MATERIALS AND METHODS

This study was conducted in the prefecture of Kozani (region of Western Macedonia, Greece), in durum wheat cultivations of the variety Bronde, in a one-hectare field during the years 2009 and 2010. Two thrips (Thysanoptera: Thripidae) species were defined: Limothrips denticornis (Haliday) and Limothrips cerealium (Haliday). Measurements were carried out during the growing season of wheat at late stages, for year 2009 sampling started from 27 of April and lasted until 15 of June and for year 2010 from 30 of April to 18 of June. Samplings of insects were taken in eight periods (every week) to cover the late stages of wheat. Measurements were made every about 100 m in the field, with five wheat plants per sample, from 10 different places (replications) in the field. Total thrips individuals (per species) on each wheat plant were counted according to Deligeorgidis et al. (2005a, 2006b, 2007). The average value of the five plants was used in calculations to make data more representative and suitable for statistical analysis (with no transformations and absence of nested analysis). ANOVA was performed according to Snedecor and Cochran (1980), analyzing the following factors: year and sampling period (separately for each insect species used). In order to perform more appropriate and representative comparisons, an additional species found in the same field was used: Lema melanopa L. (Coleoptera: Chrysomelidae) from a completely different taxonomic order, which was sampled and analyzed statistically in the same way as the two thrips species (this unrelated species was chosen to compare the behavior of seasonal population fluctuations).

RESULTS

According to the results presented in Table 1, for all three insects studied, sampling period showed the greatest differences (significance level p<0.00001). This fluctuation is shown in Fig. 1, for all three insects species, for the 2 years of study. In the first sampling period the average number of thrips individuals is 0.5 or lower and from the 4th to 6th period these numbers increase significantly. At late stages (8th sampling period), insect populations are decreasing.

L. denticornis and L. melanopa, showed a significant interaction between year and sampling periods, indicating different population fluctuations in the two years (Table 1). On the other hand, L. cerealium showed a stable population fluctuation across years. L. denticornis, showed some differences in population size across years (Table 1, Fig. 1).

Table 1:	General mean of insect individuals (GM), sources of variability analyzed, degrees of freedom (df) and significance level, for the three species: Lema melanopa, Limothrips denticornis and Limothrips cerealium

Fig. 1:	Population fluctuations across (p1-p8), for the three species: Lema melanopa (LM), Limothrips denticornis (LD) and Limothrips cerealium (LC), during the 2 years (2009: 09 and 2010: 10)

Table 2:	Total average, maximum and minimum number of individuals counted, for the three species: Lema melanopa, Limothrips denticornis and Limothrips cerealium on durum wheat plants, for each year (2009-2010)

In present study, maximum number of thrips indidividuals found on each plant was 5-6 for L. denticornis and 7-9 for L. cerealium (Table 2) but maximum average values were close to 2 for L. denticornis (in 6th sampling period) and 2.8-3.3 for L. cerealium (between 4th and 5th sampling period), as shown in Fig. 1. Total average values were lower (Table 2). Maximum values of the two species were recorded in different periods and from the 4th sampling period and after, L. cerealium was recorded in greater numbers. For L. melanopa maximum was found at 7 individuals, maximum average value was 1.5 and total average value was around 0.8 (Table 2, Fig. 1).

DISCUSSION

Sampling period for all three insects studied, showed the greatest differences. This was expected since insect populations vary within the year (Deligeorgidis et al., 2005a, 2007) and is something usual for thrips species (Webb et al., 1970). This fluctuation was present in both years of study. L. denticornis and L. melanopa, showed a significant interaction between year and sampling periods, indicating different population fluctuations in the two years. This phenomenon was also reported for other insect species (Deligeorgidis et al., 2005a, 2007, 2008). L. cerealium showed a stable population fluctuation across years. L. denticornis, showed additional differences in population size across years. This unstable behavior may be a result of some specific enemies, or host plant reaction (Deligeorgidis, 1997; Deligeorgidis et al., 2005b), or competition phenomena between the two thrips species similar to those described for Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman (Deligeorgidis et al., 2006b). According to present findings, population fluctuations of L. cerealium are more predictable across years, while population fluctuations of L. denticornis are rather unstable and unpredictable, in durum wheat cultivations and in the specific environment of the area (region of Western Macedonia). For polyphagous species like F. occidentalis, significant population fluctuations might be the final result for many reasons: high or low temperature and humidity and different host plants where thrips migrate seeking for flowers (Chyzik and Ucko, 2002).

Larsson (2005) reported a relatively large number of thrips individuals in wheat, triticale and rye cultivations. He found that the mean number of thrips per leaf sheath was around 8 in all three crops and also reported that maximum number was 27 thrips individuals per leaf sheath. In our study, maximum number of thrips individuals found on each plant was different for the two thrips species (5-6 for L. denticornis and 7-9 for L. cerealium). Maximum average values were found in 6th sampling period for L. denticornis and between 4th and 5th sampling period for L. cerealium. This differences may indicate a kind of competition (Andjus et al., 2001; Deligeorgidis et al., 2006b), since maximum values of the two species were recorded in different periods and from the 4th sampling period and after, L. cerealium was recorded in greater numbers. It should be noted that the two thrips species of the genus Limothrips (L. cerealium and L. denticornis) exhibit similar behavior and biology but they have different origin representing Europe and America respectively (Herrick, 1924; Franssen and Mantel, 1965; Chisholm and Doncaster, 1982). This competition phenomenon has been also reported for Coleoptera (Coccinellidae), between species with different origin (Europe and Asia) but similar biology (Kajita et al., 2000). For other thrips species like Frankliniella occidentalis (Pergande) and Thrips tabaci Lindeman, maximum individual numbers were recorded between 4th and 5th sampling period, at late May recordings (Deligeorgidis et al., 2005a).

Larsson (2005) reported that population density in the leaf sheath was correlated with the damage observed on the leaf sheath as percentage of damaged surface. The effect on the leaf sheath alone contributed to a significant proportion of the yield loss. Thrips in the ear and aphids also contributed to the damage. The yield loss caused by L. denticornis was about the same in all three crops (wheat, rye and triticale), with a variation between 21 and 26 kg per thrips per leaf sheath and hectare. The economic injury level was found to be about 14 thrips per straw, which is greater than our recordings. For onion cultivations, variety selection may result in thrips repulsion and yield improvement up to 19% (Malik et al., 2004). L. melanopa was found in smaller numbers, as it was expected for Coleoptera insects (Fleming, 1988; White and Lambkin, 1988).

CONCLUSION

The economic threshold for L. denticornis and L. cerealium has to be decided in combination to other insect species (Alavi et al., 2007). Larsson (2005) reported that economic damage by L. denticornis alone was 20% for all three crops studied (wheat, rye and triticale) but total economic damage for L. denticornis together to other thrips species and aphids was estimated at 40%. The relatively low populations of thrips species found in present study did not resulted in significant and apparent damages in durum wheat cultivation. According to our findings, population fluctuations of L. cerealium are more predictable across years, while population fluctuations of L. denticornis are rather unstable and unpredictable, in durum wheat cultivations and in the specific environment of the area.