks. In 1999 three blue and three yel- low traps per week were used in carrot for seven weeks. In 2000 6 blue and 6 yellow traps in two rows were used in both localities (Jokioinen 3.5 ha field and Kokemäki, 0.4 ha field) for nine weeks, and the positions of traps were reversed (yellow vs. blue) each week to reduce any bias due to positional effects. In 2001 a ten week in- vestigation was conducted on a 3.6 ha carrot field in Jokioinen with four replicate plots each hav- ing two yellow and two blue traps in 10 m dis- tances from each other. Trap catches of Lygus bugs from blue and yellow traps from 1997 to 2000 were analysed using weekly catch of each trap as a true repli- cate. Non parametric tests (Mann-Whitney U) were used for comparisons, since data were not normally distributed because of small number of observations. Data from replicated plot experi- ment in 2001 were analysed with t-test using mean plot catches. SPSS for Windows statisti- cal package was used for analyses. Results Reflectances from yellow and blue traps were 25% and 12%, respectively, of the irradiation from the sun (Fig. 1) in the integrated wave- lengths 280–700 nm (UV-radiation and photo- synthetically active visible light (PAR)). Blue traps reflected from blue to red light the peak being at blue (460 nm). Yellow traps had a great- er overall reflectance than blue traps reflecting mainly wavelengths from yellowish green (550 nm) to red (750 nm) the peak being at 665 nm. Reflectance in the UV-radiation area (< 400 nm) was very low in both trap types. The dominant Lygus species in trap catches was L. rugulipennis (Popp.) in most crops. L. kalmi (L.), which is a specialist feeding on Apiaceae (Linnavuori 1966), was found in car- rot fields in 1999 and 2000. In 1997, in the car- rot field, weekly mean catch of L. rugulipennis in blue traps 3.75 ± 1.13 (SE) was significantly (n = 8, Mann-Whitney U = 1, P < 0.001) higher than in yellow traps 0.13 ± 0.12. The accumu- lated total catch in blue traps during the moni- toring period was 29-times compared to yellow traps (Fig. 2a). In the grassland fields, in 1997, weekly mean catch of L. rugulipennis in blue traps 3.25 ± 2.00 was only marginally (n = 4, U = 2.5, P = 0.089) higher than in yellow traps, 0.25 ± 0.30 individuals. In 1998, the total catch of L. rugulipennis (and possibly the density also) in different field crops was so random and low that statistical significant differences were not found (Table 1). In 1999, blue traps in carrot caught weekly 2.29 ± 0.71 individuals. The catch was signifi- cantly (n = 48, U = 848, P = 0.004) higher than Fig. 1. Radiation (W/m2) spectrum of the sun and reflect- ances of yellow and blue sticky traps in UV area (250– 400 nm) and visible light area (400–750 nm). 280 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Holopainen, J.K. et al. Lygus spp. monitoring with blue sticky traps in yellow traps (0.19 ± 0.01) (Fig. 2b). In the growing season of 2000, bug densities were low. The mean catch of L. rugulipennis in blue traps (0.85 ± 0.40) was not significantly (n = 48, U = 1075, P = 0.503) higher than in yellow traps (0.44 ± 0.13). However, during peak density on June 26 significantly (n = 6, U = 6, P = 0.049) more L. rugulipennis adults were caught in blue (3.00 ± 0.82) than in yellow (1.00 ± 0.37) traps (Fig. 3a). In 2001, in the replicated plot experi- ment mean plot catches of blue traps were sig- nificantly higher (P < 0.05, t-test) than that of yellow traps on May 22 and June 5 and 12 (Fig. 4). When log-transformed numbers were used in the analyses, L. rugulipennis catches were significantly higher in blue traps also on May 22. In carrot fields, in 1999, after the flying pe- riod of L. rugulipennis, during three weeks in July, L. kalmi was caught in traps. Numbers in yellow traps (n = 9, 6.4 ± 2.8) did not differ (U = 33.5, P = 0.531) from catch in blue traps (6.0 ± 4.8). In 2000, L. kalmi also appeared in carrot fields in Jokioinen. The first observations were earlier than in previous year (Fig. 3b). Again the mean catch between blue (0.31 ± 0.13) and yellow (0.58 ± 0.17) traps did not differ (n = 48, U = 1037, P = 0.254). In 2001 only very few L. kalmi individuals appeared in the traps not allowing any statistical analyses. Table 1. Weekly numbers of Lygus rugulipennis per trap in yellow and blue traps (n is same for both colors) in different field crops (21 fields) in 1998. Plant species Yellow Blue n Mean(SE) Mean(SE) U P Carrot 52 0.32 (0.07) 0.46 (0.12) 1292 0.676 Cabbage 25 0.13 (0.03) 0.27 (0.05) 270 0.308 Potato 18 0.61 (0.19) 0.61 (0.21) 160.5 0.960 Spinach 12 0.22 (0.12) 0.11 (0.05) 68 0.781 Swede 30 0.13 (0.05) 0.12 (0.03) 412 0.700 Red beet 6 0.17 (0.17) 0.06 (0.07) 17.5 0.902 Fig. 2. Mean weekly catch (± SE) of L. rugulipennis in carrot fields in Jokioinen in 1997 (n = 2) (a) and 1999 (n = 3) (b). 