**2. Field and laboratory investigations of interactions of cabbage flea beetles (***Phyllotreta* **spp.) and cabbage stink bugs (***Eurydema* **spp.) with cabbage and selected trap Brassica crops**

#### **2.1. Association between glucosinolate concentration and injuries caused by cabbage stink bugs,** *Eurydema* **spp. (Heteroptera: Pentatomidae), on different Brassicas**

In 2010, our group was involved in determining [3] the content of glucosinolates in different *Brassica* species in order to determine their impact on feeding of cabbage stink bugs (*Euryde‐* *ma* spp.) and the consequent extent of damage caused by feeding. We confirmed that the level of glucosinolates depends on plant species, plant part and the time of sampling. In these samples, aliphatic glucosinolates prevailed. Glucobrassicin, an important indolic glucosino‐ late, was detected in all tested Brassica species. Its content was the highest in oil radish samples during the first assessment (30 DAS), 8.84 ±0.65 μmol g−1 ds, while the oilseed rape samples displayed lowest concentration during the last assessment (134 DAS), 4.30±0.80 μmol g−1 ds. Based on these results, we cannot confirm the stimulative activity or negative influence of a specific glucosinolate on feeding of *Eurydema* spp. Based on the results of our research, we can conclude that oil rape was the most adequate trap crop used to allure cabbage stink bugs. In future, glucosinolates should be employed to a greater extent in environmentally acceptable ways of food production, one of which is also the use of trap crops in order to reduce harmful effects of cabbage stinkbugs.

chemical factors. By exploiting natural resistance of plants, we can reduce the use of insecti‐ cides and thus avoid the pertaining resistance of harmful insects. Different factors are in‐ volved in defence of Brassicas against harmful organisms. It has been established that more solid (more compact heads) cabbage [1] and higher content of epicuticular wax [2] diminish the extent of damage done to cabbage by onion thrips (*Thrips tabaci* Lindeman). Thicker layer of epicuticular wax can negatively influence also the feeding of cabbage flea beetles (*Phyllotreta*

In our research, whose results are briefly presented on the following pages, we focused on the study of glucosinolate content in different species of Brassicas and their effects on the target

**•** Different concentrations and ratios of glucosinolates in trap crops influence their different susceptibility to cabbage stink bugs and cabbage flea beetles. Thus, we think they are of

**•** We think that glucosinolate content differs in individual organs of the same plant species, which consequently causes different extent of damage due to feeding of the studied groups

**•** We think the content of secondary metabolites (glucosinolates) in the studied Brassicas is

**•** We suppose there is a connection between the glucosinolate type and susceptibility/

**•** The extent of damage caused by the studied harmful pests depends also on the principles of good agricultural practice and on the application of agrotechnical measures. For this purpose, we will point out the connection between the average index of damage by harmful pests during the growth period and morphological properties of trap crops and cabbage as the

**•** By selecting plant species which differ also in their length of growth period, we want to influence also the bionomy of the studied groups of harmful pests, *Phyllotreta* spp. and *Eurydema* spp. We expect there are differences in susceptibility to damage also between the

**2. Field and laboratory investigations of interactions of cabbage flea beetles (***Phyllotreta* **spp.) and cabbage stink bugs (***Eurydema* **spp.) with cabbage**

**2.1. Association between glucosinolate concentration and injuries caused by cabbage stink**

In 2010, our group was involved in determining [3] the content of glucosinolates in different *Brassica* species in order to determine their impact on feeding of cabbage stink bugs (*Euryde‐*

two cultivars of cabbage, which differ in the length of their growth periods.

**bugs,** *Eurydema* **spp. (Heteroptera: Pentatomidae), on different Brassicas**

group of harmful organisms, cabbage flea beetles and cabbage stinkbugs.

influenced also by environmental factors, primarily the air temperature.

resistance of the plant species to attacks by the studied harmful pests.

spp.) and cabbage stink bugs (*Eurydema* spp.) [2].

of harmful pests.

422 Insecticides Resistance

main plant species.

**and selected trap Brassica crops**

Before the research, we made the following hypotheses:

different suitability for trapping the studied harmful pests.

