Molting site fidelity accounts for colony elimination of the Formosan subterranean termites (Isoptera: Rhinotermitidae) by chitin synthesis inhibitor baits

The potential of 20-hydroxyecdysone to accelerate termite baiting programs

While chitin synthesis inhibitor baits are environmentally friendly and sustainable measures to control subterranean termites compared to conventional termiticides, the lengthy time required to eliminate a termite colony is perceived as a drawback for both homeowners and pest control industry. Recent efforts have focused on overcoming such problem by accelerating the baiting process. To accelerate termite colony elimination, 20-hydroxyecdysone was proposed as a potential active ingredient due to its similar mode of action observed in chitin synthesis inhibitors. Termites ingesting 20-hydroxyecdysone displayed hyperecdysonism, and mortality occurred much faster than with chitin synthesis inhibitor-ingesting termites. However, trial with 20-hydroxyecdysone alone and 20-hydroxyecdysone mixed with noviflumuron baits with intact colonies of Coptotermes formosanus Shiraki failed to accelerate the colony elimination time due to the feeding cessation. To bypass this issue, lower concentrations of 20-hydroxyecdysone may be necessary and sequential feeding of chitin synthesis inhibitors and 20-hydroxyecdysone have been proposed. In this review, a few more potential approaches were summarized which can be used to accelerate termite baiting program.

Subterranean termites (Coptotermes formosanus [Blattodea: Rhinotermitidae]) colonies can readily intercept commercial inground bait stations placed at label-prescribed distance

The Formosan subterranean termite, Coptotermes formosanus Shiraki, is both an economically impactful pest and a successful invader. One method of subterranean termite control is baiting. According to the label, baits are installed surrounding the structure at a uniform interval distance of ≈3 m. However, homeowners and pest control professionals are often concerned that termites may bypass bait stations and have access to the structure. To address this concern, we experimentally duplicated field conditions using a large planar arena (3.6 m × 1.1 m) to study the optimal distribution of bait stations based on colony-wide foraging activity. We installed 2 bait stations 3 m apart as per label instructions and introduced C. formosanus colonies to allow them to explore the arena by tunneling through the sand. In this real-scale arena, all termite colonies intercepted a bait station in an average of 21 (± 8 SD) days. We assumed that termites could find bait faster if there were more bait stations by overlaying additional hypothetical baits closer than per label instruction, but the improvement was incremental, requiring 4 times more stations (0.45 m interval) to obtain a significant difference. We also revealed the characteristic behavior after intercepting bait stations, termites created a burst of tunnels that radiated from the bait station. These branching tunnels averaged 16 cm in length, suggesting immediate interceptions of additional auxiliary stations placed within 16 cm of an active station. These findings contribute to our understanding on how subterranean termites intercept inground bait stations.

How do termite baits work? implication of subterranean termite colony demography on the successful implementation of baits

In 1995, the launch of the first commercial chitin synthesis inhibitor (CSI) bait led to the transformation of the subterranean termite control industry around the world. Their slow mode of action, which relies on both their ability to be transferred among nestmates and termite molting biology, has made them cost-effective solutions for subterranean termite colony elimination while minimizing the introduction of pesticides into the soil toward an environmentally sustainable strategy. However, despite successful commercial implementations, the acceptance of their use varies within the pest control industry around the world. Notably, the nuanced complexity of how CSI baits lead to colony elimination upon feeding by termite foragers has, in part, remained elusive for the past 3 decades, allowing for long-lasting misconceptions to persist. A recent series of studies has since provided complementary elements of understanding how CSI baits utilize termites' inherent colony demography, behavior, and physiology to trigger colony elimination after a characteristic succession of events within the colony collapse process. I here provide a synthetic overview of subterranean termite colony characteristics when exposed to CSI baits using Coptotermes (Wasmann) (Blattodea: Heterotermitidae) as a primary model system. The changes in colony demography through the colony collapse reflect how the mode of action of CSI baits makes them a prime solution for sustainable subterranean termite pest management. Following decades of innovation, ongoing interactions among termite researchers, bait product manufacturers, and pest management providers must continue to bring solutions to existing and emerging termite pest problems around the world.

