Einfluss von Pyrrolizidinalkaloiden auf die Honigbiene (Apis mellifera) (English shop)

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Insights were gained into the influence of PAs on the behaviour and physiology of adult honeybees. Answers were also found to the question of whether a honeybee colony may be endangered by the toxicity of PAs when PA-containing plants occur in increased numbers in its surroundings.

In laboratory trials with adult honeybees under controlled no-choice conditions, it was found that the natural N-oxide mixture from S. vernalis exerts significant feeding-deterrent effects from concentrations of > 0.2 % onwards. In the trials with the corresponding tertiary PA mixture, similar tendencies towards a decrease in the amounts consumed were discernible, but these proved not to be significant.

By means of feeding trials with a bee colony in its natural environment, the behaviour of adult bees towards PA-containing food sources under choice conditions could be elucidated. Both tertiary echimidine and echimidine N-oxide as well as the PA mixtures (tertiary and N-oxidised) from S. vernalis tended to exert antifeedant effects at concentrations between 0.02 and 0.2 %. Only in the case of the tertiary PA mixture did these not prove to be significant. Compared with the effects under no-choice conditions, the antifeedant effects under natural conditions were higher by a factor of 10.

High concentrations of 2 % of the tertiary PA mixture from S. vernalis and of pure monocrotaline caused an increase in mortality in the adult bees to over 50 % on average. It can be ruled out that the significant mortalities observed were attributable to starvation due to feed refusal under the no-choice conditions.

The toxic effects of PAs on adult bees can be linked to the double bond in the PA ring system. In feeding trials, 1,2-dihydromonocrotaline, as a representative of the 1,2-saturated PAs, showed no toxic effects whatsoever, whereas the 1,2-unsaturated form (monocrotaline), like the tertiary PA mixture (likewise 1,2-unsaturated PA representatives), caused a significant increase in mortality at high concentrations.

The PA contents in individual bees from the feeding trial with the tertiary PA mixture showed a wide scatter. Specimens that had died early contained on average around 175 μg PAs in the body. Bees that died towards the end of the trial, by contrast, contained on average up to 250 μg PAs. Considerably lower were the mean PA contents of 50 μg in the surviving bees, in which, however, maximum values of around 200 μg per individual bee were also reached.

In the uptake and storage of the various tertiary PAs, there was no structurally determined discrimination. The alkaloid profile in the bees examined remained unchanged relative to the PA profile of the feed mixture used. The quantitative recovery of the amount of PAs consumed per group of bees in a cage was 61 %.

It was found that bees are not able to detoxify tertiary PAs by endogenous N-oxidation. The reverse direction, the reduction of N-oxides into the toxic tertiary forms, was demonstrated to an extent of 70 %. The resulting maximum contents of tertiary PAs per individual bee were 50 μg.

Once taken up, PAs are neither resorbed into the haemolymph nor accumulated in the fat body of the bee organism. With the aid of 14C-labelled standard substances, a continuous transfer of both the tertiary and the N-oxidised PAs from the gut into the rectum (faecal sac) was demonstrated.

PAs taken up into the honey sac by an adult bee are passed on to other bees during trophallaxis. In the trials, the extent of this depended on the PA concentration in the feed and decreased with increasing PA content.

The larvae of the honeybee are considerably more sensitive to PAs in their food. In the brood, tertiary PAs caused significant toxic effects at markedly lower concentrations (from 0.02 % onwards). With N-oxidised PAs, these occurred in each case at concentrations higher by a factor of 10 (0.2 %). Even the 1,2-saturated form had significant toxic effects on the larvae at concentrations increased by a further factor of 10 (2 %).

As in the adult stage, the larvae possess endogenous (intoxication) mechanisms for the reduction of N-oxides into the tertiary forms.

Detailed information was obtained on the PAs present in the forage plant E. vulgare used by bees, as well as on the distribution of the individual structures in different plant organs. The four PAs echimidine, vulgarine, acetylechimidine and acetylvulgarine were identified, each in their tertiary and N-oxidised forms. The PA pattern of the individual structures in the different organs of the plant differs greatly. A major commonality that can be highlighted is the higher presence of the N-oxides at approx. 75 %.

The quantitative PA contents of the plant organs examined also turned out to be markedly different. The bee-relevant parts pollen and nectar likewise showed more or less high PA contents, at 1.5 and 0.02 mg/g retronecine equivalents respectively.

The coexistence of PA plant and bee is a product of nature and does not constitute a problem for either of those involved. In the course of evolution, the honeybee has adapted to the occurrence of PAs in its environment. A linking of the results makes clear that the toxic PAs inevitably occur in honey and pollen pellets, the products used by humans. The extent to which PAs are carried over and spread within the human food chain can ultimately be influenced by humans alone. The PA contamination of honey can be reduced by limiting the honeybees’ access to PA plants (choice of site by the beekeeper). In addition, the spread within the food chain can be controlled by acting correctly in the case of PA-contaminated batches.