Early signs of colony collapse in termites when baiting with CSIs

Chitin‐synthesis inhibitors (CSIs) are a group of Insect Growth Regulators (IGRs) such as hexaflumuron, noviflumuron, and novaluron, that are used in subterranean termite bait stations. When ingested by foragers, they are shared with the members of the colony and slowly cause death by disrupting proper molting in workers and causing post-molting complications, such as losing hemolymph and desiccation. Additionally, such active ingredients can also affect the viability and hatch rates of the eggs (Chouvenc, 2025). Finally, the colony fails to replace dying workers with young (larvae) termites, and after a certain point, the whole colony collapses due to a low number of workers and invasion of pathogens, predators, and parasites. The general advantage of using CSI insecticides over traditional liquid insecticides includes their low risk to non-target organisms, non-repellent nature (this is shared with some liquid insecticides, though), lower risk of leaching into waterways (because bait is contained in a bait station underground), and their wider effect on the colony by causing complete or almost complete colony elimination in the field.

Based on various reports, it takes CSI baits anywhere from 1-14 months to completely eliminate termite colonies (Evans & Iqbal, 2015). Depending on the species, colony size, and other factors, colony elimination typically takes 3 or more months using CSI baits. In the field, it is very difficult to identify full colony elimination, simply because the termites are cryptic insects, hiding under ground or inside wooden objects most of the time. Termite colonies tend to feed less on the bait toward the end of the baiting process, and that makes them less visible and yet partly alive. Despite all the challenges in identifying colony collapse discussed above, several signs can help identify the onset of colony collapse or measurable weakening due to CSI exposure. The following are key indicators observed at bait stations that may signal colony decline. Ideally, two or more signs should be evident to confirm colony weakening:

Figure 1: Mites on termite bodies. Photo: David Mora.

• Presence of mites on termite bodies: In a weak colony, mites can be seen on the head and body of termites (Fig. 1). This phenomenon has been observed in Reticulitermes and Coptotermes visiting bait stations. A weak colony cannot perform sanitation practices well enough, leading to the introduction and proliferation of parasites, predators, and harmful microorganisms (Mora & Hernández-Teixidor, 2025).

Figure 2: High springtails activity in termite colonies. Photo: David Mora.

• Presence of detritivore insects in bait stations: Termite nest contains a variety of decomposable materials that can be utilized by invaders such as springtails (Fig. 2) and fungus gnats (Sciaridiae) (Chouvenc et al., 2013; Mora & Hernández-Teixidor, 2025). In a healthy colony, these invaders are kept in check by soldiers and workers. However, high presences of such insects can be an indicator of inadequate nest maintenance due to worker and eventually soldier cast mortality.
• Increased soldier cast percentage: CSI exposure removes younger worker cohorts first and therefore skews caste proportion in the colony. This increases the percentage of soldiers in the colony. For example, it was shown that the average percentage of the Asian subterranean termite (Coptotermes gestroi) soldiers in the colony can change from ~7% before baiting to over 70%, 80 days after exposure to a CSI bait (Chouvenc & Su, 2017). Please note that various termite species have different percentage of soldiers in their colonies. For example, members of the genus Coptotermes are well known for their relatively high percentage of soldiers (7-10%) while species in the genus Gnathamitermes have one of the lowest percentage of soldiers to the extent that finding even one soldier can be extremely difficult at times.
• Worker "bleaching": The abdomen of workers in colonies exposed to CSI baits can lose its fine yellowish-orange tint and become pure white (low temperature color). This is often linked to uric acid accumulation and cannibalism during colony decline.
• Reduce mobility of workers: Termites affected by the CSI bait may move more slowly than usual.
• Presence of dead soldiers: Seeing dead soldiers in bait stations is another sign of colony decline.
• Presence of secondary reproductives: As colony demography destabilizes, neotenic or secondary reproductives can appear inside bait stations, which is an early sign of colony collapse.
• Presence of eggs without larvae: Although rarely seen, the presence of eggs with no developing brood indicates reproductive dysfunction and brood-care, which is characteristic of colony collapse.
• Reduced bait consumption rate: After an initial consumption phase, stations show decreasing intake of bait due to worker caste mortality. However, this sign alone cannot accurately identify colony collapse simply because termites may switch their food choice in the field. However, long-term absence of termite feeding activity on the bait is a more reliable cue for colony collapse.

References

• Chouvenc, T. (2025). How do termite baits work? Implication of subterranean termite colony demography on the successful implementation of baits. Journal of Economic Entomology, 118(3), 997–1007. https://doi.org/10.1093/jee/toae243
• Chouvenc, T., Bardunias, P., Efstathion, C. A., Chakrabarti, S., Elliott, M. L., Giblin-Davis, R., & Su, N.-Y. (2013). Resource opportunities from the nest of dying subterranean termite (Isoptera: Rhinotermitidae) colonies: a laboratory case of ecological succession. Annals of the Entomological Society of America, 106(6), 771–778.
• Chouvenc, T., & Su, N.-Y. (2017). Subterranean Termites Feeding on CSI Baits for a Short Duration Still Results in Colony Elimination. Journal of Economic Entomology, 110(6), 2534–2538. https://doi.org/10.1093/jee/tox282
• Evans, T. A., & Iqbal, N. (2015). Termite (order Blattodea, infraorder Isoptera) baiting 20 years after commercial release. Pest Management Science, 71(7), 897–906. https://doi.org/10.1002/ps.3913
• Mora, D., & Hernández-Teixidor, D. (2025). Large-scale elimination of subterranean termite colonies of the genus Reticulitermes (Blattodea: Heterotermitidae) from town centers in Spain. Journal of Economic Entomology, 118(3), 982–996. https://doi.org/10.1093/jee/toaf039