Biology, Behaviour and Genetic Diversity of Trichogramma aurosum Sugonjaev and Sorokina (Hymenoptera: Trichogrammatidae)

Abstract

Biological control of Lepidopterous pests with egg parasitoids of the genus Trichogramma (Hymenoptera: Trichogrammatidae) failed to provide successful control of the target pests in several cases because of selection of either the wrong species or inappropriate strains. Thus, biological control programmes of the Codling moth, Cydia pomonella L. (Lepidoptera: Tortricidae), with Trichogramma spp. require pre-introductory basic research on the performance of potential candidates. In the present study, 32 strains of Trichogramma aurosum Sugonjaev and Sorokina collected from different locations in the Federal Republic of Germany and from six countries in Europe were reared in the laboratory, in order to select a suitable strain and/or strains for attempts at controlling the Codling moth. The candidate strains were examined to obtain detailed information about their biology, behaviour, and genetic diversity. Experiments on host-age preference and host preference of five strains were carried out in the laboratory at room temperature. In the choice tests for host preference, the wasps did not show a significant preference for eggs of C. pomonella compared with the eggs of both Lobesia botrana Den. and Schiff, and Agrotis segetum Schiff. In contrast, in the non-choice tests all strains parasitized a significantly higher number of eggs of C. pomonella and L. botrana compared with the other hosts tested. Experiments on host age preference were conducted on eggs of C. pomonella by directly observing the behaviour of the parasitoids. None of the strains tested showed a significant preference for either fresh or old hosts. Drilling time on old eggs (four or five days old) was longer than the time consumed on fresh ones. This could be due to the mechanical resistance of the host chorion in old eggs or to cell differentiation due to aging. Females of all strains were able to discriminate between parasitized and unparasitized eggs. The acceptance/contact ratio for unparasitized host eggs was significantly higher. Resting made out 30-60% of the total handling time, followed by cleaning and walking. Fertility life tables have received increasing attention as a tool to evaluate the antagonistic potential of Trichogramma spp., and also to compare the suitability of different factitious hosts for mass-rearing. Hence, fertility life tables were constructed at 25 °C for seven strains of T. aurosum collected from seven European countries. The mean accumulative fertility ranged from 6.9 – 14.6 eggs / female, while the realised fertility (i.e. the accumulative fertility during the first three days) ranged from 5.2 – 11.6 eggs / female. Development time from egg to adult ranged from 9.9 – 11.35 days, while the adults were only able to live from 2.8 – 6.1 days. Adult emergence rate was higher than 60%, and the female proportion ranged from 58 – 96%. The mean number of progeny emerging from the eggs ranged from 5 – 13 wasps, whereas mortality in the prepupal stage ranged from 11 – 24%. Adaptability of Trichogramma spp. to adverse abiotic factors such as high or low temperature is one of the major selection criteria in biological control. Accordingly, the impact of constant and alternating temperature regimes on the parasitization potential and the population growth parameters of five German strains of T. aurosum was studied. Accumulative fertility was positively correlated with temperature, while development time from egg to adult and female longevity were negatively correlated with temperature. The lowest development threshold temperature was 9 – 10 °C. Extreme high temperatures had a significant adverse effect on the sex ratio and the emergence rate. The values of cumulative fertility, longevity and developmental time were reduced at alternating temperatures compared with constant temperatures. This could be due to the better metabolic process of the immature stages at alternating temperatures. We conclude that rearing parasitoids at constant temperatures may overestimate their efficiency in biological control under field conditions, because the values of the biological parameters were higher at constant temperatures than at alternating temperatures. This might be due to an adaptation of Trichogramma to the constant laboratory rearing conditions. The strain from Munich (Ta13) showed the best biological properties at all alternating temperatures studied. Molecular techniques have gained increasing interest in the past ten years to assist assessing biological diversity, phenology, and population dynamics of insects. This is also the case in Trichogramma spp., especially what is concerned with the identification of morphologically close related species or those that cannot be assessed morphologically (e.g. parthenogenetically reproducing forms). In this part of the work, the ITS2 region (rDNA) of the collected strains was assessed and compared with those of other Trichogramma spp. The amplified products of the T. aurosum strains tested had all the same size (ca. 530 bp), and the sequences (411 nucleotides) possessed a high degree of homology (> 96%). However, when sequences were compared with those of strains from the USA, homology ranged from 86 to 90%. Thus, it is not clear yet, whether the collected strains and those from the USA belong to the same species. Here further work is needed. Parsimony analysis with unweighted characters resulted in 792 equally parsimonious trees each with a length of 938 (CI = 0.664, RI = 0.429). In general, strains collected from Germany and other European countries were grouped together with the strains from both Switzerland and the USA. Moreover, ITS2 sequences of T. aurosum were rendered together with T. alpha Pinto and T. sibericum Sorokina. All three species have been recovered from eggs of hymenopteran hosts, thus suggesting that they might have evolved from a single ancestor. Analysis of Amplified Fragment Length Polymorphisms (AFLPs) has been confirmed as a powerful method for the characterization of intraspecific genetic variability among populations of insects because of its high reproducibility. Therefore, this technique was used to analyse the genetic structure, genetic diversity and gene flow between strains of T. aurosum. The analysis of five AFLP primer combinations among the 31 strains of T. aurosum showed that the mean gene diversity within strains was higher than between strains. Using the Bayesian analysis the Wright’s fixation index value (Fst) it could be shown that there was very low gene flow between the populations of T. aurosum (i.e. very low genetic differentiation), as well as very little genetic variability. The homogeneity between the strains was very high; it ranged from 75 to 95% similarity. Only two strains (Ta29 and Ta30) showed a polymorphic fingerprint pattern. Finally, no clear clustering in relation to geographical origin was formed in the UPGMA dendrogram, with few exceptions. Conversely, other strains were grouped together, although they were collected from widely separated regions.