Chemical ecology and eco-physiology of the grain chinch bug, Macchiademus diplopterus (Distant) (Hemiptera: Lygaeidae: Blissidae), a phytosanitary pest of South African export fruit

Okosun, Olabimpe Olayem (Stellenbosch : Stellenbosch University, 2012-03)

Thesis (MScAgric)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: The grain chinch bug, Macchiademus diplopterus, is an endemic pest of cultivated grain crops and wild grasses in the south-western Cape region of South Africa. In early summer when host plants dry out, adult grain chinch bugs aggregate in large numbers in shelter sites in surrounding areas and enter into aestivation. These shelter sites sometimes include the stalk or calyx ends of fruit, and shelter-seeking bugs can also contaminate export fruit cartons, consequently posing a phytosanitary/quarantine risk to importing countries. Presently, there are no feasible pre- or post-harvest control measures to manage this quarantine risk. The aggregating behaviour of grain chinch bugs suggests the involvement of pheromones. Therefore, investigating the chemical ecology of grain chinch bugs for potential use in control measures is the focus of the first research chapter of this study. Gas chromatography-mass spectrometry (GC-MS) was used to identify headspace volatiles collected from aggregating bugs. Olfactometer bioassays were conducted to assess the attractiveness of each gender to separate sexes, individual compounds and a mixture of the compounds as a formulated lure. The lure was tested in field trapping trials with delta and bucket traps. In the bioassays with the live insects the response of each gender to live females was greater than the responses of each gender to live males, suggesting that females may disseminate the pheromones more efficiently than males. The following eight volatile compounds were indentified from the GC-MS analysis: hexanal, (E)-2-hexenal, (E)-2-hexenol, (E)-2-hexenyl acetate, (E)-2-octenal, (E)-2-octenol, (E)-2-octenyl acetate and tridecane. In the bioassays with individual compounds, three of these eight compounds, hexanal, (E)-2-hexenal, and tridecane, elicited attraction of both females and males. The formulated lure was attractive to both males and females in the laboratory bioassay, but this attraction was not evident in the field. In the field, there was only one occasion when a significantly higher number of bugs were caught in baited traps compared to unbaited traps. Trap catches were very low compared to the actual level of infestation in the field which was evident from corrugated cardboard bands tied around tree trunks which contained many sheltering bugs. The low trap catches seen in the field were partly due to competition between the synthetic pheromone lure and the natural pheromones emitted by aggregating live insects. Also, the characteristic shelter-seeking behaviour of grain chinch bugs influenced trap catches, as more bugs were found in places that provide shelter, like cardboard bands and walls of the delta traps. This behavior of aestivating bugs could be used to the advantage of trapping bugs by integrating sheltering sites into traps in future trials. Also, the lure needs to be improved for optimum efficiency in the field. The second research chapter also addresses the quarantine risk posed by grain chinch bugs, by investigating the thermal biology of bugs to ultimately facilitate the development of effective post-harvest treatments. Critical thermal minimum and maximum temperatures (CTmin and CTmax) of both active and aestivating bugs were subjected to critical thermal limits analysis. The CTmin and CTmax of aestivating bugs were not affected by gender (p > 0.05). There was a decrease in CTmin from the active period into aestivation for both males (2.8°C to 1.0°C (± 0.1)) and females (2.1°C to 0.6°C (± 0.1)). Also, for CTmax there was an increase in tolerance from the active period into the aestivation period for both males (49.9°C to 51.0°C (± 0.1)) and females (49.9°C to 51.5°C (± 0.1)). To determine the plasticity of grain chinch bug thermal tolerance, aestivating bugs at 27 weeks into aestivation, were acclimated at different temperatures and photoperiods [18°C (10L:14D) and 26°C (16L:8D)] for a period of seven days. Both low (18°C) and high (26°C) acclimation temperatures and photoperiods increased CTmin of aestivating grain chinch bugs at 14 weeks from 0.8°C to -1.2°C and -0.1°C (± 0.1) respectively. However, CTmax was not altered by acclimation temperatures (p > 0.82). Field temperatures at collection sites were recorded to compare to grain chinch bugs thermal tolerance levels exhibited in the laboratory. These results, as well as the effects of acclimation treatments on the CTmin of bugs, have implications for post-harvest treatments, and understanding the quarantine risk posed to importing countries. The information generated from this study can be used to further advance the development of both effective pre-harvest and post-harvest control measures to reduce grain chinch bug quarantine risk.

