Agricultura 2: 1-6 (2003)
Copyright 2003 by University of Maribor
Monitoring of western flower thrips (Frankliniella
occidentalis [Pergande], Thysanoptera) in the vicinity
of greenhouses in different climatic conditions
in Slovenia
Stanislav TRDAN1, Klemen BERGANT2 and Gábor JENSER3
1University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Slovenia
2University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Slovenia
3Plant Protection Institute, Hungarian Academy of Sciences, Hungary
To study the bionomics of western flower thrips (Frankliniella occidentalis) in Slovenia in the open, monitoring of this pest was performed on six locations in the vicinity of the greenhouses using light blue sticky boards. From the data obtained during this investigation - from September 1997 till April 1999 - it was established that the occurrence of western flower thrips in the open coincides with periods, when the air temperature is within the estimated limits of favourable conditions (15 °C - 25 °C). It was found that in districts with the temperate continental climate, the favourable period for the development of western flower thrips is between the middle of May and the middle of September. However, in districts with the submediterranean climate, this period extends from the beginning of May till the beginning of October. In the first dis-tricts, the air temperatures are above the lower developmental threshold from the middle of April till the middle of October, in comparison to the later districts where the air temperatures are above the same threshold from the end of March till the middle of November. Therefore it can be concluded that in Slovenia western flower thrips (Frankliniella occidentalis [Pergande]) does not overwinter in the open in the stage of an active adult. For the time being it can be considered as an economically important pest only in greenhouses. Agricultura 2: 1-6 (2003)
Key words: monitoring; Frankliniella occidentalis; environment; overwintering; Slovenia
INTRODUCTION
Western flower thrips (Frankliniella occidentalis [Pergande]) is one of the most important greenhouse pests in Europe (Brodsgaard 1989; Schmidt and Frey 1995; Tommasini and Maini 1995). This polyphagous species is difficult to control because of its small size, rapid reproduc-tion, and widespread resistance to pesticides (Helyer and Brobyn 1992, Immaraju et al. 1992, Gaum et al. 1994; Herron and Cook 2002). The species feeds on a number of plant stages and tissues within their hosts, ranging from pollen to leaves and other vegetative issues (Van Dijken et al. 1994, Harrewijn et al. 1996). Most of the damage caused by this thrips is based on scarring from its feeding style, besides the species acts as a vector for tomato spotted wilt virus (Chatzivassiliou et al. 2000).
The occurrence of western flower thrips (Frankliniella occidentalis [Pergande]) in some greenhouses and their
Correspondence to: Stanislav TRDAN, University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Chair of Entomology and Phytopathology, Jamnikarjeva 101, SI-1111 Ljubljana, Slovenia (tel.: 00386 1 423 11 61 ext. 225, fax.: 00386 1 423 10 88, E-mail: stanislav.trdan@bf.uni-lj.si)
vicinities in Slovenia was first confirmed in 1992 (Jane`i~ 1993). After this year, the presence of this pest was frequent in our country (Trdan et al. 1999). The occurrence of western flower thrips in last decade of 20th century, together with continuing positive trends of air temperature in Slovenia (Rakovec et al. 1998), were the reasons for our investigation of life and development (bionomics) of this pest under dif-ferent climate conditions. Especially, while western flower thrips is considered as an important outdoor pest in some parts of the world with warmer climate as the present cli-mate of Slovenia (Chyzik et al. 1995, Gonzales 1996, Grove et al. 2001). The study was performed on various locations with different climate conditions, covering most important agriculture areas of Slovenia.
So, up to now, it has been reported that in littoral part of Spain the pest remains active in the open air all over the year in present climate conditions (Lacasa et al. 1995). In the central part of Italy adults can also overwinter outside the greenhouses in a torpid (dormant) form on some hosts (Del Bene and Gargani 1989), but in Hungary it can survive in the field only during the vegetation period (Jenser 1990).
Until recently, the western flower thrips has been con-sidered as an exclusively greenhouse pest in Slovenia, so
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MONITORING OF WESTERN FLOWER THRIPS
only few payed attention to the direct influence of the environment on its occurrence in various geographic districts. The 18-months investigation of the bionomics of the western flower thrips was undertaken to get data on these infl ences.   The   bionomics   of   Frankliniella   occidenta (Pergande) in the open air was studied on six locations Slovenia, three of them lie in the temperate continental zo while three places are located in the submediterranean c mate zone.