An overview of the development of termite baits in the past 3 decades

Since the commercialization in 1995, termite baits have been introduced in 32 countries, protecting over 3.7 million homes and reducing pesticide use by more than 11,100 metric tons. Over the past 3 decades, advancements have been made to address the limitations of bait systems, including improvements in bait toxicants, bait matrices, and the frequency of required site visits. Termite baits have been applied in both commercial and experimental area-wide projects. This article discusses the factors contributing to the success of these projects and the key elements driving the progress and advancements in termite bait technology.

Feeding cessation after feeding on 20-hydroxyecdysone in the Formosan subterranean termite

BACKGROUND

To control subterranean termite pests, chitin synthesis inhibitor (CSI) baits have been widely applied. Despite CSI baits having low impacts on the environment, they require a lengthy time period to eliminate colonies. 20-hydroxyecdysone (20E) was proposed to speed up the baiting process as it showed faster mortality than CSI baits. However, the efficacy of 20E has previously not been tested at the colony level prior to applying in the field.

RESULTS

We compared the effect of 20E, 20E + noviflumuron, noviflumuron and untreated control using colonies of Coptotermes formosanus. Our result revealed that both 20E and 20E + noviflumuron did not accelerate colony elimination and termite activity remained relatively stable during the observation periods. To determine the limited effects of 20E, we further investigated feeding duration and consumption amount of 20E with different concentrations (control, 100 and 1000 ppm) for 10 days. Termites ceased feeding after 1 day in 100 and 1000 ppm treatment and 100% mortality was observed within 10 days in 1000 ppm 20E, while mortality in the 100 ppm 20E treated group was much lower than that in the 1000 ppm group. Furthermore, no termites molted in the control and termites died from hyperecdysonism in 1000 ppm 20E treatment, whereas about 20% of termites molted in 100 ppm 20E.

CONCLUSION

This study demonstrated that 20E may not be suitable as a sole active ingredient to accelerate elimination of a subterranean termite colony, while CSI baits and lower concentrations of 20E may reduce the lengthy time period in colony elimination. © 2023 Society of Chemical Industry.

Life and Death of Termite Colonies, a Decades-Long Age Demography Perspective

A eusocial insect colony represents a complex biological entity that must ensure degrees of perennity once it reaches maturity (production of dispersing imagoes over many successive years) to optimize its reproductive success. It is known that a subterranean termite colony invests differentially in different castes over time and adjusts colony functions depending on colony internal and external conditions over many years of activity. However, the current study demonstrates that Coptotermes formosanus Shiraki field mature colonies go through dramatic demographic changes and breeding structure shifts, even many years after they have reached reproductive success. By analyzing the changes in age demography of C. formosanus colonies from four field sites, we here provide a new perspective on how a colony may function over decades, which reveals that each colony demographic trajectory is unique. In a way, throughout its life, a termite colony displays its own “demographic individuality” that drives its growth, its foraging ability, its competitiveness, its age demography, its senescence and ultimately its death. This study is therefore a narrated story of the life -and death- of different C. formosanus field colonies over decades of observation.

Queen Egg Laying and Egg Hatching Abilities are Hindered in Subterranean Termite Colonies When Exposed to a Chitin Synthesis Inhibitor Bait Formulation

Subterranean termite control methods using chitin synthesis inhibitors (CSIs) aim at eliminating colonies that feed upon a bait formulation. Several benzoylurea active ingredient formulations are currently commercially available as alternative termite management strategies to liquid termiticides. Individual workers need to molt on a regular basis and CSIs interfere with such molting process, allowing sufficient time for the acquisition of a colony-wide lethal dose prior to widespread mortality. As workers progressively die, the colony eventually collapses, leaving only soldiers and primary reproductives that starve to death. One common observation is that young workers often die early owing to their relatively short molting cycle. However, the absence of brood in dying colonies raises questions about the potential fate of eggs laid by the queen. This study aims to determine if CSI baits also terminate the ability of a colony to produce a new cohort of workers by disabling the ongoing brood development. Incipient termite colonies were used to test the impact of noviflumuron on the queen's ability to lay eggs and on the eggs' ability to hatch. Our results showed that queens in colonies exposed to CSI not only initially laid less eggs than the control queens, but eggs also did not develop and were progressively cannibalized, eventually leading to colony establishment failure. This result implies that queens of mature colonies exposed to CSI would lose the ability to lay viable eggs as the colony collapses, leading to an absence of worker replacement, aiding in colony elimination.

