Evaluation of farnesol-induced changes in the haemocyte pattern of red cotton bug Dysdercus koenigii Fabricius (Heteroptera: Pyrrhocoridae)

Background Dysdercus koenigii Fabricius is a polyphagous agricultural pest of paramount importance. Farnesol is a plant-derived open-chain aliphatic sesquiterpenoid alcohol. It is an intermediate in the metabolic pathway of juvenile hormone biosynthesis. The effects of Farnesol on haemocytes of fifth instar nymphs of Dysdercus koenigii were assessed in the present investigation. The early fifth instar nymphs were treated with doses of 0.05 μL, 0.1 μL and 0.2 μL Farnesol topically and day wise change in the total haemocytes was reported. Different types of haemocytes were identified in the haemolymph based on their morphological characteristics. Difference in day wise distribution pattern of haemocytes was evaluated in relation to dose of treatment. Results In normal fifth instar nymphs, age-related variation in the haemocyte count was evident; the haemocyte number increased up to four days and subsequently decreased till six-day when the nymph moulted. The results indicate that Farnesol had an adverse effect on the total number of haemocytes; in treated nymphs the haemocyte count decreased in a dose-dependent manner. The decrease was most prominent in the nymphs treated with a dose of 0.2 μL Farnesol. Based on the morphological characteristics, six different types of haemocytes, viz. prohaemocyte, plasmatocyte, adipohaemocyte, granulocytes, oenocytoids and disintegrated cells were identified in the haemolymph. The percent composition of different haemocytes showed variation depending on the age of the fifth instar nymph and the dose of treatment. In control, the percentages of plasmatocytes decreased up to day 5 and then increased up to 7 days. The percentage of adipohaemocytes increased throughout the nymphal period of the fifth instar, except for a small dip on the fourth day. Granulocytes and oenocytoids showed no clear trend. In Farnesol-treated nymphs, an increase in percent of prohaemocytes was observed in all the treatments. Dose-dependent effects were more prominent after day 5. An increase in plasmatocytes was seen after day 3. A distinct dose-related decrease was reported in adipohaemocytes on all days. Percent of disintegrated cells also showed an increase in all the tested doses of Farnesol on all the days. Granulocytes and oenocytoids showed no definitive trend of change. Conclusions Farnesol is a plant sesquiterpenoid and acts as a precursor in juvenile hormone biosynthesis. The application of Farnesol exogenously may disturb haemocyte patterns which may challenge the immune response of insects. Therefore, Farnesol can potentially be used as an alternative approach in pest management.