#### **2.2. Glucosinolates as arsenal for defending** *Brassicas* **against cabbage flea beetles attack**

Feeding of cabbage flea beetles (Figure 1) on various *Brassica* species can reduce the plant's productivity. While progressing towards the goal of reducing the use of synthetic pesticides and promotion of environmental protection, we wish to exploit plants' natural resilience [4]. The results of our study carried out in 2010 show that glucosinolate contents vary with plant species, plant organs and period of growth. Among the indole glucosinolates, all *Brassica* species displayed the presence of glucobrassicin, whose influence on cabbage flea beetles varied according to the plant species.

**Figure 1.** Injuries caused by cabbage flea beetles (*Phyllotreta* spp.) on oil radish (photo: T. Bohinc)

We established that progoitrin (*r* = 0.51), sinalbin (*r* = 0.61) and gluconapin (*r* = 0.67) stimulate the feeding of flea beetles on oil rape, while the gluconasturtiin content in oil rape negatively (*r* = −0.99) influenced the feeding of flea beetles. The oil rape displayed the significantly highest damage done by the said harmful pest, but no significant influence of gluconasturtin and glucoiberin influence of flea beetles was detected in this species. Oil radish thus proved to be the most suitable species as a trap crop for flea beetles. We maintain that the protection of the Brassica family against flea beetles can efficiently depend on glucosinolates content in combination with other agrotechnical measures.

#### **2.3. Environmental factors affecting the glucosinolate content in Brassicaceae**

This study describes the effects of environmental factors, the average and highest daily temperatures, the average relative air humidity and the duration of the daily solar radiation on the glucosinolate content in Brassicaceae [4]. The results of our study indicate that the content of indole glucosinolate, glucobrassicin, is influenced by the average daily and highest air temperature. Indole glucosinolates were much more susceptible to environmental factors than aliphatic or aromatic glucosinolates. Although the impact of the environment on the groups of aliphatic and aromatic glucosinolates was variable, there was a significant impact of the environment on specific aliphatic or aromatic glucosinolates. We conclude that climatic conditions cannot be neglected in the future planning of cropping systems, as our results showed significant effect of environmental factors on the glucosinolate content in Brassicaceae.

This study describes the effects of environmental factors, the average and highest daily temperature, the average relative air humidity and the duration of the daily solar radiation on the glucosinolate content in Brassicaceae. The results of our study indicate that the content of indole glucosinolate, glucobrassicin, is influenced by the average daily and highest air temperature. Indole glucosinolates were much more susceptible to environmental factors than aliphatic or aromatic glucosinolates. Although the impact of the environment on the groups of aliphatic and aromatic glucosinolates was variable, there was a significant impact of the environment on specific aliphatic or aromatic glucosinolates. We conclude that climatic conditions cannot be neglected in the future planning of cropping systems, as the results of our investigation showed the important effect of environmental factors on the glucosinolate content in Brassicaceae.

#### **2.4. Trap crops for reducing damage caused by stink bugs (***Eurydema* **spp.) and flea beetles (***Phyllotreta* **spp.) on white cabbage: Fact of fantasy?**

During the years 2009 and 2010, a field experiment was carried out to determine the effect of three trap crops (oil radish, oil rape and white mustard) as a protection method against cabbage stink bug (*Eurydema* spp.) and flea beetle (*Phyllotreta* spp.) attacks on two hybrids of white cabbage. The experiment was designed as a randomised complete block with four treatments, each replicated four times. The damage caused by cabbage stinkbugs and flea beetles was estimated in 10-day intervals, considering main cash crop and trap crops. Based on statistical analysis, we can conclude that oil rape was the most effective trap crop against cabbage stink bugs. Flea beetles have shown specific preference to oil radish as a trap crop in 2010, while they did not show specific preference to any of the trap crop tested in 2009. The damage caused by cabbage stink bugs on cabbage started to increase by the beginning of July, while that caused by flea beetles started to increase at the end of May.

