1Estonian Agricultural University, Tartu, Kreutzwaldi 1, 51014, Estonia.
2The Royal Institute of Technology, Dept of Chemistry, Organic Chemistry, Group of Ecological Chemistry, KTH, 100 44 Stockholm, Sweden.
In the spring, Phyllothreta spp migrate to their host plants after feeding on spring flowers. Degradation products of glucosinolates, mainly isothiocyanates (ITC) are known to be responsible for host-plant recognition. In this study, alkylthiocyanates (TCN) were found to be highly attractive to the beetles. To avoid competition of host floral fragrance, the field tests were performed before and after the flowering period of oilseed rape. The first test was made in June on a meadow with flowering dandelions (Taraxacum spp) with no oilseed rape fields located nearby. The second test was performed in July-August. Phyllothreta spp were feeding in buttons of Sonchus arvensis L. and on Sinapis arvensis L. Sachet dispensers in closed type delta traps (Atrakon A., Flora, Estonia) were used. In June the trap catches were (insects/trap): butyl-ITC (2.6); butyl-TCN (8.2); allyl-ITC (9.4); 3-butenyl-1-TCN (149). In July-August catches were higher: butyl-ITC (5); butyl-TCN (43); allyl-ITC (44); 3-butenyl-1-TCN (240). The main part of beetles trapped was identified as Phyllothreta undulata. The influence of "green leaf volatiles" was tested. Trap catches decreased 4-10 times when combining thiocyanates and (Z)-3- hexenol, (Z)-3-hexenyl acetate had no effect on butenyl -TCN; (E)-3-hexenol and (Z)-2- hexenal had no effect on butyl-TCN. We conclude that minor decomposition products of glucosinolates may act as basic plant-insect cues and the attraction may be influenced by "green leaf volatiles" which indicates a wounded or infested plant.