Predicting ecological impacts of invasive termites

Invasive termites and their growing global impact as major urban pests

While termites play important ecological roles, a fraction of species have strong invasive capabilities and represent urban pests of economic importance worldwide. Their invasive potential is exacerbated by human activities such as maritime transport, with privately owned boats serving as key vectors for local and global termite dispersal, particularly for Cryptotermes and Coptotermes pest species. Land establishment by invasive termites can remain undetected for decades, often making eradication attempts too late to succeed. Ultimately, invasive termite species will likely continue to spread at the global scale, and recent new invasive records point toward an underestimation of their actual current invasive status.

Ongoing human-mediated spread and hybridization of two major invasive termite species

Human-mediated biological invasions can lead to introgressive hybridization events between lineages that have evolved independently, with potential for evolutionary, ecological, economic and social impacts. This study provides evidence for the ongoing spread and hybridization between two major invasive and destructive termite pest species, Coptotermes gestroi and C. formosanus in Florida. Heterospecific courtship behaviour between alates (winged reproductive caste) of the two species has led to the establishment of F1 hybrid colonies in the field, which have matured and produced F1 hybrid alates. Laboratory backcross attempts confirmed the possibility for F1 hybrid female alates to establish viable F2 colonies with males of either parental species. With the recent documentation of introgressive hybridization between the two species in Taiwan, the current study confirms its independent occurrence in Florida, demonstrating that both Coptotermes species can hybridize in areas where their distributions overlap. In Florida, the proximity of field-established hybrid colonies to the large leisure boat industry implies that Florida populations of C. gestroi, C. formosanus and their hybrids will continue to serve as a bridgehead source of propagules for further dispersal beyond their current distribution, with a potential for F1 hybrids to spread outside of Florida.

Phylogenetic relationship of subterranean termite Coptotermes gestroi (Blattodea: Rhinotermitidae) inhabiting urban and natural habitats

Coptotermes termites were collected from forestry habitats at University Sains Malaysia, Penang, while urban samples were collected from residentials from Penang and Kedah, Malaysia. Mitochondrial DNA markers, Cytochrome Oxidase 1 (CO1), and 16S ribosomal RNA (16S rRNA) genes were amplified and sequenced to confirm the species of the termite samples as Coptotermes gestroi. Through Blastn, all 25 CO1 and 16S rRNA sequences, respectively from urban and natural habitats were found to be 99.54-100.00 % similar to C. gestroi reference sequences from previous studies in Peninsular Malaysia. The phylogenetic trees constructed using Neighboring-joining (NJ) and Maximum Likelihood (ML) methods resulted in CO1 sequences clustering in two clades and 16S rRNA sequences clustering in a single clade. The overall mean distance was low for the C. gestroi populations from natural habitats and urban settings (FST = 0.004). Analysis of natural habitat populations using CO1 sequences revealed two haplotypes within the population, with a haplotype diversity (Hd) of 0.045 ± 0.005, while the urban population shared a common haplotype with the natural habitat populations and there was no haplotype diversity recorded between the populations. Urban and natural habitats included only one haplotype for 16S rRNA sequences, indicating a lack of nucleotide diversity. Based on the findings, a non-significant difference between the natural habitat and urban population suggests C. gestroi inhabiting both regions likely originated from a similar source and underwent population homogeneity in different settings facilitated by anthropogenic dispersal.

Mobilome characterization of the beetle Euchroma gigantea (Buprestidae) uncovers multiple long range Tc1-Mariner horizontal transfer events

Transposable elements (TEs) are mobile repetitive DNA sequences that can transfer horizontally between species. Due to their mutagenic characteristics, TEs are associated with different evolutionary events, including chromosomal rearrangements that are abundant in the beetle Euchroma gigantea. In order to understand more in depth the impact of TEs on the genomic evolution of E. gigantea, we characterized the E. gigantea mobilome and evaluated the horizontal transfer of Tc1-Mariner elements. Genomic sequencing data was generated on the Illumina Hiseq plataform, from a specimen (Northeast lineage) collected in Recife, Pernambuco - Brazil. The TEs were characterized by two independent approaches based on the clustering and assembly of highly repetitive sequences, the RepeatExplorer and dnaPipeTE. The sequences obtained were further characterized using ORFfinder and CD-Search, to obtain the TEs' potential coding proteins and verify the presence and integrity of known TE domains. Evidence for horizontal transfer was evaluated by nucleotide and protein genetic distance between TEs from E. gigantea and other species and phylogenetic incongruences detected between TEs and hosts phylogenetic trees. The mobilome of E. gigantea represents about 21 to 26% of its genome. This mobilome is composed of TEs from 31 superfamilies, belonging to different classes and most known orders of TEs. Several types of TEs with intact domains were observed with emphasis on Tc1-Mariner suggesting the presence of potentially autonomous elements. This superfamily also stands out for having the greatest abundance and diversity, with TEs being classified into four families. When compared to TEs deposited in databases, Mariner TEs stood out as having the highest nucleotide identity (above 90%) with TEs from phylogenetically distant species, such as ants and bees. Altogether these results suggest that E. gigantea Mariner TEs underwent multiple horizontal transfer events to other insect species.

