Huhu, Prionoplus reticularis White (Coleoptera: Cerambycidae), are native New Zealand borers of quarantine importance. Huhu do not attack living trees, although their eggs can be laid on freshly-felled timber and larvae may be found in older export timber. To control insect pests, many logs from New Zealand are currently fumigated with methyl bromide (MBr), which has led to research to find alternatives for this ozone-depleting chemical. Treatments using heat and controlled atmospheres require a large amount of insect mortality data, often collected on a trial and error basis, which is time consuming and expensive. Insect physiological responses to treatment stress may complement mortality studies. These physiological studies on huhu sought to determine how huhu larvae respond to stresses caused by elevated temperature and low oxygen atmospheres. The purpose was to identify metabolic responses in huhu that could assist with development of commodity, insect disinfestation strategies.
An artificial diet for huhu larvae was developed and tested for rearing uniform insects for physiological studies. Larvae were reared at 20 and 25°C and two day-lengths to identify suitable rearing conditions. The artificial diet was suitable for rearing huhu larvae at 20°C with either day-length. Rearing at 25°C shortened development time initially, but ultimately, larval weight decreased and many died.
Huhu larvae had standard and active metabolic rates at 20°C of 1.66 and 4.42 μmol.kg⁻¹.s⁻¹, respectively as measured by manometric compensatory respirometry. Larval metabolic rate increased 1.72 times in response to a 10°C increase in temperature, over the range of 20 to 42°C. At temperatures greater than 35°C, larval movement ceased, consistent with stress responses reported for other insects.
An automated gas analysis system was constructed and calibrated, which redirected the focus of planned research using the manometric system. SMR for huhu at 20- 40°C was lower than predicted values, but within the range of non-flying adult coleopterans and had a mass exponent between 0.62 and 0.67. The respiratory quotient (RQ) increased with temperature, suggesting a shift from metabolising fat to include other substrates. Activity increased larval metabolism but RQ was independent of activity at 20°C. Temperature sensitivity was similar when measured by gas analysis or manometrically.
Huhu oxygen consumption ( ṀO₂ ) and carbon dioxide production ( ṀCO₂ ) were measured with the automated gas analysis system. .When larvae were exposed to extreme constant temperatures, 35-45°C, larval ṀO₂ increased until a critical temperature (42/43°C) resulted in falling ṀO₂ , which was exacerbated by time. ṀCO₂ reached a thermal maximum, suggesting the elimination of carbon dioxide may be a limiting factor. These changes in metabolism were consistent with mortality studies on huhu exposed to heat. Larval ṀCO₂ was reduced in 1.8% oxygen and movement ceased, suggesting huhu use metabolic depression to survive hypoxia. Recovery from anoxia increased ṀO₂ as larvae repaid an oxygen debt.
Huhu haemolymph pH and lactate levels were measured after exposure to stress. Extreme temperature, reduced oxygen atmospheres and their combinations increased anaerobiosis, indicated by increased haemolymph acidosis and lactate. The metabolic response depended on the severity of the atmosphere-temperature treatment and exposure time.
This study has successfully measured the metabolism of a larval cerambycid, and added valuable knowledge to the relative paucity of studies in this subject area. In response to stress, huhu larvae can utilise anaerobic pathways leading to severe acidosis with large amounts of lactate, in contrast to much of the published literature. The respiratory responses and increased anaerobic respiration found in this study, when an insect is exposed to stress, will assist the development of commodity disinfestation protocols and will contribute to a reduction the use of MBr.
