Spring 2015 Colloquium: Kevin Ulrich

The alarm-defense system of Cimex lectularius and its implications for pest management
Post by Lisa Kuder

Figure 1: Cimex lectularius

Bed bugs (Hemiptera: Cimicidae) . . . the very mention of these small blood sucking parasites is enough to make most feel squeamish. While all cimicids have mammalian hosts, only three species have a strong preference for humans. Often considered a public health concern, there is no evidence that they are disease vectors. However, bed bugs can illicit allergic reactions, discomfort, anemia, and illnesses associated with insecticide treatments.

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Kevin Ulrich recently defended his thesis on potential Integrated Pest Management (IPM) strategies of Cimex lectularius (Figure 1) (Ulrich 2015), the best known species of bed bug. Innovation in the bed bug pest control industry is needed for several reasons: 1) bed bugs have developed widespread resistance to conventional pyrethroid insecticide treatments, 2) direct treatment or contact with the bug is likely necessary for insecticides to be effective but is difficult to achieve, and 3) affordable, non-chemical controls are desired for environmental and health reasons (Ulrich 2015). Before exploring alternative control options, Dr. Ulrich performed a survey of current commercial treatments (chemical vs. heat) determining how effective they were based on whether retreatments were needed.

Field site data from the DC Metro area were analyzed to determine how current chemical controls (contact aerosol sprays, liquid residuals, and dusts) compare to that of heat treatment (mobile heating units). Building types (single-family homes, row homes, multifamily apts., and commercial properties) were included in the analysis to determine if certain building types have greater potential for reinfestation (Ulrich 2015). Of the 4,258 units evaluated over a 3 year period, heat treatment required a lower percentage of retreatment than did chemical controls, 9.5% and 20.8% respectively, and multifamily apts. were the most susceptible to reinfestation (Ulrich 2015). Overall heat treatments were more effective at eliminating bed bugs. However, neither was 100% successful and both methods have several major disadvantages (Ulrich 2015). Thus the impetus for developing novel bed bug detection and treatment solutions.

Dr. Ulrich’s research focused on multiple aspects of bed bug management, including alternative control strategies. Specifically he investigated how bed bug communication pheromones, in particular their alarm-defense system, interact with a promising candidate for pest control, a naturally occurring insect pathogenic fungus. In addition, he determined whether these pheremones could be used as a monitoring tool for determining whether bed bugs are present. Bed bug communication pheremones have previously been isolated with two common aldehyde major components: (E)-2-hexenal and (E)-2-octenal (Siljander et al. 2008, Gries et al. 2014). Both aldehydes have antibacterial and antimicrobial properties, therefore they have the potential to inhibit the growth of fungal spores. Metarhizium anisopliae, a ubiquitous soil dwelling fungus, has been extensiveily studied as a tool for regulating pest insects in agrosystems and greenhouses (Tiago et al. 2014). While deadly to insects, primarily beetle larvae, it is non-toxic to humans and mammals (EPA 2003). To determine whether M. anisopliae impacts bed bug survival, ingestion assays were performed using 4 different concentrations of M. anisopliae spores added to blood. As seen in Figure 2 below, bed bugs are indeed susceptible to M. anisopliae, particularly at higher spore concentrations (Ulrich et al. 2014).

Figure 2: Results from the ingestion assay show that the insect-pathogenic fungus M. anisopliae has an effect on bed bug mortality and that it is concentration dependent; Higher concentrations of fungal spores were associated with increased mortality rates (Ulrich et al. 2014)

Previous studies have shown that temperature and relative humidity are important factors in fungal spore germination (El Damir 2006). Thus Dr. Ulrich tested the effectiveness of M. anisopliae at different levels of relative humidity (RH) on bed bug mortality. Both spray and contact applications at ambient temperatures [32% RH and 74% RH (Figure 3)] had low mortality rates, approx. 20% (Ulrich et al. 2014). Fatalities at high saturation levels (98% RH) were impressive (80 – 100%), unfortunatley RH’s > 80% are impractical in the field. Since bed bugs feed on a liquid diet and have a tendency to group together, could they increase the humidity of their microclimate enough to improve fungal growth? The next round of studies explored whether this is possible. Aggregation assays showed that blood-fed bugs did not elevate humidity high enough to increase mortality in fungal treated bed bugs (Ulrich et al. 2014).