On the reproductive strategies post-colony foundation: major termite pest species with distinct ecological habits differ in their oviposition dynamics

Termite colony foundation precedes the incipient stage, when the first oviposition cycle takes place, followed by months of reproductive inactivity. The royal couple is supposed to cease oviposition during this period, investing energy to care for the first brood. When a suitable number of alloparents differentiate, egg-laying resumes. Here we followed oviposition dynamics, embryo development and queen/king body changes in laboratory colonies of the major pest species Coptotermes gestroi (Rhinotermitidae) and Cryptotermes brevis (Kalotermitidae) during 9 months. We show that they differ in these oviposition dynamics, as C. gestroi queens displayed an uninterrupted oviposition whereas C. brevis laid a cohort of eggs and ceased oviposition during a 3-month period (lag phase). C. gestroi oviposition dynamic was remarkable and suggests that occurrence of progeny was not a limiting factor, thus queens and kings were able to concomitantly invest energy in reproduction and parental care. These findings contrast those reported for rhinotermitids from temperate areas, and we discuss the likely reasons for such a condition, including endogenous rhythms, avoidance of a high mortality rate of the first progeny and adaptation to the weather conditions of the Neotropical region. Oviposition dynamic in C. brevis resembled those of several termite species, in which the royal couple cease reproduction to care for the first brood. Rearing conditions did not influence oviposition dynamics (egg-laying cycle followed by a lag phase), thus our results on the oviposition of C. gestroi and C. brevis correspond to different reproductive strategies post-foundation adopted by these pest species.

Functional Morphology and Development of the Colleterial Glands in Non- and Egg-Laying Females of the Pest Termite Coptotermes gestroi (Blattaria, Isoptera, Rhinotermitidae)

Colleterial glands of female insects are accessory glands responsible for producing secretions associated with egg-laying. Within Dictyoptera, they synthesize compounds of the ootheca. However, their morphology and role in termites are poorly understood. Here, we compared the morphology, development, and secretory activity of the colleterial glands between non- and egg-laying females of the pest termite Coptotermes gestroi under light and transmission electron microscopy. We also provide the first description of these glands for Rhinotermitidae. The glands are paired, divided into anterior and posterior units, which join in a common duct via basal trunks. They are highly developed within egg-laying females, especially the posterior gland, secreting glycoproteins to lubricate the genital chamber and/or stick the eggs together. Ultrastructure revealed glandular epithelia composed of bicellular units of Class 3, whose secretory activity varied between groups and units. Posterior gland of egg-laying females showed richness of mitochondria, rough endoplasmic reticulum, and secretory vesicles, including electron-dense secretory granules, indicating synthesis and transport of contents, especially proteins. The basal trunks were enfolded by muscles, supporting their role in conducting secretion. Morphophysiological modifications occur in the colleterial glands as females mature and lay eggs, and the mechanisms underlying the secretory cycle of the glands are discussed.

Exuviae Recycling Can Enhance Queen Oviposition and Colony Growth in Subterranean Termites (Blattodea: Rhinotermitidae: Coptotermes)

Wood-feeding termites have a nitrogen-poor diet and have therefore evolved nitrogen conservation strategies. However, termite workers molt periodically, and throughout the lifetime of a colony, millions of exuviae, a nitrogen-rich resource, are produced by the colony. In Coptotermes Wasmann, workers foraging at remote feeding sites must return to the central part of the nest to molt, where the queen, king, eggs, and larvae are located. It was hypothesized that this molting-site fidelity is an efficient way to recycle nitrogen for reproduction and colony growth, as nestmates involved in exuviae consumption can directly transfer such resources to individuals engaged in reproduction (the queen) or growth (larvae). This study investigates whether incipient colonies of C. gestroi (Wasmann) can gain additional biomass when they are fed supplementary exuviae. Incipient colonies were reared in nitrogen-poor or nitrogen-rich conditions, and 0, 1, 5, or 10 exuviae were added to 3-month-old colonies. After 6.5 months, colonies reared in nitrogen-poor environments gained significantly more biomass when exuviae were added than colonies with no added exuviae. However, the addition of exuviae had no effect on colony growth for colonies reared in nitrogen-rich environments. In a second experiment, queens from colonies in which exuviae were effectively removed laid fewer eggs than queens from colonies in which exuviae were not removed. Therefore, consumption of exuviae from molting individuals by nestmates is an important part of the nitrogen recycling strategy in Coptotermes colonies, as it facilitates queen oviposition and colony growth, especially when such colonies have limited access to nitrogen-rich soils.

