Determining the host range and metabolic responses to starvation and feeding in a novel outbreak pest of sugarcane, Cacosceles newmannii (Coleoptera: Cerambycidae, Prioninae)

Date
2020-12
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: To gain insight into the rapid emergence of Cacosceles newmannii (Thomson 1877) (Coleoptera: Cerambycidae) as a crop pest in sugarcane, a better understanding of its biology is required. My study had two main aims. First, I aimed to understand how this species was previously integrated in its trophic network, to understand its potential host range. Therefore, I first sought to determine the host plants that C. newmannii fed on prior to its detection on sugarcane in 2015. To do this, I compared δ13C and δ15N ratios of adult beetles retrieved from South African museum collections, collected between 1891 and 2016 with samples from infested fields collected in 2017 and 2019, as well as diverse, plausible host plants from infested fields and surrounding patches of indigenous and commercial forest vegetation. I then used isotope mixing models to infer the relative contribution of the different plants to the diet of C. newmannii. Second, I aimed to determine the metabolic responses to starvation and feeding in C. newmannii at two temperatures. This was done by estimating multiple parameters related to digestive physiology. Additionally, in individuals that showed cyclic or discontinuous gas exchange patterns, I compared gas exchange characteristics and how these were influenced by variation in temperature. The results showed that pre-outbreak, C3 plants contributed strongly to the larval diet while post-outbreak, C4 plants were the largest component of their diet. There was some C4 plant contribution to their diet pre-outbreak suggesting that C. newmannii did not undergo a dramatic host shift but rather experienced a rapid increase in the proportion of C4 plants in their diet. Furthermore, I found that the metabolic rate of C. newmannii is sensitive to a change in temperature when fed with sugarcane, but relatively temperature insensitive under starved conditions. For the larvae that showed cyclic gas exchange, there was no significant difference in burst rate, volume or duration, or interburst rate or volume, between 20 °C and 30 °C, although interburst duration was significantly longer at 20 °C compared to 30 °C. I conclude that C. newmannii larvae have physiological characteristics that could be advantageous for coping in resource poor, seasonal or fluctuating environments and ultimately have increased their ability to invade a novel environment and become pests of high-energy host plants like sugarcane.
AFRIKAANSE OPSOMMING: Om ‘n beter begrip the kry van die skielike verskyning van Cacosceles newmannii (Thomson 1877) (Coleoptera: Cerambycidae) as ‘n gewasplaag in suikerriet, is ‘n beter begrip van sy biologie nodig. My studie het twee hoofdoelstellings gehad. Die eerste doelwit was om te verstaan hoe hierdie spesie voorheen in sy trofiese netwerk geïntegreer was, om sy potensiële gasheerreeks te verstaan. Hier het ek dus probeer vasstel op watter gasheer plante C. newmannii gevoed het voor die skuif na suikerriet in 2015, deur die gebruik van δ13C en δ15N verhoudings van volwasse kewers. Kewers vanaf Suid-Afrikaanse museums versamel tussen 1891 en 2016 is vergelyk met kewers versamel in besmette velde in 2017 en 2019, asook met diverse, potensiële gasheerplante uit besmette velde en nabygeleë inheemse en kommersiële woude. Ek het gebruik gemaak van isotopiese mengmodelle om die relatiewe bydrae van verskillende plante tot die diëet van C. newmannii te bepaal. Die tweede doelwit was om die metaboliese reaksies tot verhongering en voeding te bepaal by twee temperature. Dit was gedoen deur verskeie parameters wat met verteringsfisiologie verband hou, te meet. Verder is die gaswissellings-eienskappe van individue wat sikliese of diskontinue gaswisselling getoon het, sowel as die invloed van temperatuur op hierdie eienskappe bepaal. Die resultate het gewys dat voor die skuif na suikerriet, het C3 plante ‘n groot gedeelte van die larwes se diëet opgemaak. Daarteenoor het C4 plante die grootste komponent van hulle diëet uitgemaak na die plaag uitbraak. ʼn Klein gedeelte van die larwes se dieët het bestaan uit C4 plante voor die uitbraak en dit dui dus daarop dat die kewers nie ‘n dramatiese gasheer verskuiwing gehad het nie, maar eerder ‘n toename in die proporsie van C4 plante in hulle diëet. Verder het ek gevind dat die metaboliesetempo van C. newmannii sensitief is vir 'n verandering in temperatuur as larwes met suikerriet gevoer word, maar relatief onbeïnvloed onder verhongerde toestande. Vir die larwes wat sikliese gaswisseling gewys het, was daar geen verskil in “burst” tempo, volume of tydsduur, of in “interburst” tempo of volume tussen 20 °C en 30 °C nie, alhoewel die “interburst” tydsduur aansienlik langer was by 20 °C in vergelyking met 30 °C. Samevattend het ek gevind dat C. newmannii larwes fisiologiese eienskappe het wat voordelig kan wees vir die hantering van omgewings met min hulpbronne of seisoenale of wisselende omstandighede, en dit het uiteindelik bygedrae tot hulle vermoë om 'n nuwe omgewing binne te dring en ‘n suikerrietplaag te word.
Description
Thesis (MScConsEcol)--Stellenbosch University, 2020.
Keywords
Insect -- Feeding and feeds, Metabolic rate, Isotope analyses, Sugarcane -- Diseases and pests, Cacosceles newmannii, UCTD
Citation