The main objective of our study was to investigate t possibility for overwintering of western flower thrips as active adult in the open air in present climate condition Possible extensive outdoor overwintering of this speci should result in radical changes of the control strategy, whi Frankliniella occidentalis (Pergande) belongs among qua antine pests in EU (Karnkowski and Trdan 2002), whi tomato spotted wilt virus is one of the most important qua antine virus in the same region (Smith 1999).
MATERIAL AND METHODS
The fluctuations in the number of western flower thri specimens in the open air were studied in connection weather conditions all over the year. Because of the inte esting geographic position of Slovenia, agricultural land located in three major districts with different climate (su pannonical/temperate continental in the eastern part, su alpine in the central lower part and submediterranean in t western part of the country). These districts also exhibit se eral local climatic differences (Ogrin 1998, Rakovec a Vrhovec 1998). The goals of our study were to investiga the survival of western flower thrips (Frankliniella occide talis [Pergande]) in the open air during the winter perio and to find possible connections between the fluctuations the adults and different weather conditions during veget tion period.
The monitoring of the species was performed usi light blue sticky boards. They are worldwide as well as in Slovenia most frequently used tool for such purposes during the last decade (Cabello et al. 1991, Br¸dsgaard 1993; Trdan 1999a; Trdan 1999b). Rectangular sticky traps (11 x 13 cm) were used in the period from September 1997 till April 1999 on the following locations: Ljubljana, Ko~evje, Jur{inci, Vrtojba, [empeter and Koper.
The meteorological data for this locations were used from the nearest meteorological stations. The pairs of exper-imental locations and representative meteorological stations, together with their geographical information, climate, long-term averages of mean annual air temperature and cumula-tive rainfall, are presented in Table 1. The experimental locations were chosen on the base of pronounced popula-tions of western flower thrips in greenhouses reported dur-ing the years preceding the monitoring.
During the chosen periods the sticky traps were placed round the greenhouses: 16 boards on the top of pillars app. 1 m high, 4 of them immediately at the greenhouse, 4 at the distance of 10, 4 at the distance of 20 and 4 at the distance of 50 m from the greenhouse. Among 4 traps on each dis-tance from the greenhouses, a single trap was located on each cardinal point. The time of exposure was different for different periods of the year, in summer usually shorter than
Table 1:    Locations where the monitoring of western flower thrips (Frankliniella occidentalis [Pergande]) was performed, the nearest
Location	Nearest meteorological station	<p[N]	ME]	z [m]	T[RC]	R [mm]	Climate*
Koper	Portorož	45R 32'	13R 34'	92	13.4	989	SM
Bilje	Vrtojba	45R 54'	13R 38'	55	12.2	1531	SM
Sempeter	Nova Gorica	45R 57'	13R 39'	113	12.0	1446	SM
Juršinci	Starše	46R 28'	15R 46'	240	9.8	954	TC
Kočevje	Kočevje	45R 38'	14R 52'	461	8.6	1476	TC
Ljubljana	Ljubljana	46R 02'	14R 31'	299	10.2	1368	TC
* SM - submediterranean, TC - temperate continental
The favourable temperature conditions for development of western flower thrips (Frankliniella occidentalis [Pergande]), i. e. from 15 oC up to 25 oC, were estimated on the basis of our previous study that included laboratory experiments with rearing chamber (Trdan and Milevoj 2000), different literature sources (Shipp and Gillespie 1993; Boissot et al. 1998), and data collected during this study. The lower developmental threshold of western flower thrips, i. e. 9.5 oC, were determined on the basis of literature sources (Gaum et al. 1994, Katayama 1997).
RESULTS AND DISCUSSION
On all the locations (Fig. 1-6) the first females were observed already in September or October 1997. The temporal pattern of the number of specimens caught on the boards, which were placed at different distances from green-houses, was rather uniform for all experimental locations. The only exception was location Ljubljana, where the population of pest went through a mass gradation in the green-house some month before monitoring started and remained in it also during first two observation periods. Afterwards, the number of female adults in the greenhouse was drasti-cally reduced, because of the removal of the host – the
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MONITORING OF WESTERN FLOWER THRIPS
Fig. 1. A number of females caught per day (N) on experimental
location Ko~evje in different periods (top figure) together with simultaneously observed mean daily air temperatures (T in oC) solid line, and cumulative daily precipitation (P in mm) – black bars on the nearest meteorological station (bottom fiure). White bars for single period present mean N at the various distances – 1 m, 10 m, 20 m, and 50 m, and line in their centers the corresponding standard error of mean. Shaded area on bottom figure presents the favourable temperature conditions, and dashed line the lower developmental threshold of western flower thrips (Frankliniella occidentalis [Pergande]).
cucumbers. In general, the occurrence of western flower thrips coincides with periods, when the air temperature is within the estimated limits of favourable conditions.