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.

High Exploration Behavior of Termite Propagules Can Enhance Invasiveness

Social life is usually associated with enhanced propagule pressure, which increases the chance of introducing several individuals during a single introduction event. Social insects are therefore among the most successful invasive species, benefiting from rapid establishment and increased foundation success in new habitats. In termites, propagule pressure may also be increased by the development of reproductive individuals from a small group of foraging workers. This suggests that enhanced exploration activity may increase propagule pressure through an elevated chance of transporting isolated groups of foragers. Here, we analyzed the exploration behavior of three termite species of the Reticulitermes genus, comparing the invasive species Reticulitermes flavipes (testing both native and introduced populations) to the native species Reticulitermes grassei and Reticulitermes lucifugus. Different features representative of the exploration capacity were measured during 48 h, including: the number of tunnels, the length of tunnels, the number of foragers, and the interindividual distance of foragers in a straight line or through tunnels. Our results show that compared to the native Reticulitermes species, R. flavipes foragers from both populations dug more tunnels with a longer total length, and individuals were more spatially dispersed and covered a larger exploration zone. These findings suggest that the enhanced exploration ability of R. flavipes may have played a role in its invasion success, by increasing its propagule pressure through a higher chance of human-mediated transport. In addition, the absence of differences between the native and introduced populations of R. flavipes suggests that the exploration behaviors facilitating the worldwide invasion of this species originated in its native range.

Land use effects on termite assemblages in Kenya

Termites perform key ecological functions and they also cause crop damage. Land use change resulting from agricultural intensification can result in changes in termite species diversity and abundance. Termite species occurring in natural vegetation, maize monocrop and maize-beans intercrop macrohabitats were investigated in Embu and Machakos Counties, Kenya. Influence of soil properties and seasons was also evaluated. Across the two Counties, seven termite species were recorded with Machakos County having the highest number. Additive diversity partitioning of species richness and Simpson diversity showed that, α component contributed to 98.3% and 99.1% of the total diversity, respectively. Population densities of three termite species significantly varied between land use types in Machakos County but there were no differences in termite species abundance in Embu County. In addition, there were no significant differences in species richness between macrohabitats within each County. In Embu, season significantly influenced the abundance of Macrotermes subhyalinus, M. herus, and Coptotermes formosanus which occurred in greater numbers during the wet season. There was a significant influence of land use on Trinervitermes gratiosus and C. formosanus in Machakos with both species occurring in higher numbers in natural vegetation. Trinervitermes gratiosus was negatively associated with Mn and positively correlated to pH and sand. Macrotermes subhyalinus and M. herus showed a positive association with P and silt while C. formosanus was positively correlated to Ca and Mg. These findings provide an insight into the effects of land use change from natural vegetation to maize agro-ecosystems on termite diversity. It also provides a baseline for further studies on termite diversity in Kenya and their ecological significance.

Soil organic matter is essential for colony growth in subterranean termites

Intrinsic dinitrogen (N₂) fixation by diazotrophic bacteria in termite hindguts has been considered an important pathway for nitrogen acquisition in termites. However, studies that supported this claim focused on measuring instant N₂ fixation rates and failed to address their relationship with termite colony growth and reproduction over time. We here argue that not all wood-feeding termites rely on symbiotic diazotrophic bacteria for colony growth. The present study looks at dietary nitrogen acquisition in a subterranean termite (Rhinotermitidae, Coptotermes). Young termite colonies reared with wood and nitrogen-rich organic soil developed faster, compared to those reared on wood and inorganic sand. More critically, further colony development was arrested if access to organic soil was removed. In addition, no difference of relative nitrogenase expression rates was found when comparing the hindguts of termites reared between the two conditions. We therefore propose that subterranean termite (Rhinotermitidae) colony growth is no longer restricted to metabolically expensive intrinsic N₂ fixation, as the relationship between diazotrophic bacteria and subterranean termites may primarily be trophic rather than symbiotic. Such reliance of Rhinotermitidae on soil microbial decomposition activity for optimal colony growth may also have had a critical mechanistic role in the initial emergence of Termitidae.