Subterranean Termite (Coptotermes gestroi (Blattodea: Rhinotermitidae)) Colony Elimination Through Exposure to a Novaluron CSI Bait Formulation in Laboratory

The development of baits for subterranean termite control over the past 25 yr has provided cost-effective alternatives to liquid termiticide treatments. Current bait products use one of the few available benzoylurea chitin synthesis inhibitors (CSIs) labeled for subterranean termites. These insecticides are used because of their nonrepellency, their slow-acting mode of action, and their dose-independent lethal time. Although many studies have provided ample evidence of the efficacy of CSI baits for subterranean termite colony elimination, most have focused on hexaflumuron and noviflumuron. However, bait products using alternative CSIs have not received the same level of scrutiny, limiting the amount of evidence proving their efficacy. One such compound is novaluron, the active ingredient currently used in the Trelona ATBS-Advance Termite Baiting System bait product. The current study independently tested the efficacy of this commercially available bait formulation against whole colonies of Coptotermes gestroi (Wamann) (~63,910 workers) in the laboratory, using an extended experimental setup to simulate a 15-m foraging distance from the central part of the nest to the bait, while having access to alternative food sources. Treated colonies progressively ceased to feed on wood items within 45 d after being provided access to the novaluron bait formulation, with a subsequent progressive collapse of the population, leading to colony elimination by 91 d. This study therefore confirms the efficacy of novaluron baits against subterranean termites, and currently remains one of the few CSIs that can be applied for the successful control of Coptotermes infestations.

Application strategies by selective medium treated with entomopathogenic bacteria Serratia marcescens and Pseudomonas aeruginosa as potential biocontrol against Coptotermes curvignathus

The success of microbial termiticides in controlling termites depends on the ability of microbes to grow in different media and the functionality of the microbes as a resistant barrier or toxic bait. This study was conducted to understand the mortality rate and behaviour changes of the subterranean termite Coptotermes curvignathus Holmgren introduced with different concentrations of Serratia marcescens strain LGMS 1 and Pseudomonas aeruginosa strain LGMS 3 using wood and soil as bacterial transfer medium. In general, higher concentration of bacteria in soil caused a reduction in tunnelling activity and wood consumption and an increase in mortality. However, application on wood revealed a different outcome. Wood treated with S. marcescens of 10⁶ CFU ml^-1 concentration proved to be more efficient as bait than higher concentration applications as it caused a high mortality rate while still highly palatable for termites. Wood or soil treated with S. marcescens concentration higher than 10⁹ CFU ml^-1 creates a high toxicity and repellent barrier for termites. Pseudomonas aeruginosa of 10⁹ CFU ml^-1 concentrations applied on wood served as a slow-acting toxic bait. However, the ability for S. marcescens and P. aeruginosa to survive on wood is low, which made the bait unable to retain a useful level of toxicity for a long period of time and frequent reapplication is needed.

Concentration-Dependent Feeding Deterrence to 20-Hydroxyecdysone for Three Subterranean Termite Species (Blattodea: Rhinotermitidae)

Simple Summary

Subterranean termite colonies can be eliminated using baiting systems. However, for a given bait to be effective, the active ingredient must be lethal at concentrations that are also palatable to termites. The insect molting hormone, 20-hydroxyecdysone (20E), has potential for use in termite baits, but its palatability to termites has not been examined. The purpose of this study was to determine what concentrations of 20E, if any, cause termite workers to feed less readily. To test this, paper disks were treated with various concentrations of 20E. Groups of 1000 termites of three different species; the Formosan, the Asian and the Eastern subterranean termite; were placed in arenas. The termites had the option of following a path to feed on either a paper disk containing the 20E, or an untreated disk, and the amount of paper consumed was then compared. The results showed that the Asian subterranean termite had the least tolerance for the 20E, the Formosan subterranean termite had a reduced tolerance, and the presence of the 20E had no impact on the Eastern subterranean termite.