AFRIKAANSE OPSOMMING: Die graanstinkluis, Macchiademus diplopterus, is 'n endemiese plaag van aangeplante graangewasse en wilde grasse in die Suidwes Kaap-provinsie van Suid-Afrika. In die vroeë somer wanneer gasheerplante uitdroog, soek groot getalle volwasse graanstinkluise skuiling in die omliggende gebiede en gaan in ʼn somerrusperiode. Hierdie skuilplekke sluit soms die stam of kelk eindes van vrugte in en graanstinkluise kan ook uitvoer-vrugte kartonne kontamineer. Gevolglik word lande wat vrugte uit Suid-Afrika invoer, aan die fitosanitêre kwarantynrisiko van stinkluisbesmetting blootgestel. Tans is daar nie haalbare voor- of na-oes beheermaatreëls om hierdie kwarantyn risiko te bestuur nie. Die aggregasiegedrag van graanstinkluise dui op die betrokkenheid van ʼn feromoon. ‘n Ondersoek van die chemiese ekologie van die graanstinkluis vir moontlike gebruik in beheermaatreëls is die fokus van die eerste gedeelte van hierdie studie. Gaschromatografie-massaspektrometrie (GC-MS) is gebruik om die vlugtige organiese verbindings in die bodamp van die saamgetrosde stinkluise te identifiseer. Olfaktometriese biotoetse is uitgevoer om die aantreklikheid van die insekte vir die teenoorgestelde geslag te bepaal, asook van die individuele verbindings en 'n mengsel van die verbindings as 'n geformuleerde lokmiddel in lokvalle. Die lokmiddel is getoets in veldproewe met deltatipe en emmertipe lokvalle. In die olfaktometriese biotoetse met die lewende insekte is die reaksie van beide geslagte teenoor lewende wyfies groter as die reaksie van die geslagte teenoor mannetjies, wat daarop dui dat wyfies die feromoon meer doeltreffend as mannetjies versprei. Die volgende agt verbindings is geïdentifiseer met behulp van GC-MS-analise: heksanaal, (E)-2-heksenaal, (E)-2-heksenol, (E)-2-heksenielasetaat, (E)-2-oktenaal, (E)-2-oktenol, (E)-2-oktenielasetaat en tridekaan. In die biotoetse met individuele verbindings het drie van die agt verbindings, hexanal, (E)-2-hexenal, en tridecane, lokaktiwiteit vir beide geslagte getoon. Die geformuleerde lokmiddel was aantreklik vir beide geslagte in laboratorium toetse, maar soortgelyke lok is nie in die veld gevind nie, waar daar net een keer 'n aansienlike groter getal graanstinkluise met lokmiddel gevang is in vergelyking met lokvalle sonder lokmiddel. Die getal graanstinkluise in lokvalle was baie laag in vergelyking met die werklike vlak van besmetting in die veld, wat duidelik geblyk het uit die getalle graanstinkluise wat skuiling gesoek het in die geriffelde karton bande wat om boomstamme vasgemaak was. Die lae lokvalvangste in die veld was deels te wyte aan die kompetisie tussen sintetiese feromoon en die natuurlike feromoon van saamgetrosde insekte. Die kenmerkende aggregasiegedrag van graanstinkluise het lokvalvangste beïnvloed, aangesien meer stinkluise gevind is in plekke wat skuiling bied, soos die kartonbande en die binnekant van die delta-lokvalle. Hierdie skuilings van graanstinkluise kan in toekomstige proewe uitgebuit word deur vir meer skuilplek in lokvalle voorsiening te maak. Die formulering en die aanbieding van die lokmiddle moet ook verbeter word vir 'n optimale doeltreffendheid in die veld. In die tweede hoofstuk word die kwarantynrisiko van die graanstinkluis aangespreek deur die ondersoek van die termiese biologie van stinkluise om uiteindelik die ontwikkeling van doeltreffende na-oes behandelings te fasiliteer. Kritiese termiese minimum en maksimum temperature (CTmin en CTmax) van beide aktiewe en rustende graanstinkluise is bepaal deur analise van die kritiese termiese beperkings van die insek. Die CTmin en CTmax van rustende graanstinkluise is nie geraak deur geslag nie (p > 0.05). Daar was 'n afname in CTmin van die aktiewe tydperk tot in rus, vir beide manlike (2.8°C tot 1.0°C (± 0.1)) en vroulike insekte (2.1°C tot 0.6°C (± 0.1)). Ook vir die CTmax was daar 'n verbetering in toleransie vanaf die aktiewe tydperk tot in die rusperiode vir beide manlike (49.9°C tot 51.0°C (± 0.1)) en vroulike insekte (49.9°C tot 51.5°C (± 0.1)). Om die aanpasbaarheid van die termiese toleransie van die graanstinkluis te bepaal, is graanstinkluise 27 weke na aanvang van die rusperiode geakklimatiseer by verskillende temperature en fotoperiodes [18°C (10L: 14D) en 26°C (16L: 8D)] vir 'n tydperk van sewe dae. Beide lae (18°C) en hoë (26°C) akklimatiseringstemperature en fotoperiodes het onderskeidelik die CTmin van rustende graanstinkluise op 14 weke verhoog van 0.8°C tot -1.2°C en -0.1°C (± 0.1). Daar is egter geen effek op CTmax deur akklimasie temperature nie (p > 0.82). Veldtemperature is ook bepaal om te vergelyk met graanstinkluis termiese toleransie vlakke wat in die laboratorium bepaal is. Hierdie resultate, sowel as die gevolge van die akklimasie behandelings op die CTmin van graanstinkluise, het implikasies vir na-oes behandelings, en begrip van die kwarantyngevaar wat dit inhou vir vrugte-invoerlande. Die inligting wat uit hierdie studie voortvloei, kan gebruik word om die ontwikkeling van beide effektiewe voor-oes en na-oes beheermaatreëls te bevorder en om die kwarantynrisiko wat graanstinkluise inhou, te verminder.

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