Effects of copper and zinc oxide nanoparticles on German cockroach development, indoxacarb resistance, and bacterial load

Abstract

Background: The German cockroach, Blattella germanica, is a ubiquitous and medically significant urban pest. The ongoing development of insecticide resistance in global populations of B. germanica has complicated control efforts and created a need for improved tools. We previously reported that disruption of the gut microbiota by oral administration of the antimicrobial doxycycline reduced resistance in an indoxacarb resistant field strain and also delayed nymphal development and reduced adult fecundity. However, the application of doxycycline for cockroach control in the field is impractical. Here, we sought to determine whether two metal nanoparticles with known antimicrobial properties, copper (Cu) and zinc oxide (ZnO), have similar effects to doxycycline on the physiology of B. germanica and could provide more practical alternatives for control.

Results: We found that dietary exposure to 0.1% Cu nanoparticles, but not ZnO, significantly delays the development of nymphs into adults. However, neither of the nanoparticles altered the fecundity of females, and ZnO surprisingly increased resistance to indoxacarb in a resistant field strain, in contrast to doxycycline. Semi-quantitative polymerase chain reaction (qPCR) further revealed that prolonged dietary exposure (14 days) to Cu or ZnO nanoparticles at the low concentration readily consumed by cockroaches (0.1%) does not reduce the load of the bacterial microbiota, suggesting alternative mechanisms behind their observed effects.

Conclusions: Together, our results indicate that ingestion of Cu nanoparticles can impact German cockroach development through an undetermined mechanism that does not involve reducing the overall load of the bacterial microbiota. Therefore, Cu may have some applications in cockroach control as a result of this activity but antagonistic effects on insecticide resistance should be considered when evaluating the potential of nanoparticles for cockroach control. © 2023 Society of Chemical Industry.

Keywords: German cockroach; bacteria; copper; development; indoxacarb resistance; microbiota; nanoparticle; zinc.

Figure 1.

Effects of nanoparticles on feeding by Blattella germanica. Over-night consumption of a proprietary gel bait matrix containing copper (Cu) or zinc oxide (ZnO) nanoparticles by groups of cockroaches was quantified in a side-by-side choice assay. Gel bait matrix with no nanoparticles added served as the control. Three independent replicates were performed (N = 3), and data were analyzed by ANOVA with Tukey’s post hoc test. Shown are the mean, individual data points, and standard error. The P-values are adjusted for multiple comparisons.

Figure 2.

Development of Blattella germanica nymphs exposed to nanoparticles in the diet. Groups of first instar nymphs were provided an experimental dog chow gel formulation with 0.1% copper (Cu) (A, B) or zinc oxide (ZnO) (C, D) as a sole food source. Cockroaches provided the same dog chow gel without nanoparticles served as the control group. The cockroaches were monitored throughout development and the number of individuals molting to adulthood was recorded at regular intervals. Two independent replicates were conducted for each nanoparticle, and in each replicate development curves were compared to the control using a Mantel–Cox log-rank test.

Figure 3.

Fecundity of adult female Blattella germanica exposed to nanoparticles in the diet. Groups of cockroaches were provided an experimental dog chow gel formulation containing 0.1% copper (Cu) or zinc oxide (ZnO) as a sole food source. Cockroaches provided the same dog chow gel without nanoparticles served as the control group. The number of viable offspring produced from the oothecae of individual females was recorded upon hatching. Data were combined from two independent experiments (N = 7–11 total hatched oothecae per treatment) and analyzed by ANOVA with Dunnett’s post hoc test to compare fecundity between nanoparticle treated and control females. Shown are the mean, individual data points, and standard error. The P-values are adjusted for multiple comparisons.

Figure 4.

Indoxacarb resistance in Blattella germanica exposed to nanoparticles. Enclosures containing mixed stages and sexes were administered proprietary gel baits containing 0.6% indoxacarb (Indx) with or without 0.1% copper (Cu) or zinc oxide (ZnO) nanoparticles. A control enclosure that received blank bait matrix without indoxacarb or nanoparticles was also included in each experiment. Over a period of 14 days, dead cockroaches (defined as exhibiting no movement when physically stimulated) were counted in each experimental group. Data were collected from three independent replicates and were analyzed by ANOVA with Tukey’s post hoc test to compare the percent mortality at the termination of the 14 days experimental period. Shown are the mean, individual data points, and standard error. The P-values are adjusted for multiple comparisons.

Figure 5.

Effect of nanoparticle exposure on the load of the bacterial microbiota. Groups of first instar nymphs were provisioned an experimental dog chow gel formulation with 0.1% copper (Cu) or zinc oxide (ZnO) as a sole food source for 14 days. Cockroaches provided the same dog chow gel without nanoparticles served as the control group. DNA was then isolated from individual insect bodies and used in a semi-quantitative real-time polymerase chain reaction (qPCR) assay targeting a conserved region of the bacterial 16S rRNA gene. Cycle threshold (CT) values were obtained for individual insects as a proxy for the bacterial load and analyzed by ANOVA with Dunnett’s post hoc test. Shown are the mean, individual data points, and standard error. The P-values are adjusted for multiple comparisons.