Repellent Activity of Cymbopogon citratus Essential Oil Against Four Major Stored Product Pests: Plodia interpunctella, Sitophilus oryzae, Acanthoscelides obtectus and Tribolium castaneum

Summary The purpose of this study is to assess the repellent activity of lemongrass (Cymbopogon citratus) essential oil (EO), grown in Serbia under greenhouse conditions, against four prevalent stored product pests: Plodia interpunctella (larvae), Sitophilus oryzae, Acanthoscelides obtectus and Tribolium castaneum (adults). The lemongrass EO repellency was tested using filter paper in Petri dishes and a Y-tube olfactometer. According to the repellency index (RI), the lemongrass EO repellency was divided into 5 classes. Prior to biotesting, the chemical characterization of lemongrass EO was performed and the following main compounds were detected: myrcene (31.0%), geranial (30.0%), and neral (23.6%). The C. citratus EO considered was found to exhibit the Class III repellent activity against P. interpunctella larvae only at the highest concentration (namely 0.5%). This is the very first report on the C. citratus EO repellent activity against this pest. The lemongrass EO examined showed strong repellency (Class IV) against S. oryzae (0.2% and 0.5% of EO), A. obtectus (0.1% and 0.2%), and T. castaneum (0.05-0.1%). Moreover, higher lemongrass EO concentrations (0.5%) were found to exhibit extreme repellency (Class V) against A. obtectus and T. castaneum. The results obtained were confirmed in the bioassays performed, indicating the great potential of lemongrass EO as a bio-repellent when applied in higher concentrations to all the insects considered, regardless of the exposure period.

The Evolution of Host Specialization in an Insect Pathogen

Niche breadth coevolution between biotic partners underpins theories of diversity and co-existence and influences patterns of disease emergence and transmission in host-parasite systems. Despite these broad implications, we still do not fully understand how the breadth of parasites’ infectivity evolves, the nature of any associated costs, or the genetic basis of specialization. Here, we serially passage a granulosis virus on multiple inbred populations of its Plodia interpunctella host to explore the dynamics and outcomes of specialization. In particular, we collect time series of phenotypic and genetic data to explore the dynamics of host genotype specialization throughout the course of experimental evolution and examine two fitness components. We find that the Plodia interpunctella granulosis virus consistently evolves increases in overall specialization, but that our two fitness components evolve independently such that lines specialize in either productivity or infectivity. Furthermore, we find that specialization in our experiment is a highly polygenic trait best explained by a combination of evolutionary mechanisms including conditionally positive fitness asymmetries and mutation accumulation. These results are important for understanding the evolution of specialization in host-parasite interactions and its broader implications for co-existence, diversification, and infectious disease management.

Influence of Temperature and Photoperiod on the Fecundity of Habrobracon hebetor Say (Hymenoptera: Braconidae) and on the Paralysis of Host Larvae, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae)

Studies were carried out in the laboratory to understand the optimum environmental conditions at which the ectoparasitoid, Habrobracon hebetor Say (Hymenoptera: Braconidae), can paralyze and lay eggs when reared on the larvae of the stored product pest, Plodia interpunctella Hübner (Lepidoptera: Pyralidae). At the four temperatures investigated (20, 25, 30, and 35 °C), optimum temperatures for oviposition were found to be 25 and 30 °C, while 35 °C was the least favorable temperature. No significant differences were found between the percentages of diapausing and non-diapausing larvae paralyzed by the wasp at the temperatures of 20, 25, 30, 35 °C within 5 days. However, in another experiment that investigated the effect of photoperiods at different temperatures that included 15, 19 and 28 °C, the number of paralyzed larvae was highly reduced at low temperatures (15 °C) but photoperiods had no significant impact on the number of host larvae paralyzed. In addition, observations at short time intervals also showed that lower temperatures slowed down host larvae paralysis. The results suggest that H. hebetor can paralyze host larvae of P. interpunctella more efficiently and deposit more eggs at temperatures within the range of 20–30 °C.

Cold storage-mediated rearing of Trichogramma evanescens Westwood on eggs of Plodia interpunctella (Hübner) and Galleria mellonella L.

The egg parasitoid Trichogramma evanescens Westwood is considered as an efficient biological control agent for managing several lepidopteran pests and it is widely distributed throughout the world. Mass rearing protocols of parasitoids that are currently in use in biocontrol programs require a meticulous quality control plan, in order to optimize their efficacy, but also their progeny production capacity. In this paper, the effect of different factors on the quality control in mass rearing of T. evenescens, using Plodia interpunctella (Hübner) and Galleria mellonella L. as host species, were investigated. The impact of egg agewas significant in the rates of parasitism, for both host species tested. Significantly highest percent of parasitoid emergence was noticed in two day-old eggs for both host species, while one day-old eggs day exhibited the maximum emergence when both species were used togetherin the same trials. Age-dependent storage egg preservation at either 4 or 9°C significantly influenced the parasitism percentages on both species. The highest parasitism percentage was recorded in two day-old G. mellonella eggs that are kept for 15 days at 9°C while the lower in one day-old P. interpunctella eggs for 60 d storage. Moreover, the highest parasitoid mortality was recorded in T. evanescens reared either on P. interpunctella or G. mellonella at 20°C. Rearing of the parasitoid on a mixture of eggs of both host species resulted in higher parasitism, but not always in higher rates of parasitoid emergence. The results of the present work provide useful information that can be further utilized in rearing protocols of T. evanescens.