Measuring and interpreting the energy budget of the tick, Ixodes ricinus

Abstract

Precise and accessible techniques for measuring metabolic responses to environmental stress are essential to allow the likely impacts of climate and climate change on tick distribution, abundance and phenology to be predicted. Here, in Chapter 2, a series of biochemical protocols employing spectrophotometric methods are described and used to determine the entire energy budget of ticks. Protein, total lipid, neutral lipid, the total water-soluble carbohydrates and glycogen were measured in individual Ixodes ricinus nymphs and adults. Two key trends were identified: in adults, protein was relatively more abundant than in nymphs, whereas in nymphs, glycogen and soluble carbohydrates were more abundant than in adults, with glycogen alone composing 39% of the mass of metabolites in nymphs compared to 15 and 10% in females and males, respectively. The results demonstrate that the spectrophotometric approaches deliver relatively rapid and reliable estimates of the total energetic budget and can be used to quantify the metabolic profiles of individual ticks. The work described in Chapter 3 aimed to investigate the effects of temperature on the rate of depletion of energy reserves by nymphal and adult I. ricinus. A cohort of nymphs, males and females were collected and divided into incubators at a range of temperatures. The protein, total lipid, neutral lipid, soluble carbohydrates and glycogen levels were measured over time. In nymphs, the rate of soluble carbohydrates and glycogen utilisation was higher than in males or females and the concentrations of neutral lipids were affected significantly by higher temperatures. In adults, the concentrations of protein and structural lipid (phospholipid) responded rapidly to changes in treatment time and the ambient temperature. Nymphs and adult I. ricinus were sampled from the field each month from February 2018 to January 2019 and, in Chapter 4, the changes in energy source contents over the year are presented. The data suggest that there exists a well-defined cohort of relatively well-fed nymphal ticks in the early spring, most probably derived from larvae that fed the previous summer and moulted the previous autumn. They start to quest as temperatures rise sufficiently in spring to permit activity. Those that are unable to find a blood-meal continue questing but gradually exhaust their reserves. By mid-summer the only larvae left questing are close to starvation. For females, the population at the start of the year is composed of a cohort that started questing early in the year that were relatively hungry; they may have been derived from nymphs that fed relatively early the previous year moulted to become adults that were unable to feed, so overwintered and started questing early the following year in an already resource depleted state. By April, the cohort of questing females has now been joined by individuals that moulted to become adults late the previous year or early in the year and are in relatively well resourced. However, those that do not feed gradually start to exhaust their resources. In the second half of the year, females with high levels of metabolic resource appear – presumably from nymphs that early in the year of the study, digested their blood meals, moulted to become adults and are now re-joining the questing population. In males the patterns of changing energy sources were much less distinct than seen in nymphs or females. In the final Chapter, the data are discussed generally, and further applications of the spectrophotometric techniques presented here are considered.