Plant water stress is commonly cited as the most important factor predisposing lawns to chinch bug; it can even be regarded as a required condition for the initiation of chinch bug damage. [C1, C3] Water stress makes the grass less tolerant to chinch bug because they damage the plant water conducting system. Water stress may also make the grass more nutritious to sucking insects which increases their reproductive rate.[V1]
Conditions that are particularly conducive to water stress are in general conducive to chinch bug. These include:
low rainfall, especially in July and August; exceptionally low rainfall in July, 2003 (31.8 mm compared to 30 year average of 107 mm) was probably a major factor in the chinch outbreaks in Halifax Regional Municipality;
high temperatures, which increase loss of water from plants and soil;
Chinch damage at this site occurred mainly by the curb and on a south facing slope.
windy weather or sites, which increase loss of water from plants and soil;
compaction and/or surface sealing of soil, which reduces the rate at which rain can be absorbed during a heavy rainfall;
excess thatch, which limits infiltration of water during short rainfalls and encourages shallow rooting of grass;
low soil WHC (water holding capacity), which lowers the amount of rain that can be absorbed in a rainy period. Low WHC could be associated with one of
thin soil
low soil organic matter (humus)
sandy texture
rapid drainage, e.g. on a slope or in turf established over porous fill;
a combination of most of these conditions is not unusual in new developments in Halifax Regional Municipality where often only a few inches of sandy soil, low in organic matter are laid down over fill or other base in highly exposed situations;
high demand of grass for water, often created by excessive nitrogen fertilization. [L29]
Within a lawn, drier, full sun areas, especially on south facing slopes, mounds, edges of turfs and the sides of buildings where heat is radiated are likely to be the most prone to water stress and chinch bug damage. [C1-C3]
Chinch bug generally is not a problem in shaded areas and, rarely if at all, in well watered lawns.
Thatch in a lawn attacked by chinch bug. Click on image for larger version
"Thatch is light brown, fibrous organic matter that looks like peat moss. It is located between the soil and grass blades and is made up of compressed surface roots, stems, and runners. Thatch is hidden, so most people do not realize it is there. Excessive thatch feels soft and springy underfoot. "
-Matthew J. Fagerness [13]
Some thatch on a lawn is normal and a good thing. However extension bulletins[C2, C3] and formal observations and experiments[C6, C7] indicate that excess thatch is highly conducive to chinch, although not a required condition for chinch bug damage.[C8]
Thatch is considered excessive when it is more than 12 mm (1/2 inch) thick.
Excess thatch tends to stimulate chinch bug for at least two reasons:
More thatch provides more habitat for chinch bugs. Nymphs stay in the thatch layer most of the time, probably because it provides protection against dessiccation and predators.[C6] Adults may overwinter in thatch.[C1]
Excess thatch increases water stress. Once dry, thatch does not wet easily, and acts as a barrier to movement of water into soil during short rainfalls. On the other hand, once it gets wet, it holds water and encourages shallow rooting of the grass, which then become much more drought susceptible in dry weather.[L15]
Excess thatch results from an imbalance between production of organic matter and its breakdown (decomposition). High production of thatchy materials or reduced rate of breakdown of thatch (or both) leads to excess thatch.
BREAKDOWN OF THATCH
Checking for Thatch
Checking for thatch is easy. All you need is a sturdy jackknife. Start by cutting a pie-shaped wedge out of the lawn, making sure to get down into the soil.
Pull the wedge out, and examine the brown mat of thatch just above the soil surface.
Replace the wedge and examine other places in your lawn. Just because thatch is present or absent in one place doesn't mean the same will be true in another. Thatch thickness typically varies throughout the lawn. Places where thatch has gotten thick feel soft and spongy when you walk across them.
Source: Info Source Division of Cooperative Extension of the University of Wisconsin-Extension,
Item # 535 Thatch
(Viewed 6 Apr. 2004)
Thatch consists of the more difficult to decompose components of leaves and surface roots, notably the vascular tissues.[L13] These tissues are rich in lignin, a complex molecule that only certain fungi can break down efficiently.[V2] Earthworms and arthropods grind up the tissues, increasing the surface area for fungi to grow and penetrate the tissues.