We established that progoitrin (*r* = 0.51), sinalbin (*r* = 0.61) and gluconapin (*r* = 0.67) stimulate the feeding of flea beetles on oil rape, while the gluconasturtiin content in oil rape negatively (*r* = −0.99) influenced the feeding of flea beetles. The oil rape displayed the significantly highest damage done by the said harmful pest, but no significant influence of gluconasturtin and glucoiberin influence of flea beetles was detected in this species. Oil radish thus proved to be the most suitable species as a trap crop for flea beetles. We maintain that the protection of the Brassica family against flea beetles can efficiently depend on glucosinolates content in

This study describes the effects of environmental factors, the average and highest daily temperatures, the average relative air humidity and the duration of the daily solar radiation on the glucosinolate content in Brassicaceae [4]. The results of our study indicate that the content of indole glucosinolate, glucobrassicin, is influenced by the average daily and highest air temperature. Indole glucosinolates were much more susceptible to environmental factors than aliphatic or aromatic glucosinolates. Although the impact of the environment on the groups of aliphatic and aromatic glucosinolates was variable, there was a significant impact of the environment on specific aliphatic or aromatic glucosinolates. We conclude that climatic conditions cannot be neglected in the future planning of cropping systems, as our results showed significant effect of environmental factors on the glucosinolate content in Brassicaceae.

This study describes the effects of environmental factors, the average and highest daily temperature, the average relative air humidity and the duration of the daily solar radiation on the glucosinolate content in Brassicaceae. The results of our study indicate that the content of indole glucosinolate, glucobrassicin, is influenced by the average daily and highest air temperature. Indole glucosinolates were much more susceptible to environmental factors than aliphatic or aromatic glucosinolates. Although the impact of the environment on the groups of aliphatic and aromatic glucosinolates was variable, there was a significant impact of the environment on specific aliphatic or aromatic glucosinolates. We conclude that climatic conditions cannot be neglected in the future planning of cropping systems, as the results of our investigation showed the important effect of environmental factors on the glucosinolate

**2.4. Trap crops for reducing damage caused by stink bugs (***Eurydema* **spp.) and flea beetles**

During the years 2009 and 2010, a field experiment was carried out to determine the effect of three trap crops (oil radish, oil rape and white mustard) as a protection method against cabbage stink bug (*Eurydema* spp.) and flea beetle (*Phyllotreta* spp.) attacks on two hybrids of white cabbage. The experiment was designed as a randomised complete block with four treatments, each replicated four times. The damage caused by cabbage stinkbugs and flea beetles was estimated in 10-day intervals, considering main cash crop and trap crops. Based on statistical analysis, we can conclude that oil rape was the most effective trap crop against cabbage stink bugs. Flea beetles have shown specific preference to oil radish as a trap crop in 2010, while

**2.3. Environmental factors affecting the glucosinolate content in Brassicaceae**

combination with other agrotechnical measures.

424 Insecticides Resistance

content in Brassicaceae.

**(***Phyllotreta* **spp.) on white cabbage: Fact of fantasy?**

**Figure 2.** The field experiment at the laboratory field of the Biotechnical Faculty in Ljubljana (photo: T. Bohinc).

**Figure 3.** Mating cabbage stink bugs (*Eurydema oleracea*) on a blossom of white mustard (photo: T. Bohinc).

#### **2.5. Sowing mixture of Brassica trap crops is recommended to reduce** *Phyllotreta* **beetles injury to cabbage**

We studied the extent of damage caused by cabbage flea beetles on four different *Brassica* species in a 2-year field experiment (2009–2010) at two locations in Slovenia. The entire experiment was based on testing oilseed rape, white mustard and oil radish as potential trap crops to protect cabbage from cabbage flea beetles [5,6]. A significant influence of the *Brassi‐ ca* species on the feeding by the flea beetles was confirmed at both the locations. The damage index on oil radish was the highest throughout most of the growth period, whereas oilseed rape and white mustard were preferred only during a certain growth period. The initial damage caused by the cabbage flea beetles occurred in the first half of May, whereas the heaviest damage occurred at the beginning of July. This research shows that the onset of cabbage flea beetle feeding can be controlled in a medium–late cabbage cultivar using trap cropping. However, since none of the tested trap crops strongly attracted the flea beetles throughout the entire growing period of the crop, we recommend sowing mixtures of crops for cabbage production such that each of three *Brassica* species would attract phytophagous insect during a particular part of the cabbage growing season.
