Mulches and Insect Problems
September 25, 2009
  • /Crop Sciences
Urbana -- Many different types of mulching materials are available, said a University of Illinois Extension horticulture educator.

"There are two main categories of mulch, inorganic and organic," explained Martha Smith. "Both have their advantages and disadvantages. The reasons we mulch, however, remain the same.

"First, it helps control weeds by reducing germination of weed seeds. Mulching maintains soil moisture by reducing evaporation. And mulching keeps the soil temperature constant, insulating against temperature fluctuations."

Inorganic mulches include various types of stone, crushed rock, volcanic rock, ground tires, synthetic fabrics, and numerous other materials. These mulches don't break down over time and therefore last longer in our landscapes.

In addition, they generally are more fire resistant. However, they don't improve soil quality or provide nutrients. Inorganic mulches can be heavy and typically require a landscape fabric placed underneath to avoid sinking into your garden soil.

"Organic mulches include leaves, wood chips, hardwood and softwood bark, cocoa hulls, compost, pine needles, and a variety of other materials derived from plants," she said. "Organic mulches decompose in the landscape at different rates depending on the material. Those that decompose faster must be replenished more often. This decomposition improves soil quality and fertility, both desirable features despite the added maintenance and expense."

Smith noted that a common question regarding mulches is, will organic mulch located next to my house result in insect and/or termite problems?

"The answer is no ... and maybe," she said.

"Organic mulches decompose, that is, they break down resulting in an earthy, black soil-like substance called humus or compost. They don't break down all by themselves, however. Insects help this process."

During seasons of high rainfall and moisture, more of these decomposers may be attracted to your mulched area. Sow bugs, millipedes, earwigs and centipedes may appear in higher numbers than normal.

"The organic mulch, by itself, does not attract these insects," Smith explained. "Rather it is in combination with either wet weather that season, or the inability of a particular mulched area to dry out quickly. During a drier season, you may not see these decomposers in your landscape — at least not in abundant numbers."

Termites live in the soil. They need high moisture levels and dry out quickly when exposed to fresh air. They feed on wood and wood products if the moisture level in a particular location remains high for an extended period of time.

A two to three-inch layer of coarse organic mulch does not have the capability of sustaining this constant level of moisture. The finer the mulch, the greater the chance of tight compaction that can hold in moisture, thus creating the right environment that is necessary for termite activity.

"Bark nuggets, hardwood mulches, and cocoa hulls are coarse and don't compact easily as, for example, sawdust," she said. "Organic mulches should never be more than three inches deep.

"Deeper mulch layers have the potential of maintaining high moisture levels, creating the ideal termite environment."

If the area in your landscape is naturally moist, keep the mulch layer shallower at about two inches. Periodically rake the area to aerate the mulch. Never mound organic mulch up against the foundation of a building or against untreated wood.

"Allow for six inches of exposed foundation between any woodwork and the soil," Smith said. "If you are concerned, make sure water drains away from your foundation and put a six to 12-inch wide layer of stone mulch against the foundation and use organic mulch throughout the rest of the landscape bed."

Organic mulches will not bring termites to your landscape, she emphasized.

"Termites, in most circumstances, are already in the vicinity of your home and will be attracted to the proper environment," she said. "If you find termites don't panic. Evaluate your situation carefully and contact a pest control professional to discuss options and treatments available."