Excess nitrogen, pesticides and fungicides slow breakdown of thatch. Reduced breakdown of thatch is associated largely with intensive use of fertilizers and pesticides.[L3, L13B, L29]
Excess nitrogen fertilizer acidifies the soil which reduces microbial activity, and through acidification or other effects, reduces earthworm and arthropod populations that are important in breaking down thatch;[L4, L5, L29 ]. High levels of inorganic nitrigen also interfere biochemically with the breakdown of lignin[V3, V4]
Insecticides, applied to kill pests (often stimulated by high plant nitrogen), kill the earthworms and arthropods that break down thatch.[L4]
Fungicides, applied to control diseases (often stimulated by high plant nitrogen), kill the fungi which are essential for breaking down the lignin molecules in thatch.[L13B, V10]
Even some herbicides have detrimental effects on earthworms[L13B]and 2,4-D stimulates certain pests and pathogens. [V8]
Excess water can also contribute to thatch formation. The fungi that break down lignin require oxygen. A layer of water on the soil surface reduces movement of oxygen into the thatch and to fungi, slowing decomposition. During extended rainfalls water can accumulate on the surface of heavy soils (high clay content) or on poorly drained soils. Even on a well drained site, improper watering can lead to an excess of water, e.g., by delivering water much faster than the soil can absorb it, or by watering well past the time needed to saturate the soil. .
PRODUCTION OF THATCH
Grass species
"High maintenance programs that stimulate rapid growth contribute to thatch buildup. Excess fertilization, irrigation and chemical applications are all contributors. It sounds ironic, but the higher the maintenance level the greater the likelihood of a thatch problem." S. Barton and J. Windsor[L3]
Some grass species produce more thatch than others. The heavy thatch producers are species that spread by stolons and rhizomes, which are rich in lignin. These include Kentucky bluegrass, creeping red fescue and creeping bentgrass. The most vigorously growing cultivars produce the most thatch. The bunch type grasses (ryegrass, tall fescue) on the other hand produce much less thatch.[L12]
Unfortunately most sod sold locally is 80 to 100% Kentucky bluegrass, and can produce heavy thatch very quickly.
Clippings and mowing
It is sometimes suggested that high production of clippings, stimulated by excess nitrogen, is a factor in thatch accumulation. (It used to be advised to remove clippings). On this issue, Barton and Windsor[L3] comment
"...these clippings decompose rapidly and do not usually affect thatch accumulation. However, if a thick layer of thatch builds up and clippings do not come in contact with the microorganisms in the soil, they may contribute to the thatch layer."
In other words, Barton and Windsor maintain that high clipping production can make an existing problem worse, but does not create the problem.
A Recipe
for
Producing Thatch
1. Lay down 100% Kentucky bluegrass sod.
2. Fertilize it heavily with concentrated, inorganic fertilizers to keep it dark green.
3. Use pesticides and fungicides to prevent pests and diseases.
4. Mow close to the ground.
5.Water the grass lightly but frequently during dry periods.
However, the mowing regime is a factor.[L12] When grass is mowed regularly at a relatively high level (2.5-3"), only small clippings are produced and they come from the tips of the grass blades which have the least lignin and are high in water. These clippings fall quickly to the soil surface and decompose rapidly. When grass is mowed infrequently, long clippings containing more lignin are produced and they tend to get caught in the grass canopy; both factors slow their breakdown. If grass is mowed very close to the ground, it removes older tissues near the base of the plant that are high in lignin and slow to decompose.