In the beginning of November no female adults were found any more in the central part of Slovenia (Fig. 1-3), only single specimens were found on the three locations in Slovenian Primorje (Fig. 4-6). The unusually warm December 1997 - February 1998 period made us proceed with the monitoring in Ljubljana and Jur{inci (Fig. 3), but no thrips were found on the boards there. Frankliniella occi-dentalis (Pergande) did not reach its lower developmental threshold at that time in spite of the very warm period, so the thrips did not leave the nearby of greenhouses, where they overwintered very probably as adults or as non adult stages in soil. No female adults were found in Vrtojba (Fig. 4), [empeter (Fig. 5) and in Koper (Fig. 6) during the winter months. The rare specimens caught very probably originat-ed from the greenhouses nearby.
In the spring 1998 the first females were observed in the period from the end of February till the beginning of March in Koper. This has been expected, as the average temperatures on this location were the highest and so these females were the first to enter the active period. The greenhouse on this location was empty during the winter months and we assume that the specimens would have been found even earlier if not so.
In [empeter the female adults were detected app. one month later, they were found also 50 m from the greenhouse, which could mean that they could overwinter on the host plants (trees, bushes or wild growing grasses) or in soil. In Vrtojba the first females were observed in the period between the end of April and the beginning of May. This may be due to non heated greenhouse on this location which is also consid-
Fig. 2. The same as Figure 1, but for experimental location Ljubljana
Fig. 3. The same as Figure 1, but for experimental location Jur{inci
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Fig. 4. The same as Figure 1, but for experimental location Vrtojba
erably more exposed to wind compared to the location in [empeter.
In Jur{inci first females were observed app. at the same time as in [empeter. They came very probably from the green-house where ornamental flowers were grown at that time. In
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MONITORING OF WESTERN FLOWER THRIPS
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Fig. 6. The same as Figure 1, but for experimental location Koper
Ko~evje and in Ljubljana the first females were found app. at the same time as in Vrtojba.
In the period from May 1998 till September 1998 the number of females caught per day drastically differs among different experimental locations. There are various reasons for these differences. Different amount of precipitation on chosen experimental locations could be the decisive reason. The agrotechnical measures undertaken also play an impor-tant role. So, intensive insecticide treatments of chrysanthemums in the greenhouse in Ko~evje (Fig. 1) resulted in a greater number of thrips further from the geenhouse, where plenty of suitable host plants were available. Using the her-bicides to control weeds in the greenhouses or turf in their vicinity can drastically reduce the number of the specimens at small distances from the greenhouses. This was true for Ko~evje, Jur{inci and Vrtojba. The pronounced decline of the pest population in the greenhouse in Ljubljana is a result of the spring weed control in it. Namely, during autumn 1997 and winter 1997/98 the species could comfortably sur-vive as adults on the weeds present (e. g. Stellaria media (L.) Vill. and Convolvulus spp., only single specimens were found on the latter).
The constantly present uniform population in Jur{inci had plenty of vegetables to feed on, the abundant foliage
was a shelter from unfavourable weather conditions (above all the heavy rainfall).
The results for comparable locations Vrtojba and [empeter show pronounced maximum in the number of the western flower thrips caught on the light blue sticky boards in Vrtojba. These can be attributed to the less abundant pre-cipitation in the period before the boards were placed. On such exposed sites, rainfall have a pronounced influence on the bionomics of the species in the open air. The low num-ber of the specimens caught in Koper was very probably due to relatively empty greenhouse during the entire vegetation period.
The thrips fed on wild growing plants (Jenser 1990, Chellemi et al. 1994, Trdan et al. 1999), so they were found also at greater distances from the greenhouse. The occur-rence of western flower thrips (Frankliniella occidentalis [Pergande]) in [empeter was rather uniform, due to the great variety of plants grown there, so the pests have many host plants to choose from, when they look for shelter in time of unfavourable weather conditions.
In September 1998 the pest became less frequent on all the locations, in October only single specimens were found on some locations. During the last period of this investiga-tion (March 1998) first females were already found on the sticky boards, on these location (e. g. [empeter) they appeared at the same time as during the previous year.
