Environmental physiology of Eldana saccharina (Lepidoptera:Pyralidae) in South Africa : implications for pest management

Thesis (PhD)--Stellenbosch University, 2014.

ENGLISH ABSTRACT: Eldana saccharina Walker (Lepidoptera: Pyralidae) is a stem borer and food crop pest of economic importance. Temperature and moisture availability possibly influence E. saccharina distribution and abundance, however, the thermal biology and desiccation physiology of E. saccharina are not fully understood. Furthermore, physiological adaptation probably facilitates the invasion success of E. saccharina into novel environments and this too remains unstudied. Here, the thermal- and desiccation-trait variation of E. saccharina were studied and population responses were modelled. The results of this work provided insights into novel physiological outcomes of E. saccharina that is coupled with its environmental climatic stress resistance, overwintering ability and population fitness in general. In determining thermal limits to activity and survival of E. saccharina results showed that chill coma onset temperature (CTmin) and critical maximum temperature (CTmax) of E. saccharina moths collected from sugarcane (Saccharum spp. hybrids) were significantly lower than those from Cyperus papyrus L. (CTmin = 2.8 ± 0.4 vs. 3.9 ± 0.4 °C; CTmax = 44.6 ± 0.1 vs. 44.9 ± 0.2 °C, P < 0.0001 in both cases). These results holds important implications for habitat management (or ‘push-pull’) strategies in the sense that host plant may strongly mediate lower critical thermal limits. Results for pronounced variation in adult CTmin (± 4 °C) across the geographic range of E. saccharina in South Africa was found and it was significantly positively correlated with the climatic mean minimum temperature. Slower developmental time in the most low-temperature tolerant population suggests lower CTmin adaptation has come at a cost to fitness, but allows greater survival and activity in that environment. There are a significant reduction of phenotypic plasticity in the laboratory population and a strong genetic component to CTmin trait variation. Physiological acclimation within a single generation, during immature life stages, resulted in altered adult water balance physiology to enhance fitness. Results from a biophysical population model showed that over-wintering life stage and climate significantly affected the number of E. saccharina generations, predicted stress, relative moth fitness and relative adult abundance. Larval overwintering led to less generations and more frequent cold- and heat stress at a cold field site compared to a warm one. This in turn reflected on the relative adult fitness and –abundance. Larval presence predictions overlapped well with positive scout records averaged across a matrix of sugarcane ages and cultivars. The results from this work are important on which to base integrated pest management strategies and are applicable to a large audience across agricultural landscapes and in the sugarcane industry of South Africa.

AFRIKAANSE OPSOMMING: Eldana saccharina Walker (Lepidoptera: Pyralidae) is ‘n stamboorder en voedselgewas pes van ekonomiese belang. Alhoewel ons nie die termiese biologie en uitdrogings-fisiologie van E. saccharina verstaan nie, beïnvloed temperatuur en lugvog inhoud waarskynlik die verspreiding en teenwoordigheid van E. saccharina. Fisiologiese aanpassing fasiliteer waarskynlik die indringing sukses van E. saccharina in nuwe omgewings en navorsing op hierdie verskynsel is ook nog nie gedoen nie. Die variasie in termiese- en uitdrogingseienskappe van E. saccharina is hier genavors en populasie reaksies is gemodeleer. Die resultate van hierdie werk gee insig tot nuwe fisiologiese uitkomste van E. saccharina wat gekoppel is met omgewings-klimaat stres weerstand, oorwintering vermoë en die algemene populasie geskikdheid. Die termiese limute vir aktiwiteit en oorlewing van E. saccharina resultate het getoon dat temperature waar koue-koma opgemerk word (CTmin) en kritieke maksimum temperature (CTmax) van E. saccharina motte wat uit suikerriet (Saccharum spp. hibriede) versamel is, betekenisvol laer is in vergelyking met díe gemeet vir motte uit Cyperus papyrus L. versamel (CTmin = 2.8 ± 0.4 vs. 3.9 ± 0.4 °C; CTmax = 44.6 ± 0.1 vs. 44.9 ± 0.2 °C, P < 0.0001 in albei gevalle). Hierdie resultate het belangrike gevolge vir habitat (of ‘push-pull’) strategië in die sin dat gasheerplant lei tot veranderede kritieke temperatuur-limute van E. saccharina. Daar is betekenisvolle variasie in volwasse mot CTmin (± 4 °C) oor die geografiese verspreiding van E. saccharina in Suid-Afrika en dit is betekenisvol positief gekorreleerd met die gemiddelde minimum temperatuur. ‘n Stadige ontwikkelingstyd in die mees koue-tolerante populasie opper dat ‘n laer CTmin aanpassing tot ‘n fiksheidskoste lei, alhoewel dit aktiwiteit en oorlewing in die omgewing verbeter. Daar is ‘n betekenisvolle afname van fenotiepe-plastisiteit in die laboratorium kolonie en ‘n sterk genetiese komponent aan die variasie in die CTmin eienskap gekoppel. Fisiologiese akklimasie binne ‘n enkele generasie, deur die onvolwasse lewensstadia, het gelei tot veranderde water balans fisiologie om die geskikdheid van volwasse motte te verbeter. Resultate van die biofisiese populasie model het getoon dat die oorwinteringstadium en ook klimaat ‘n betekenisvolle effek het op die aantal generasies, voorspelde stres, relatiewe-geskikdheid en - voorkoms van E. saccharina. Larwale oorwintering het gelei tot minder generasies en meer dikwelse koue- en hitte stres in ‘n kouer gebied in vergelyking met ‘n warmer gebied. Hierdie waarnemings in die model voorspellings het op die volwasse fiksheid en –teenwoordigheid weerspieël. Die voorspelde teenwoordigheid van larwes het goed met positiewe veldopnames oorvleuel oor ‘n matriks van suikerriet ouderdomme en kultivars. Die resultate van hierdie werk is belangrik en moet gebruik word om geïntegreerde plaagbestuur strategië op te basseer. Die resultate is van toepassing op ‘n wye gehoor oor landbou in die algemeen en veral vir die suikerriet industrie van Suid-Afrika.

Doctoral