281 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 277–284. Discussion In the years when L. rugulipennis population densities are high enough to cause extensive damage to cultivated plants, yellow traps do not indicate bug densities efficiently (Holopainen and Rikala 1990). Our replicated plot experiment in 2001 with reasonable high L. rugulipennis density confirmed the preliminary trap-based observations in the years from 1997 to 2000 that blue sticky traps caught significantly more L. ru- gulipennis than yellow traps. Therefore, blue sticky traps are suitable for monitoring of L. ru- gulipennis. However, L. kalmi was collected in equal numbers in yellow as in blue traps. This suggests that different Lygus species have a dif- ferent visual response in orientation to host plants. A North American species, L. lineolaris has been observed to prefer yellow sticky traps to bright blue traps (Prokopy et al. 1979) and yellow water-pans traps to red and green traps (Landis and Fox 1972). The variation of colour detection among in- sect families is poorly known, but e.g. some Heteroptera species do not perceive orange col- ours (Hénaut et al. 1999). Blue sticky traps have been observed to be efficient in the monitoring of some thrips species in greenhouses (Gillespie and Vernon 1990). In natural ecosystems Thy- sanoptera is the only insect order that is more efficiently caught by blue than yellow traps (Ho- Fig. 3. Mean weekly catch (± SE) of L. rugulipennis (a) and L. kalmi (b) in carrot fields in Jokioinen in 2000 (n = 6). Fig. 4. Mean weekly catch (± SE) of L. rugulipennis per trap using plot means in carrot fields in Jo- kioinen in 2001 (n = 4). Asterisks above blue trap catches indicate significant (P < 0.05) difference from yellow trap catches. (o indi- cate P = 0.067). . 282 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Holopainen, J.K. et al. Lygus spp. monitoring with blue sticky traps back et al. 1999). Hoback et al. (1999) demon- strated that attractiveness of blue and yellow traps could be variable inside one insect genus as in Frankliniella spp. thrips. Our reflectance analysis demonstrated that blue traps are more efficiently reflecting short blue light waves (400– 500 nm) than yellow traps, which are more effi- ciently reflecting in the green area (500–560 nm). This might be one explanation for the higher catch of blue traps for L. rugulipennis, since this species is not an absolute herbivore. The om- nivorous diet could include insect eggs and lar- vae (Varis 1972), and possibly green colour of the plant foliage is not as attractive to this spe- cies as to other species in the genus Lygus. It is interesting to note, that Frankliniella occiden- talis is also an omnivore (Agrawal et al. 1999) and is also trapped in greater numbers on blue traps than on yellow (Gillespie and Vernon 1990). Colour preference of Lygus bugs might be different also in different crop types. Prokopy et al. (1979) found that for L. lineolaris that dam- age flowers in apple trees, bright blue sticky traps were not as attractive as non-ultraviolet-light reflecting white, yellow or plexiglas rectangles, hung vertically at about 0.7 m above ground. We did not find yellow traps to catch L. rugulipen- nis more efficiently than blue traps in any of the studied fields. Blue traps are probably most effective for Lygus monitoring on crops where they damage plants on seedling stage, and the foliage of young plants do not shade the soil surface e.g. carrot, conifer seedlings, transplanted cabbage seed- lings. Kostal and Finch (1996) found that the cabbage root fly Delia radicum (L.), avoided landing on blue traps when background was green, while brown background resulted in the relative increase of catches in the non-preferred violet and blue traps. Prokopy and Owens (1978) and Prokopy et al. (1979) found that white sticky traps were more efficient than yellow sticky traps to col- lect L. lineolaris. Therefore efficiency of white sticky traps to collect L. rugulipennis should be tested and compared to blue sticky traps. How- ever, at the moment blue traps are significantly more efficient than yellow traps for L. ruguli- pennis monitoring. Since L. rugulipennis frequently damage the apical meristem in the production of vegetable seedlings