Abstract

Effective active ingredients in toxicant bait formulations must be non-deterrent to insect feeding behavior at lethal concentrations. This study evaluated feeding deterrence for Coptotermes formosanus Shiraki, C. gestroi (Wasmann), and Reticulitermes flavipes (Kollar) when provided access to cellulose impregnated with various concentrations of the insect molting hormone, 20-hydroxyecdysone (20E). Termites were exposed to 20E concentrations of 200, 500, 1000 and 2000 ppm and to noviflumuron at 5000 ppm in a 24 h choice-test, and the mass of substrate consumption from treated and untreated media pads was compared for each treatment. 20E feeding deterrence was detected at 500, 1000 and 2000 ppm for C. gestroi, and at 2000 ppm for C. formosanus. No significant differences in consumption of treated and untreated substrate was detected at any concentration for R. flavipes. Potential methods for reducing deterrence are discussed.

Area-Wide Elimination of Subterranean Termite Colonies Using a Novaluron Bait

We investigated the use of termite baiting, a proven system of targeted colony elimination, in an overall area-wide control strategy against subterranean termites. At two field sites, we used microsatellite markers to estimate the total number of Reticulitermes colonies, their spatial partitioning, and breeding structure. Termite pressure was recorded for two years before and after the introduction of Trelona® (active ingredient novaluron) to a large area of one of the sites. Roughly 70% of the colonies in the treatment site that were present at the time of baiting were not found in the site within two months after the introduction of novaluron. Feeding activity of the remaining colonies subsequently ceased over time and new invading colonies were unable to establish within this site. Our study provides novel field data on the efficacy of novaluron in colony elimination of Reticulitermes flavipes, as well as evidence that an area-wide baiting program is feasible to maintain a termite-free area within its native range.

A Reproductives Excluder for Subterranean Termites in Laboratory Experiments

As a social insect, termites have different castes and division of labor in a colony. Investigating the social behavior of subterranean termites is a challenge due to the cryptic nature and large colony size. Planar arenas are commonly used to study these termites under laboratory conditions, and have provided several advantages. However, there is no means to designate areas such as a royal chamber or central nest from foraging sites because reproductives can move freely across arenas. In this study, we examined the minimum passing size of different castes of Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae), in order to develop a reproductive excluder and correlated minimum passing size with head widths and heights. We found that workers and soldiers of C. formosanus were able to pass through a gap greater than or equal to 0.7 mm. Our results showed that there are significant differences in the head width and height based on castes and head height was more critical than head width to determine passing size. We further confirmed feasibilities of the reproductive excluders using incipient colonies of C. formosanus. Confining reproductives using the excluder in laboratory experiments will provide more chances to study the royal chamber and central nest independently of foraging sites.

Fused Colonies of the Formosan Subterranean Termite (Blattodea: Rhinotermitidae) for Laboratory Experiments

Laboratory studies of Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae) often employ the use of field-collected foraging populations of individuals as defined colonies. The biological relevance of this practice is often called into question, because these colonies lack a full composition of reproductive castes and brood, which may have physiological and behavioral consequences. Rearing intact laboratory colonies can be done; however, it is time-consuming and labor-intensive. The artificial fusion of field-collected foraging populations with a young, laboratory-reared incipient colony may provide whole, intact colonies for laboratory research. The current study measures survivorship of fused colonies using laboratory-reared complete incipient colonies ranging in age from 0 to 5 mo, fused with 100 workers and 10 soldiers from field-collected populations of different colonial origin. Results indicate that 60% of colony fusion was successful when the incipient colony introduced is 5 mo of age. This method of colony fusion will provide researchers with intact colonies using minimal resources.