Figure 3: Spray and contact assays were performed at 3 different relative humidities (RH) to see if RH alters the effectivness of M. anisopliae on bed bug mortality (Ulrich et al. 2014)

Might bed bug communication pheremones have an effect on fungal growth? Dr. Ulrich sought to answer the following questions: Can aldehydes inhibit M. anisopliae growth? How long must fungus be exposed? And are aldehyde effects specific to developmental stage of conidia? Disks impregnated with synthetic aldehydes identical to those emitted by bed bugs were placed on fungal inoculated plates. As Figure 4 shows, both aldehydes inhibit the growth of M. anisopliae at concentrations > 0.5 mg, which is equivalent to pheromone emissions from 12 bed bugs (Ulrich et al. 2014). Aldehyde exposure of 0.5 hours was enough to inhibit fungal growth. Fungal development stage also proved to be a critical factor.  Once spores began to germinate, aldehydes no longer acted as a fungicide (Ulrich et al. 2014).

Inhibition of M. anisopliae by aldehydes

Figure 4: Results from petri dish assays indicate that two common alarm pheremones (2-hexenal and 2-octenal) in bed bug communication inhibit the growth of M. anisopliae at certain concentrations indicated by minus signs (spore germination stopped) and plus symbols (no inhibition observed) (Ulrich et al. 2015).

The aldehydes released in bed bug communication might also be incorporated into an effective lure, an attractive proposition given that current monitoring tools perform poorly. To be commercially viable, a lure will attract bed bugs of all life stages and at low densities in addition to being low maintenance and cost effective (Ulrich et al. 2015). With this goal in mind, Dr. Ulrich determined the concentrations of  2-hexenal and 2-octenal that are attractive bed bugs, as well as the length of time these aldehydes remain effective. Setting up plates as an arena with aldehydes present in one quadrat, the movement of bed bugs were tracked with a video recorder (Ulrich et al. 2015). The results? Males were significantly attracted to low concentrations of the aldehyde mixture (0.04 µg) within the first 2-hours of its release (Ulrich et al. 2015). Thus for commercial application, a slow-release formulation would need to be developed.

In conclusion, commercial treatments are not entirely effective and have several major drawbacks. Thus, there is a real need and demand for novel solutions in the bed bug control industry. M anisopliae, a naturally occurring insect pathogenic fungus, shows some promise as an alternative method for controlling bed bug infestations (Ulrich et al. 2014). However, as demonstrated by the present research, several hurdles exist. Bed bugs are susceptible to M. anisopliae but only at high relative humiditities (> 98%), levels impractical in the field (Ulrich et al. 2014). Also, two common alarm pheremones (2-hexenal and 2-octenal) in bed bug communication inhibit the growth of M. anisopliae at field relevant concentrations (Ulrich et al. 2015). Despite their drawbacks for management, these pheremones, 2-hexenal and 2-octenal, did show potential as monitoring tools for determining bed bug presence, as bed bugs were attracted to both aldehydes at low concentrations (Ulrich et al. 2015). Given the desire for alternative treatment methods, Dr. Ulrich’s findings are encouraging and will likely inspire further investigations into innovative solutions to controlling the commmon bed bug.

References

El Damir, M. “Variation in germination, virulence and conidial production of single spore isolates of entomopathogenic fungi in response to environmental heterogeneity.” Journal of Biological Science, 6 (2006), pp. 305–315

Gries, Regine, et al. "Bed Bug Aggregation Pheromone Finally Identified." Angewandte Chemie 127.4 (2015): 1151-1154.

Siljander, Eric, et al. "Identification of the airborne aggregation pheromone of the common bed bug, Cimex lectularius." Journal of chemical ecology 34.6 (2008): 708-718.

Tiago, Patricia Vieira, Neiva Tinti de Oliveira, and Elza Áurea de Luna Alves Lima. "Biological insect control using Metarhizium anisopliae: morphological, molecular, and ecological aspects." Ciência Rural 44.4 (2014): 645-651.

Ulrich et al. “Exposure of bed bugs to Metarhizium anisopliae at different humidities.” Journal of Economic Entomology 107 (2014): 2190-2195.

Ulrich et al. “Inhibition of the entomopathogenic fungus Metarhizium anisopliae in vitro by bed bug defensive secretions (E)-2-hexenal and (E)-2-octenal.” BioControl doi (2015): 10.1007/s10526-015-9667-2.

Ulrich, Kevin. “The alarm-defense system of Cimex lectularius and its implications for pest management” UMD thesis defense Dept. of Entomology (2015).

US Environmental Protection Agency Office of Pesticide Programs. “Biopesticides registration action document -- Metarhizium anisopliae strain F52 (PC Code 029056)” (2003).

About Lisa: She is a 1st year PhD student in the Dennis vanEngelsdorp’s bee lab.  She will be studying the biology and expansion of Anthophora plumipes, a species that has recently naturalized in the Mid-Atlantic region, and more generally exploring options for maximizing nesting sites for ground-nesting bees.