Thiourea as a xenobiotic, showing its adverse effects on mortality, behaviour and metamorphosis and on histopathological and cytological changes in the developing ovaries of Dysdercus similis.

The effect of thiourea (applied topically, in different concentrations, to the eggs and all the nymphal instars, and to adults also by feeding) on the red cotton-bug Dysdercus similis was studied. The eggs were severely affected (i.e. mortality was high), while the nymphal instars showed an adverse effect on ecdysis and adults which emerged from the treated last nymphal instar were characterized by high mortality, abnormal behaviour and reduced fecundity and viability. They were also smaller, weaker and morphologically abnormal compared with the controls and were differently coloured. They were likewise unable to reproduce, owing to atrophy of the gonads. The ovaries contained a large number of immature, pathological oocytes with degenerating follicular epithelium, which was thin with a regular outline in the early stages, but later became multilayered, with pyknotic nuclei, and displayed active destruction of glycoproteins and lipoproteins. Fibrogenesis and thickening of the tunica propria were clearly discernible. Although treated females were kept together with untreated males, the two showed no signs of being attracted to one another and displayed no mating behaviour, although some females showed a very low degree of ovulation. Only a few survived the Ist nymphal instar and none survived up to adult emergence, despite being kept under normal laboratory conditions.