2018 Highlights of Urban Entomology

The field of urban entomology is primarily associated with the study of ants, bed bugs, cockroaches, termites, and other occasional invader pests that are found within or near human-made structures. A wide array of peer-reviewed studies were published in 2018. The topics of these articles ranged from genomes and basic biology of urban insects to various applied aspects of pest management. Key findings of these papers are presented and discussed from the perspective of the contributions they make to the discipline of urban entomology. Additionally, potential future research opportunities that are evident from these publications have been outlined.

Development of Baits for Population Management of Subterranean Termites

The objective of bait application envisioned by early researchers was to eliminate the source of infestation, the colony, but because of the lack of adequate evaluation tools, results of field trials with mirex baits in the 1960s were mostly inconclusive. On-the-ground monitoring stations and mark-recapture protocol developed in the 1970s marked the turning point in the field studies of termite baits. Results of field studies with metabolic inhibitors and chitin synthesis inhibitors (CSIs) in the 1990s indicated that a bait toxicant has to be slow-acting and nonrepellent, and its lethal time has to be dose independent. A recent discovery that termites return to the central nest to molt and CSI-poisoned termites die near the royal pair further explains the success of CSI baits in eliminating colonies. Owing to the availability of durable baits that require less-frequent site inspection, more termite control professionals have adopted baiting systems in recent years.

Comparative Impact of Chitin Synthesis Inhibitor Baits and Non-repellent Liquid Termiticides on Subterranean Termite Colonies Over Foraging Distances: Colony Elimination Versus Localized Termite Exclusion

This study evaluated the impact of a non-repellent liquid termiticide (fipronil) and a chitin synthesis inhibitor (CSI) termite bait (noviflumuron) on whole colonies of Coptotermes gestroi (Wasmann) (Blattodea: Isoptera: Rhinotermitidae) in laboratory conditions, over a 12-m foraging distance. The protocol simulated the implementation of remedial treatment of an infested structure, where the colony has multiple access routes, and where only a portion of the population was directly exposed to the treatment. Within 2 wk after the implementation of fipronil, all termites within 1.5 m away from the treatment died. The accumulation of cadavers near the treated area resulted in secondary repellency and the colonies avoided the treated area for the remaining 10 wk of the experiment, using alternative foraging galleries. At the end of the 12 wk, colonies exposed to fipronil did not have any difference in population size compared with control colonies. Comparatively, colonies exposed to noviflumuron had no change in foraging activity for the first ≈40 d, but then termites progressively ceased their activity throughout their foraging territory. By 12 wk, noviflumuron-exposed colonies were near-elimination, with only a few workers, soldiers remaining, and all colonies were eliminated by 95 d. This study shows that subterranean termite colonies with access to CSI baits are inevitably eliminated, regardless of the position of the bait, while colonies exposed to fipronil are only locally excluded from the area near the treatment, but may maintain their foraging activity in untreated areas, and retain their potential risk for structural damage in the long term.

Mortality Time Trends of Three Subterranean Termite Species When Exposed to 20-Hydroxyecdysone, Alone and in Combination With Noviflumuron

Workers of three termite species, Coptotermes formosanus Shiraki (Blattodea: Rhinotermitidae), Coptotermes gestroi (Wasmann) (Blattodea: Rhinotermitidae), and Reticulitermes flavipes (Kollar) (Blattodea: Rhinotermitidae), were force-fed with cellulose media pads treated with 20-hydroxyecdysone (20E) alone and in combination with noviflumuron for 3 d to examine their mortality time trends up to 14 d (including 3-d exposure time). Termites exhibited symptoms of hyperecdysonism before the onset of death. The times required for 20E to fully express its effects were 10-13 d for C. formosanus, 13-14 d for C. gestroi, and 11-13 d for R. flavipes. Higher 20E concentrations resulted in higher mortalities at 14 d, and the addition of noviflumuron generally yielded higher mortalities. The lethal time of 20E was similar to chitin synthesis inhibitors such as noviflumuron than metabolic inhibitors, and the 10-14 d lethal time may be sufficient for 20E-affected termites to return to the central nest before the onset of hyperecdysonism and ultimate death. As an active ingredient in baiting systems, 20E has shown potential to reduce the colony elimination time.