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Investigation of onion thrips' (Thrips tabaci) population dynamics by yellow sticky traps in tobacco, Hungary 2015

Published: 12/03/2015


Based on the mandate of Universal Leaf Tobacco Hungary Ltd, studies were conducted in 2015 by yellow sticky traps in seven tobacco plantations, namely Kunadacs, Pócspetri, Hajdúhadháza, Apagy, Ófehértó, Debrecen and Encsencs.
The purpose was to determine when and in what number T. tabaci individuals can settle into the tobacco plantations. Onion thrips is one of the vector species responsible for transmitting tomato spotted wilt virus (TSWV). The primary objective of the study was to determine the proper timing of chemical treatments. Three yellow sticky traps per sampling place were put on the upper part of tobacco plants. The catching results of 315 yellow sticky traps were monitored from 14th of April till 28th of July. The traps were weekly changed by Universal Leaf Tobacco Hungary field extension service with Nyidoter Kft (Producer’s Group) agronomist , then these were sent to the Plant Protection Research Institute in the beginning and later to the Plant Health and Molecular Biology National Reference Laboratory in Budapest. The onion thrips individuals were investigated by stereo binocular microscope LEICA.

Results and discussion

In 2015, 9906 T. tabaci adults were caught by 315 sticky traps used on the seven tested locations. In 2015 onion thrips settled into the investigated tobacco plantations in the highest number, compared to the results of the previous years. This tendency is associated with the unusually hot and dry meteorological conditions during all the vegetation period. According to the meteorological data, the 2015 growing season was the poorest in precipitation, and had the highest average temperature compared to the previous five-year period. This way, after the colonization, the damaging risk of onion thrips population theoretically increased because the reproduced thrips population could cause more direct (by sucking) and indirect (by virus transmission) damages in the tobacco plantations. In conclusion, it could result in producing more serious quality and quantity deterioration for tobacco. Table 1 includes the total catching results of the different sampling sites. According to the catching results, the highest level of thrips immigration into the tobacco plantation could be found in Kunadacs.

The swarming and colonization of onion thrips into the sampling sites took place almost simultaneously during the investigation period (Fig 1). The first immigrant T. tabaci individuals appeared right after the placement of the traps (14 April) in Apagy, Ófehértó and Encsencs. Until the beginning of June thrips were caught in lower numbers, probably because of the chemical treatments applied during the planting period and also due to the rather rainy weather.

 

Table 1: Catching result by yellow sticky traps at the sampling sites (2015)




 

Figure 1. Population dynamics of Thrips tabaci based on the catching results by yellow sticky traps in the different sampling sites (2015)

Until the middle of June the number of immigrants sharply increased, partly due to the relatively low precipitation and partly due to the very high temperatures. In June, a total of 554 specimens were recorded on 16th of June at the sampling site of Debrecen. Due to the chemical treatments carried out in mid-June, the number of thrips declined until the end of June, then until mid-July the number of migrants rapidly increased again, primarily in Kunadacs orchard. The peak number of caught individuals was registered on 7th of July. Based on the catching results, 1033 specimens were counted on the traps in Kunadacs orchard. In other investigated places the specimen number did not increase significantly during this period. However, the next chemical treatments carried out in mid-July successfully reduced the onion thrips population in all sampling sites. The number of thrips adults began to rise again in late July (Fig 1), but due to the harvesting of tobacco leaves, these individuals may have migrated towards new breeding plants.

It is important to mention that the immigrants can cause population reproduction in the plantations, hence it can lead to the expected direct and indirect damage by thrips in tobacco. However, another important feature of thrips migration is that during the rainy period they look for shelter, so right before the arrival of the rainy season, the swarming could take place forcefully in the height of a few meters above the ground surface. As a result of this, not all individuals will become real pests settling into the plantations, they may not reproduce there, later they preferably continue to migrate and search for another food plant. Onion thrips is a highly polyphagous species, it means that this species has numerous breeding and food plants. The migration between the surrounding weeds or other breeding plants and the tobacco orchards is continuous during the entire vegetation period. This way the species can sometimes migrate in a very large number of individuals.

As mentioned above, it can be concluded that the increased number of onion thrips individuals on the traps does not necessarily mean an increased damage in the plantations. In spite of the fact that the lowest number of thrips was recorded on the traps in Apagy (Table 1), the visual study carried out there on the 1st of September showed that direct damages on tobacco leaves caused by thrips were approximately 50%.


Further planned investigations

There is a so called knocking method used to determine what the connection is between the number of thrips individuals caught by traps and the real level of damage in tobacco plantations. From the end of July till the beginning of September this method was used as a preliminary investigation in every second week. The purpose of this method was to collect thrips adults and larvae directly from the leaves. For this purpose, ten tobacco plants were selected in each fracture zone of the sampling sites, and from each plant it was necessary to shake randomly one leave pattern per plant above a white sheet. Thrips individuals fallen onto the sheet could be collected by a thin brush to small plastic vials containing alcohol 70%. The result of the year 2015 is only informative, but it was determined that the collected number of thrips individuals by knocking was negligible compared to the specimens caught by yellow sticky traps. However, statistically significant results can be obtained after at least three years, while knocking method is carried out in parallel with the investigation by sticky traps.

In 2015 there were only a few other thrips species besides onion thrips on the yellow sticky traps. About five-six other thrips species appeared on the traps in relatively low number. However, in the next years it could also be important to deal with other thrips species, because they can also contribute to causing direct damages on tobacco leaves. The knocking method will reflect on their role in causing damage.

Predatory thrips (Aeolothrips intermedius) settled in the highest number into the tobacco plantation in Encsencs during the entire growing season, compared to the other sampling sites. A. intermedius can feed on mostly thrips larvae, its preferable food on tobacco is onion thrips larvae. This species can immigrate to tobacco primarily from the surrounding weeds and other host plants. It is recommended to investigate the occurring and population dynamics of A. intermedius at the sampling sites, used by both trapping and knocking method in the upcoming years. It is also worth examining what kinds of features can contribute to the reproduction of these beneficial species in the tobacco plantations.


 

Dr. Szilvia Orosz

entomologist

Plant Health and Molecular Biology National Reference Laboratory

National Food Chain Safety Office

Directorate of Plant Protection, Soil Conservation and Agri-environment

H-1118 Budapest

Budaörsi út 141-145, Hungary

E-mail: OroszSz@nebih.gov.hu



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