Termites & Mulch

Mulch is a staple for gardeners and landscapers, available by the bag  or the truckload. In addition to the traditional favorites of shredded hardwood and pine bark, there are regional specialties like pine straw, cypress and cedar. Food-processing byproducts, such as buckwheat, corn cobs, coconut shells and grape seeds, are also available as mulches. Stones and gravel of all shapes and sizes, as well as ground tires, shredded carpet, tarpaper and newspapers can also be applied as groundcover. Mulches of every texture and color are on the market, including artificially dyed wood products in red, black, blue, green and gold.

Mulches are popular for the tidy, fresh look they give gardens, but there are at least three genuine benefits these groundcovers provide: 1) Water conservation. Mulches slow soil drying by reducing evaporative water loss. They also reduce soil compaction, thus allowing more rainwater to soak into the ground; 2) Insulation. Air trapped within the mulch creates an insulated buffer zone at the soil surface, keeping the soil cooler on hot days and warmer in cold weather. Mulches also reflect solar radiation, which provides further relief in the heat of the summer; and, 3) Weed reduction. A layer of mulch inhibits weeds from germinating in the garden.

Conventional wisdom has long held that inorganic groundcovers, such as stones and gravel, are inherently unattractive to subterranean termites and generally discourage insect harborage by speeding soil drainage and drying. Many structures have a band of gravel around the perimeter to aid drainage and discourage rodent burrowing. Conversely, wood and bark mulches are suspected of attracting foraging termites because of their potential as a food source and because of the temperate, moist conditions they create in the underlying soil. Considering that mulch groundcovers are ubiquitous in the urban and suburban landscape and in many cases are applied directly against a building’s foundation, we examined the validity of these two common assumptions.

THE RESEARCH. In the field, we examined the impact of three types of bark mulch and one gravel mulch on the foraging patterns of the subterranean termite, Reticuli-termes virginicus (Banks). We set up mulch beds at several sites in College Park, Md., around the University of Maryland, where we knew termite colonies were actively foraging. We tested three of the local garden center’s top- selling mulches: shredded hardwood, miniature pine bark nuggets and pea gravel. We also examined shredded eucalyptus bark, a specialty product advertised as being resistant to garden pests. We did not test the borate-containing mulch, Term-A-Rid,™ for fear that borate would leach into the surrounding soil and affect the environment beneath the other mulches in the test. We applied the mulches to a depth of about 7 cm in accordance with National Gardening Association recommendations. Bare earth served as our control treatment.

To quantify termite foraging activity, we placed rolls of corrugated cardboard beneath each experimental mulch plot. The cardboard rolls were slipped into 12 cm deep holes under the mulch, much like those created for termite bait and monitoring stations. To determine whether foraging termites traveled through the mulch itself, we placed squares of cardboard within the mulch layer.

Throughout the spring, summer and fall of 1998 and 1999, we visited the sites every three weeks to collect the cardboard monitors and replace them with fresh rolls. Termites observed within the monitors were counted in the field and returned immediately to the same monitor hole. Cardboard monitors with evidence of termite feeding were taken to our lab, cleaned, dried and weighed. We calculated the quantity of cardboard consumed by the termites by subtracting the monitors’ post-feeding weights from their initial weights. This consumption data allowed us to compare the relative attractiveness of the areas beneath the mulches to foraging termites (see chart on page 66).

THE RESULTS. We found that conditions on the soil surface, either bark mulch, pea gravel or bare earth, had no significant effect on the number of monitors discovered by foraging termites. However, termites consumed significantly more cardboard from the monitors beneath the pea gravel. Temperatures at the soil surface were significantly cooler beneath all four types of mulch compared to the surface of the bare soils (Long et al. 2001). Our findings confirm earlier work (Ettershank et al. 1980, Smith and Rust 1994) that suggested objects on the soil surface may provide a cool refuge where termites prefer to forage during the heat of the day. Moisture meters placed in the soil 12 cm under each mulch and bare ground plot showed that soils beneath the pea gravel were slightly wetter than beneath the bark mulches. This condition, combined with the cooler temperatures, apparently created a microhabitat conducive to termite foraging.

We also documented termite activity within the mulch layer, confirming earlier cautions that mulches may provide a bridge across soil termiticide barrier treatments (Forschler 1998). The termites were equally active within all four types of mulch. In the lab, termites that were offered a choice of mulches fed upon all three types of bark equally. During a 12-week lab experiment in which the termites were given only bark mulch to eat, the termites readily consumed the mulches but died at rates comparable to termites that were given no food. This indicates that these bark mulches provided insufficient nutrition to support the termites.

Our data provide no indication that landscape mulches increase the rate of initial termite scouting in an area. However, our research suggests that gravel mulches create a hospitable environment where termites are likely to spend more time feeding on resources such as roots, stumps or construction debris than they would if these resources were buried beneath either bark mulches or bare ground. Our results also infer that installing termiticide bait or monitoring stations beneath stone mulch beds may not increase the chance that the stations will be discovered by foragers, but such placements might increase the duration of feeding experienced by those stations once they have been found by the termites.

Because it is rare that structure-infesting Reticulitermes colonies exist in isolation from the surrounding soils, efforts to make structures’ perimeter soils as inhospitable as possible may decrease the likelihood that those structures will become infested or will sustain existing infestations.

Unless interior water resources, such as leaking plumbing connections, are available, foragers must maintain a connection to the exterior soil in order to gather moisture. Although colonies can be wholly contained within a building, most structure-infesting termites constitute just a portion of a larger colony network that may forage across several acres of land. Therefore, most structure-infesting termites are traveling through and are affected by, the soil surrounding a building’s foundation.

Although our results were influenced by the seasonal and regional climates and the depth to which we applied our mulches and placed our monitors, we believe they provide valuable insight into Reticulitermes’ foraging response to soil conditions beneath four popular mulch groundcovers. Bare, dry soil against foundations is preferable but if material must be placed close to a structure for aesthetic purposes, pest management professionals should recommend to their clients that the mulch be spread in a thin, single layer so that the underlying soils can dry quickly. Creating this relatively dry band of soil, combined with the removal of potential food resources from the structure’s perimeter, such as stacked firewood, flower boxes, trellises and stair railings, will create a micro-environment inhospitable to foraging termites.

References:

Long, Catherine, et al. 2001. The effect of organic and inorganic landscape mulch on subterranean termite (Isoptera: Rhinotermitidae) foraging activity. Environmental Ent. 30: 832-836.

Forschler, B.T. 1998. Subterranean termite biology in relation to prevention and removal of structural infestation. In: NPCA research report on subterranean termites. NPMA, Dunn Loring, Va.

Smith, J.L. and M.K. Rust. 1994. Temperature preferences of the western subterranean termite Reticulitermes hesperus Banks. J. of Arid Environments 28:313-323.

Ettershank, G., J.A. Ettershank and W.G. Whitford. 1980. Location of food sources by subterranean termites. Environ. Ent. 9(5):645-648.