As far as temperature is considered, in the districts with the temperate climate (Ko~evje, Ljubljana and Jur{inci), in present climate conditions the favourable period for the development of the pest in the open air is generally from the middle of May to the middle of September. In districts with the submediterranean climate (Vrtojba, [empeter and Koper), the period with the favourable conditions is a bit longer. It extends from the beginning of May till the begin-ning of October. In the first districts, the lower developmen-tal threshold is exceeded from the middle of April till the middle of October, in comparison to the latter districts where the same threshold is exceeded from the end of March till the middle of November. This conclusions base on our
Table 2: Long-term averages (1971-2000) of mean monthly air tem-peratures (T in oC) and cumulative precipitations (P in mm) for mete-orological stations near experimental locations.
	Portorož		Bilje		Nova Gorica		Starše	
	T	P	T	P	T	P	T	P
Jan	5.0	63	3.4	99	3.0	95	-0.6	41
Feb	5.5	54	4.4	75	4.1	74	1.4	44
Mar	8.3	68	7.8	84	7.4	83	5.5	60
Apr	11.8	74	11.3	119	11.0	114	9.9	69
May	16.6	76	16.1	135	15.9	122	14.9	83
Jun	20.2	94	19.3	160	19.3	145	18.1	109
Jul	22.7	64	21.7	113	21.7	101	19.9	110
Aug	22.5	78	21.2	124	21.1	114	19.2	105
Sep	18.7	114	17.2	152	16.8	158	15.1	94
Oct	14.3	118	12.5	178	12.2	164	9.8	91
Nov	9.5	104	7.5	163	7.3	150	4.3	84
Dec	6.3	82	4.2	129	3.8	126	0.5	63
Kočevje		Ljubljana	
T	P	T	P
-0.9	80	-0.1	71
0.4	84	1.8	71
4.1	106	6.1	87
8.1	115	10.0	103
13.1	115	15.0	113
16.2	148	18.1	154
18.2	118	20.4	117
17.5	127	19.8	134
13.7	146	15.5	131
9.0	161	10.3	147
3.8	157	4.5	137
-0.1	117	0.7	103
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MONITORING OF WESTERN FLOWER THRIPS
monitoring of the western flower thrips on experimental locations and simultaneous observed values of mean daily air temperatures on the meteorological stations near the experi-mental locations (Fig. 1-6). Also the long-term averages of mean monthly temperatures measured on representative meteorological stations (Table 2) were considered.
The results of similar investigations in the northeastern part of the USA which have a similar climate as Slovenia can be used to confirm the conclusions presented in this work.
They state, that the first adults usually are found in May and they overwinter in soil in the open air in spite of the fact that the temperatures during the winter can be below zero as long as 35 days in succession (Felland et al. 1993, 1995). The climate in Slovenia is milder even in the continental part of the county. The data for January, the coldest month in Slovenia, are as follows: in Ko~evje and in Maribor the low-est temperatures are not below zero for more than 26 days and in Ljubljana not for more than 24 days in succession (Hydrometeorological Institute of Slovenia 1996). Though western flower thrips can be active in the open all the year round in the littoral region in Spain, the climate in the Slovenian Primorje is obviously colder and gives no such opportunities.
The overwintering in an non active form, though in the open air, does not give the pest any possibility to develop a resistance to lower temperatures on the short term. Trials of systematic adaptation of active larvae and adults to lower temperatures which were conducted in England, failed. The insects were not able to overwinter in an active form (McDonald et al. 1997).
Though the results of the monitoring show that the pest can survive during the winter period either in protected places (e. g. greenhouses etc.) or in soil, no serous damages by this pest are probable on its hosts which are grown in the open. Meaning that in Slovenia, the western flower thrips (Frankliniella occidentalis [Pergande]) can be considered an economically important pest only in greenhouses, at least until it develops strains which would be resistant to lower temperatures or untill the climate in Slovenia has changed considerably.
ACKNOWLEDGEMENT
Authors are thankful to the Environmental Agency of Republic of Slovenia for kindly providing us with meteoro-logical data. Special thanks to Unichem d.o.o., Sinja gorica, for sticky traps that were used in monitoring and to Ass. Prof. Milica Ka~ (Biotechnical Faculty, Dept. of Food Technology, Ljubljana) for her comments on the manu-script.
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Received September 11, 2002; Accepted in final form February 26, 2003
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