during spring in greenhouses, in addi- tion to the thrips monitoring, the use of blue traps for timing of chemical control of Lygus bugs is recommendable. The blue sticky traps seem also to be suitable monitoring tool for the forecast system of L. rugulipennis that is under develop- ment in MTT Agrifood Research Finland. Vege- table growers make observations and countings of bug densities in their fields and send the in- formation by the short message service of their mobile phones to the central database of MTT. As a feedback the growers receive on the dis- play screen of they mobile phone the latest fore- cast of bug densities in their area. Acknowledgements. We want to thank Outi Järvinen for insect counting and identification and Timo Oksanen for the help in reflectance measurements. 283 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 10 (2001): 277–284. References Agrawal, A.A., Kobayashi, C. & Thaler, J.S. 1999. Influ- ence of prey availability and induced host-plant re- sistance on omnivory by western flower thrips. Ecol- ogy 80: 518–523. Fleischer, S.J., Gaylor, M.J. & Hue, N.V. 1988. Dispersal of Lygus lineolaris (Heteroptera: Midridae) adults through cotton following nursery host destruction. Environmental Entomology 17: 533–541. Fye, R.E. 1984. Damage to vegetable and forage seed- lings by overwintering Lygus hesperus (Heteroptera: Miridae) adults. Journal of Economic Entomology 77: 1141–1143. Gillespie, D.R. & Vernon, R.S. 1990. Trap catch of west- ern flower thrips (Thysanoptera: Thripidae) as affect- ed by color and height of sticky traps in mature green- house cucumber crops. Journal of Economic Ento- mology 83: 971–975. Hénaut, Y., Alauzet, C., Dargagnon, D. & Lambin, M. 1999. Visual learning in larval Orius majusculus a polypha- gous predator. Entomologia experimentalis et Appli- cata 90: 103–107. Hoback, W.W., Svatos, T.M., Spomer, S.M. & Higley, L.G. 1999. Trap color and placement affects estimates of insect family-level abundance and diversity in a Ne- braska salt marsh. Entomologia Experimentalis et Applicata 91: 393–402. Holopainen, J.K. 1986. Damage caused by Lygus ruguli- pennis Popp. (Heteroptera, Miridae), to Pinus sylves- tris L. seedlings. Scandinavian Journal of Forest Re- search 1: 343–349. – & Rikala, R. 1990. Abundance and control of Lygus rugulipennis (Heteroptera: Miridae) on Scots pine (Pi- nus sylvestris L.) nursery stock. New Forests 4: 13– 25. – & Rikala, R.1991. Luteet ja männyn taimien kas- vuhäiriöt. Summary: Lygus bugs and growth distur- bances in Scots pine seedlings. Metsäntutkimuslai- toksen tiedonantoja 379. p. 1–35. – & Varis, A.-L. 1991. Host plants of the European tar- nished plant bug Lygus rugulipennis Popp. (Heterop- tera: Miridae). Journal of Applied Entomology 111: 484–498. Johnson, M.P. & Mueller, A.J. 1988. Three-cornered al- falfa hopper response to six sticky trap colors. South- western Entomologist 13: 101–105. Kostal, V. & Finch, S. 1996. 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Annales Agriculturae Fenniae 11: 1–56. 284 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Holopainen, J.K. et al. Lygus spp. monitoring with blue sticky traps SELOSTUS Siniset liimapyydykset ovat keltaisia liimapyydyksiä tehokkaampia peltoluteen tarkkailussa Jarmo K. Holopainen, Sakari Raiskio, Anu Wulff ja Kari Tiilikkala MTT (Maa- ja elintarviketalouden tutkimuskeskus) ja Kuopion yliopisto Lygus-suvun luteet (Heteroptera, Miridae) ovat usei- den viljelykasvien merkittäviä tuholaisia. Luteiden tarkkailua ja ennustemenetelmien kehittämistä on haitannut luteiden vähäinen hakeutuminen keltaisiin liimapyydyksiin. Määritimme keltaisista liimapyy- dyksistä ja ripsiäisten tarkkailuun kehitetyistä sini- sistä liimapyydyksistä auringon valon kokonaishei- jastumisen, ja vertailimme pyydysten ludesaaliita vii- tenä peräkkäisenä kasvukautena pääasiassa porkka- nakasvustossa. Lisähavaintoja kerättiin nurmelta, kaalilta, perunalta, pinaatilta, lantulta ja punajuurik- kaalta. Porkkanalla siniset liimapyydykset pyydysti- vät selvästi tehokkaammin peltoludetta Lygus rugu- lipennis (Popp.) kuin keltaiset liimapyydykset. Har- vinaisemmalle, mutta porkkanapelloille tyypilliselle Lygus kalmi (L.) -ludelajille siniset ja keltaiset pyy- dykset osoittautuivat yhtä houkutteleviksi. Tulokset osoittavat, että siniset liimapyydykset sopivat peltolu- teen tarkkailuun ja niiden avulla voidaan osoittaa lu- teiden saapuminen kasvustoon etenkin vuosina, jol- loin luteet esiintyvät runsaslukuisina. Title Introduction Material and methods Results Discussion References SELOSTUS