Subterranean Termite Baiting: System Options and Seasonal Considerations

Bait station systems for use against subterranean termites (Blattodea: Rhinotermitidae), which usually employ chitin synthesis inhibitors (CSI, a class of insect growth regulators) as active ingredients, have been around for decades. These systems have been shown to be highly effective, killing worker termites during molting, indirectly starving soldiers and reproductive castes and often eliminating entire colonies of hundreds of thousands to millions of termites. Our developing field research on baits in California supports their efficacy: None of the unique (as per DNA) termite colonies accessing CSI baits at our 15 single-family sites were ever detected again at those sites. New research shows that egg laying by termite queens and egg viability also can be significantly reduced after CSI consumption, speeding up the colony elimination process. Pest control operators are considering bait systems as alternatives to traditional liquid termiticide barrier treatments and using these tools to manage termites for their customers. This is especially true at environmentally sensitive sites, with customers demanding green products, or when dealing with aggressive pest species, such as the invasive Formosan subterranean termite, Coptotermes formosanus.

Casey Hubble

Some regions may lag behind in bait station system adoption, however. This is true for California, where operators cite prohibitive licensing requirements, disturbance-related avoidance by termites and excessive time required for control as major barriers to adoption. To the first point, some (older) bait systems include monitoring phases, which require pest identification and, therefore, a mid-tier Field Representative license in California, before CSI bait can be added to stations. To the second point, some termites, especially certain Reticulitermes species, have been considered “skittish,” sometimes disappearing for months or longer due to disturbances such as bait station inspections. Finally, customers with infested structures often expect immediate results when hiring termite control companies, and many months may pass before termites find and attack new bait stations. Similar concerns may exist in other regions. Newer bait systems and new product labels may have removed some of these barriers, however. Three systems registered for use in California (Sentricon Always Active by Corteva Agriscience, Advance Termite Bait System by BASF, and Exterra by Ensystex; see Table 1) now allow for CSI bait on day one, bypassing the monitoring phases (provided activity by the target species has been documented at the property) and allowing for installation and replenishment service by entry-level applicator licensees. Product labels also now allow for longer service intervals (up to six months for Exterra, and up to a year for Sentricon and ATBS), minimizing disturbances that may repel foraging termites. The third bait system concern, that it simply takes too long to control infestations, is seemingly more difficult to alleviate.

A TIME-TO-ATTACK THEORY. One explanation for time-to-attack (the amount of time between installation and the first observation of bait consumption by the target species) problems in California has to do with the state’s unique Mediterranean climate (hot summers with little to no rain and cool winters that typically produce the entire annual precipitation amount) and prevailing soil textures (high proportions of clay). Termite foraging at or near the soil surface may be limited or even nonexistent during summer months, especially when areas are not irrigated. Some research supports this idea: Reticulitermes hesperus, the western subterranean termite, has been observed to forage near the surface, mostly during winter months in its native habitat in Southern California. This suggests that bait stations installed in summer may sit uninvestigated for six months or more. To test this hypothesis, and to observe whether time-to-attack could be reduced by targeting specific seasons for installation, we established five research plots directly on top of known termite colonies during 2019 at the University of California, Berkeley Richmond Field Station. Naturally occurring subterranean termites (Reticulitermes spp.) had been observed as foraging workers or brood chamber aggregations and collected at the center of each plot.

Susan Hare, used with permission

Around these five areas of documented termite activity, we established three concentric rings of bait stations at three distances from the center, installing one station from each of three registered systems along each of the rings at the beginning of each season over one year, for a total of 36 bait stations per plot. We didn’t want to kill the termites in these plots because that would significantly confound our data, so we used cellulose bait matrices from manufacturers that did not contain the CSI active ingredients. We also installed a monitoring device (Isopthor EZE station housing containing wooden monitoring blocks) at the center of each plot and along each of the three distance rings. By the end of the year, we had installed 200 stations for this investigation, all by use of a ratcheting hand auger! We then checked each station every two months (about every 60 days) after its installation for two years, opening and inspecting up to 100 stations per month. We concluded this ambitious project in December 2021.

