The Sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae), is the most important pest of wheat and barley in wide areas of the world. Different aspects of the insect's life history have been studied, but to date nothing is known about their microbial symbionts. Here, the contribution of symbiotic bacteria to the fitness of the bug was investigated by combining two different approaches to manipulate the host's microbial community: the supplementation of antibiotics into the insects' diet and egg surface sterilization. First, bacteria cultured from gut homogenates were subjected to antibiotic screening tests using 20 different antibiotics. Norfloxacin was the most effective antibiotic, with the greatest inhibition zone among all antibiotics tested. Feeding norfloxacin to adult E. integriceps individuals significantly impaired growth and development of the offspring in a dose-dependent manner, i.e., higher antibiotic doses increased the negative effects on nymphal growth and development. Total developmental time from first nymphal instars to adult emergence in control animals was 30.1 days, but when adults had been offered diets with 10, 20, and 30 µg antibiotic per mg diet, the offspring's developmental time was prolonged to 32.8, 34.0, and 34.8 days, respectively. In the highest two doses of norfloxacin, all of the nymphs died before reaching the fifth nymphal instar. Similar results as for the antibiotic treatment were obtained when egg surface sterilization was used to manipulate the microbial community of E. integriceps. These results indicate that bacterial symbionts play a crucial role in the successful development of the host.
Introduction
Insects are able to occupy a vast array of habitats, including marine and terrestrial environments (Grimaldi and Engel 2005). A major factor that contributed to the insects' success in occupying a variety of habitats is their adaptation to feed on different diets (Dow 1986). This flexibility of feeding habits is often provided by endosymbionts, and some estimates suggest that up to 20% of all insect species harbor obligate symbiotic microorganisms (Ishikawa 2003; Moran 2006).
Herbivorous insects are facing an imbalanced diet because of excess carbohydrates in comparison with those insects feeding on a carnivorous diet containing sufficient amounts of nitrogenous compounds. One strategy to overcome such limitation is to change the feeding habit towards a more predatory life-style (Denno and Fagan 2003). However, feeding on such diverse food sources needs physiological and morphological specialization. Thus, a different adaptation is to exploit the metabolic potential of endosymbionts, microorganisms that live as symbionts in the insect body. These symbionts enable insects to feed on food resources that are rich in some nutrients and lack or are scarce in some others. Such adaptation is seen in aphids feeding on plant saps (xylem or phloem), which are rich in carbohydrates but contain only trace amounts of some nitrogenous compounds (notably the essential amino acids) that are essential for the insect's growth and development (Baumann 2005).
Earlier studies have shown that some insect