**4. Conclusions**

stink bugs prefer to appear on plants of cabbage which are farthest from the trap crop, while the influence of a plant's distance on damage by cabbage flea beetles was not established. 'Hinova' in our conditions of production proved to be the hybrid which yielded a bigger average crop, the selection of the early hybrid 'Tucana' proved as unsuitable. According to the results of our research we can say this is congruent with the findings of the authors [31–33], who report that alternative methods (intermediate crops, green covers) did not reach the

By more intensive care for the main crop (watering during the growth period, dressing with fertilisers...) we could provide bigger crops. It is well known that the trap crop method can be carried out in several ways. One of these is to collect the trap crops in the middle of the growth period where we carried out the experiment [34]. At the same time we would risk that the studied group of harmful pests massively moved to the main crop [34]. We could treat the trap crop with insecticides and thus provided the so-called dead-end trap crop. Yet by using insecticides we would bring more damage than benefits [35]. The species which were used as trap crops were for a part of their growth period also blossoming and thus attracting useful organisms. The reasons for different susceptibility in the studied species of Brassicas can be found in natural resistance of plants. Glucosinolates, which are considered by some as an important factor of plants' resistance against harmful organisms, and by others as negligible in this regard, can in the protection of plants act stimulatory or deterring. While gluconasturtiin in oilseed rape acts as a deterrent (*r* = −0.99) for feeding by cabbage flea beetles, we cannot attribute the said glucosinolate the same property in case of white mustard and oilseed radish. According to the research, we can conclude that the effects of these secondary metabolites on harmful insects are very complex. We thus cannot talk about some universal influence of the three groups of glucosinolates on harmful organisms, and the influence of each glucosinolate

Gluconasturtiin in oilseed rape acts negatively on feeding by cabbage flea beetles (*r* = −0.99) and cabbage stink bugs (*r* = −0.98). Feeding by cabbage flea beetles (*r* = −0.80) and cabbage stink bugs (*r* = −0.99) on the said plant species is strongly influenced by epiprogoitrin content. Glucoiberin content in the samples of oilseed rape negatively influenced the feeding by cabbage flea beetles (*r* = −1) and cabbage stink bugs (*r* = −1), progoitrin in the samples of oilseed rape stimulated the feeding by cabbage stink bugs (*r* = 0.51) and cabbage flea beetles (*r* = 0.51), while the activity in the samples of 'Tucana' negatively influenced the feeding by cabbage stink bugs (*r* = −1.0). Gluconapiin is the only glucosinolate which in the plants oilseed rape acts stimulatory on the feeding by cabbage stink bugs (*r* = 0.64) and cabbage flea beetles (*r* = 0.67). We detected no pronounced influence of the studied glucosinolates on feeding by the species from the genus *Phyllotreta* in the samples of oilseed radish. We can conclude that the barely perceptible presence of gluconasturtiin, glucoiberin and gluconapiin most probably contrib‐ uted to the higher index of damage by cabbage flea beetles on the studied species of Brassicas.

Because of the thus far collected findings that glucosinolate content changes during the growth period and depends on the plant species, we wanted to compare the extent of damage by

cabbage flea beetles also in regard to the location of the experiments.

desired goals.

430 Insecticides Resistance

should be analysed separately.

In the years 2009–2010, we were in two field experiments establishing the efficiency of trap crop methods for reducing the extent of damage by cabbage flea beetles (*Phyllotreta* spp.) and cabbage stink bugs (*Eurydema* spp.) on the main plant species, cabbage. Because the extent of damage was significantly higher on the trap crops and because the damage was first noticed on the trap crops, we can say that the chosen alternative method of plant protection proved as efficient.

The first appearance of cabbage flea beetles was in our research detected in the beginning of May, while cabbage stink bugs began appearing in the second half of May. The said findings are congruent with the results of bionomy of the studied groups of harmful pests in the central part of Europe. Feeding of cabbage flea beetles and cabbage stink bugs was noticed on all selected plant species. On the basis of the collected data, we have found out that cabbage stink bugs display particular preference to feeding on oilseed rape, while cabbage flea beetles were feeding mostly on oilseed radish, which proved the most attractive host for them.

The trap crop method that we studied in our research will be feasible especially in the systems of mid-late cabbage production. The average mass of cabbage was influenced primarily by the selection of a hybrid. Mid-late hybrid 'Hinova' in our research proved to be more productive and consequently more suitable for production in our growth conditions.