FINDINGS. So, what did we find? To begin with, our experiences and observations with all three bait systems over these past two years allow us to compare their potential advantages and disadvantages. We believe all three systems will be effective against subterranean termites and can be used to eliminate entire colonies, but some may be easier to install, easier to inspect or more durable in the field. Economic barriers to adoption, such as cost, technical assistance from product representatives or licensing agreements required, also may differ. Our objective in this work was not to highlight differences among these bait systems, however, but rather to find generalized trends common to all three, especially considering the effect of installation season on time-to-attack. As expected, air temperature and soil moisture varied widely from season to season. Summers were hot and dry, as expected, and rainfall occurred exclusively during cooler months. First significant rains were recorded during late November in 2019 and 2020, and during late October in 2021. Most of our termite foraging activity in stations was observed in winter and spring, with a marked activity plateau during the February to June period. Smaller activity peaks were observed during the summer, especially in September, which is immediately prior to the majority of Reticulitermes swarming observed in the San Francisco Bay area. Generally, however, activity declined throughout the year, tapering to almost nothing during November and December (see Figure 1). Of the 180 bait stations and 20 monitoring stations installed, 78 bait stations and nine monitoring stations had been hit by the end of the two-year project period, representing an overall hit rate of 44 percent. Three stations were attacked within 60 days after installation, and 10 stations were attacked within 120 days. Overall, however, the average time-to-attack was 367 days, a full year after installation. This result supports the general claims of California’s pest control operators that baiting may take too long for most remedial termite control jobs. There were no significant differences between the three bait systems, with average time-to-attack for all three between 327 and 383 days. We did not detect any significant differences in time-to-attack among the three distance rings. Proximity to adjacent stations and type of adjacent stations were considered as potential factors influencing time-to-attack, but there were no measurable effects detected.

There were statistically significant differences detected among the five different sites, which included differences in soil type and irrigation regimes. Average time-to-attack at Site 5, which was in unirrigated sandy soil near a wood building, was 292 days, significantly less than average time-to-attack at Site 4 (462 days), which was in clay loam soil within an irrigated landscape bed dominated by coastal redwood trees. The other three sites, which were along a sporadically irrigated linear grove of pine and oak trees, were intermediate in terms of time-to-attack and did not differ statistically from sites 4 or 5. Our experiment was not designed to determine the site factors that may influence time-to- attack, but one hypothetical explanation for the site differences observed is that there may have been much more cellulose debris (dead wood and decomposing mulch) in the landscape bed under the redwood trees than in the sandy and mostly unvegetated area adjacent to the old building, providing ample food for foraging termites and making the bait matrices comparatively less attractive.

Andrew Sutherland

Our study’s main question was whether installation season significantly impacts time-to-attack due to seasonal differences in termite foraging in California. To answer this, we pooled data from all five sites and all three bait systems and then considered just the first year of observations. The result was striking: Time-to-attack for stations installed at the beginning of winter was more than 100 days less than for stations installed at the beginning of summer (194 days versus 296). This result was statistically significant. Installations at the beginning of spring and beginning of autumn were intermediate (282 and 268, respectively) and statistically inseparable from the other two seasons (see Figure 2). This effect was strongest at sites 1-3, along the pine and oak grove. In fact, none of the stations installed at Site 3 at the beginning of summer were hit during the first year of our observations. These data show that the lengthy time-to-attack many California operators have experienced may be reduced by targeting installations for the beginning of the wet season, usually during November to December. Also, this time of the year is usually the easiest time to dig in the region, with clay soils soft and muddy as compared with hard and dusty during the dry season.

FINAL THOUGHTS. Overall, these findings may help pest control operators to optimize their use of bait station systems as subterranean termite control tactics by targeting specific installation seasons, especially in areas with pronounced wet-dry or hot-cold periods. One way to extrapolate these results would be to consider the season known to be associated with the highest termite activity in your area and then to install bait stations immediately prior to (but not during) this season. For some, this may seem like a common-sense best practice. For others, especially those new to termite baiting, targeting the right season for new installations may facilitate early success.

Sutherland is an urban pest management researcher and educator at University of California Division of Agriculture and Natural Resources (UC ANR). Hubble is a staff research associate, and Barber is a lab assistant in Sutherland’s UC ANR program.