Toxicity and Physiological Effects of Essential Oil Components against the German Cockroach, Blattella germanica (L.) (Ectobiidae)

Abstract

The overarching goal of this research is to extend the scholarship on and provide new evidence that rationalizes the inclusion of low toxicity, plant-based essential oils (EOs), and their components (EOCs) into integrated pest management systems (IPM) for insect pests such as the intractable urban insect, the German cockroach, Blattella germanica (L.). In Chapter One, I attempted to disentangle the myths surrounding the origin of B. germanica, provided a commentary on their co-evolution with humans, reviewed associated public health concerns, and provided genetic insights into B. germanica resistance to conventional insecticides. Additionally, I suggested that EOCs could be used to either delay or circumvent insecticide resistance or even suppress B. germanica cockroaches. As a corollary, in Chapter Two, I provided a synthesis on EOs and EOCs employed in the management of urban insects. Notably, I summarized key data on the use of EOs, EOCs, and commercially available EO formulations against ants, bed bugs, cockroaches, fleas, head lice, stored product moths, silverfish, and termites. This synthesis highlights insecticidal activities of EOCs against a broad range of urban insect pests, which I selected for my subsequent investigations against B. germanica in later chapters. Further, I discussed knowledge gaps, conundrums, and offered probable insights into how laboratory/field-based investigations of EOs/EOCs should be approached if eventual integration into urban insect management is sought. In Chapter Three, I evaluated the toxicity profiles of eight EOCs against susceptible and multi-resistant B. germanica cockroaches. The results demonstrated that limonene (aliphatic), carvacrol, eugenol, and tropolone (aromatic) were the most toxic against B. germanica. Structural-activity relationship revealed that vapor pressure and molecular weight are important metrics of EOCs that moderate toxicity. The higher the molecular weight and vapor pressure of an EOC, the more toxic it is likely to be against B. germanica. The use of EOCs in controlling urban and structural pests is limited because of their high volatility. Consequently, in Chapter Four, I explored superabsorbent polymer (SAP) gels as a carrier to deliver selected EOCs in a bid to prolong their insecticidal activity against B. germanica. The results provided several new insights on how these EOCs can suppress the reproductive fitness of B. germanica, including reductions in (i) female reproductive period (ii) oothecal hatchability, and (iii) fecundity. The findings highlight the potential use of SAP gels to prolong the bioavailability of EOCs, thereby solving the problem of high volatility and achieving extended insecticidal effects against targeted pests. The use of SAP carriers also demonstrates the potential of incorporating EOCs as baits. In Chapter Five, I investigated the physiological effects of limonene, carvacrol, and β-thujaplicin on the DGC of insecticide-susceptible and multi-resistant B. germanica. Two observations were noteworthy: (1) the EOCs resulted in B. germanica abandonment of DGC, and (2) increased respiratory water loss following treatment with an EOC. In toto, the cost of this DGC loss substantiates previous findings and my meta-analytic conclusion: DGC serves to reduce respiratory water loss in insects. Importantly, it suggests that EOCs probably achieve kill via lethal desiccation. In Chapter Six, I conducted a meta-analysis to resolve the controversies surrounding the hypotheses postulated to explain why insects shut their spiracles periodically. This included data from 46 insect species in 24 families across nine orders. Insects breathe with the aid of thin capillary tubes that open out to the exterior of their body as spiracles. These spiracles are often modulated in a rhythmic gas pattern known as the discontinuous gas exchange cycle (DGC). Several explanations have been put forward to rationalize this process, but two controversial ones gain the most support: the rhythmic pattern is to (1) reduce water loss or (2) facilitate gaseous exchange in environments with high carbon dioxide and low oxygen. The meta-data supports that DGC reduces water loss in insects. However, DGC does not facilitate gaseous exchange in a high carbon dioxide or low oxygen environment. This investigation is the first meta-analytic attempt to resolve the controversies surrounding the merit of adaptive hypotheses in insects. In Chapter Seven, I summarized the major findings of my dissertation research. As existing cockroach control strategies are not always sufficient, future studies should seek to investigate formulations that could be used to deliver EOs and EOCs in field-based studies. In addition, possible synergistic combinations of EOs/EOCs with currently used conventional insecticides is an under-researched area. Taken together, the outcomes of my research contribute to the pest management industry by providing alternatives to synthetic insecticides, delaying resistance development, creating environmentally conscious pest management tools, providing solutions for public health pests, and creating affordable options. Chapter 2 of this dissertation was submitted to the Journal of Economic Entomology in January 2022. Chapters 3 and 4 were published in 2020 in the Journal of Economic Entomology (113: 896-904 and 113: 2436-2447, respectively). Chapter 5 was submitted to Current Research in Insect Science Journal in December 2021. Chapter 6 was published in Insects in January 2022 (13, 117-121).