On the basis of our research results, which were obtained at two different locations, we can conclude that air temperature has an important effect on the extent of damage by cabbage flea beetles on Brassicasas. Rising average daily temperature of air caused higher extent of damage by cabbage flea beetles on the studied species of Brassicas.

The different preferences of the studied groups of harmful pests are conditioned also by natural resistance of plants. One of the factors of natural resistance in Brassicas is also glucosinolate content. Glucosinolates in our research proved to be an important, yet variable factor of natural resistance of Brassicas to attacks by cabbage flea beetles and cabbage stink bugs. We found out that variability of glucosinolates was conditioned by plant species, and the content of these substances differs considerably also between different organs of the same plant species. Our research also revealed variability in the content of glucosinolates between individual geno‐ types of the same plant species, i.e., between individual genotypes of cabbage. In the samples of oilseed radish, we detected the largest amount of glucoraphanin (8.66±1.81 μmol/g mass of dry seed); sinalbin was present in the significantly largest amount in the samples of oilseed rape and white mustard. Although the analysis confirmed sinalbin as the most frequent glucosinolate in the samples of white mustard, the further analysis of the data revealed weak correlation (*r* = 0.36) between its content versus Brassicas and the content of damage by cabbage stink bugs on them.

Sampling of different plant parts showed that the content of certain glucosinolates, for example glucobrassicin and glucoraphanin in blossoms of oilseed radish or sinalbin and epiprogoitrin in blossoms of white mustard, is much higher than in the leaves of the studied plant species. We found out that cabbage flea beetles were at the time of blossoming very intensively feeding on plants of white mustard and oilseed radish.

We have found out that glucosinolate content in plants is influenced primarily by temperature extremes. The results of past studies concerning activities of these secondary metabolites support the notion that they act in a manner that is specific for an individual group of these secondary metabolites. However, we found out that there are differences in their activities also within individual groups of glucosinolates. Glucobrassicin, the only detectable glucosinolate in all studied plant species, belongs to indole glucosinolates. We have found out that this glucosinolate in oilseed radish affects negatively the extent of damage by the species from the genera *Phyllotreta* and *Eurydema*. The said substance belongs to the group of those which were most significantly influenced by environmental parameters, primarily the average daily and highest daily air temperature. Gluconasturtiin and epiprogoitrin in oilseed rape had on both studied groups of harmful pests negative effects, sinalbin had negative effects on the feeding of cabbage stink bugs in the samples of oilseed rape, while it stimulated cabbage flea beetles.

The obtained data at the two different locations showed that the extent of damage by cabbage flea beetles can be successful controlled by using the mixed crops of Brassicas, which were used in our experiment. The said combination of plant species would be in view of our findings suitable for both locations at which we were carrying out the experiment as well as for other Slovenian regions in which cabbage is produced and where producers encounter the harmful pests studied in our research. By seeding plants trap crops before the main crops in both years of the experiment, we considerably influenced the fact that damage by the species from the genera *Phyllotreta* and *Eurydema* first appeared on the trap crops.

The selected trap crop method successfully controlled the appearance of the species from the *Phyllotreta* spp. and *Eurydema* spp. on cabbage. Since little or no synthetic preparations are available for suppressing cabbage flea beetles or cabbage stink bugs, alternative methods is a potential option. Since agricultural land intended for production of Brassicas has been in recent years expanding, new methods for protecting crops represent new possibilities for environ‐ mentally acceptable production of cabbage. The said method is useful primarily in ecological and integral production. Commercial consumption teaches us that we increasingly trust the locally grown food, if possible without insecticides. It is true that our experiment did not produce the extent of crops which would be of commercial interest, yet we produced cabbage without using insecticides, and this will enable us to further arouse the interest of consumers, who want healthy food. In order to achieve reduction in the population of the species from the genera *Phyllotreta* and *Eurydema* on cabbage, the use of mixed crops of Brassicas seem quite realistic. As our research shows, the method is more efficient in systems of production where mid-late genotypes of cabbage are grown. As we found out, due to the spectrum of different factors (among others also glucosinolate content [45,46]), susceptibility of plants to damage by harmful organisms during the growth period varies. By using mixed crops as a means to reduce damage on the main crops, the said method could be used in different regions of Slovenia and thus override the influence of environmental factors on trap crops as well as the bionomy of the said harmful species itself [47]. Other species of Brassicas can be used as trap crops, but that is the subject of another research.