Excess nitrogen fertilizer is a key culprit. As well as causing excessive grass growth and water consumption and contributing to thatch accumulation as described above, high levels of nitrogen have a direct stimulatory effect on many pests and pathogenic fungi. In the case of insect pests, this is commonly attributed to increased plant nitrogen making the plant tissues more nutritious to insects (plant tissues are generally very deficient in nitrogen compounds relative to insect needs). High nitrogen also tends to make the tissues more succulent and easier to penetrate. [V1, V6, L29]
Chinch bugs are amongst the many plant-eating insects that are stimulated by nitrogen fertilization. Studies in the 1930s and 40s, summarized by Hale [C20] showed that southern chinch bugs produce more eggs and live longer with increasing levels of nitrogen in sorghum and St. Augustine grass. Busey and Snyder [C18] noted that sudden increases in southern chinch bug often followed heavy fertilization. In experiments, a highly soluble N fertilizer (ammonium nitrate) increased chinch bug densities on St. Augustinegrass by 65% compared to unfertilized control plots; the increase was 28% with IBDU (a slow release nitrogen fertilizer) and 0% with milorganite (an organic fertilizer). Horn and Pritchett [C21] had earlier reported similar findings.
Chinch bugs need food! Unfortunately, they feed on fine fescues, perennial ryegrasses and Kentucky bluegrass, which are the main types of grasses used to establish lawns in this region. Kentucky bluegrass is usually reported to suffer less damage than ryegrass and fescues.* Creeping bentgrass, which often invades pure Kentucky bluegrass turfs locally (L6) is reported to be particularly susceptible.[C1] However a Quebec study found the reverse, i.e. that it was negatively associated with chinch bug abundance.[C8]
Trials have shown that some cultivars of Kentucky bluegrass, ryegrass, and fescues are more resistant to chinch bug damage than others.[C1, C2] This type of resistance is genetically based and could be associated with one or more of a variety of plant characteristics.
*On the one hand, Kentucky bluegrass is a heavy thatch producer which encourages chinch bug; on the other it is more resistant than other grasses to chinch bug feeding. So should one plant more Kentucky bluegrass or less? The (simple) answer is, 'everything in moderation'.
A different sort of resistance is conferred by endophytic fungi, which associate with certain grass species or cultivars. These mutualistic fungi grow in the plant tissues and produce toxins that repel or kill chinch bug and other pests. They are considered to be 'mutualistic' because both partners of this association benefit, the grass by being more pest resistant, the fungi by gaining habitat and food. The fungi are carried in the seed. Certain cultivars of fine and tall fescues and ryegrass have been selected to carry endophytic fungi and are in general highly resistant to chinch bug.[C2, E1, E2] Such cultivars are sold locally (e.g. a ryegrass cultivar called Affinity) and are included in some turf mixtures. However, from an ecological perspective there are some downsides to the endophytic cultivars. (See Debunking Endophytes and Endophytic Grasses for Turfs for discussion of this issue.)
The chinch bug does not feed on broadleaf plants (clover, weeds) and lawns with more broadleaf plants (clover, weeds) have much less chinch bug damage.[C4, C7] Numbers of chinch bugs are not necessarily reduced by broadleaves[C4]; rather they may prevent dense aggregations from developing and/or have other effects on chinch bug that restrict damage in spite of high numbers.
Hot dry weather accelerates development of chinch bug and provides optimal conditions for survival and reproduction.[C1] The first signs of damage may occur days earlier or even one to two weeks earlier than usual when late spring and early summer temperatures are higher than normal [C4]. High temperatures also increase water stress on grass, as discussed above. High temperatures and dry weather through mid summer can result in very large infestations.
Warm wet weather increases mortality of chinch bug by stimulating growth of Beauvaria bassiana, a naturally occurring fungus that is pathogenic to chinch bug.[C1]
Large water droplets can physically damage nymphs, which "drown in great numbers during spring rains".[C3] Protective hairs on adults protect them from such damage.[C2]
Cold, dry winters increase mortality of overwintering adults through desiccation effects. [C3]
Pesticides and fungicides, besides killing target pests and pathogenic fungi, kill insects and fungi that are natural enemies of chinch bug and the insects and fungi involved in the breakdown of thatch. The result: more habitat and fewer natural enemies for the chinch bug!
Many insects (e.g. ground beetles, big-eyed bug, lacewings, ladybird beetles), spiders, and birds (e.g. robins, song sparrows) prey on chinch bug. It is also attacked by predatory mites and at least one parasitic wasp (Eumicrosoma benefica). [C1]
A naturally occurring fungus, Beauvaria bassiana, attacks chinch bug and can cause large mortalities under warm, high moisture conditions.[C1] It leaves the bodies covered with white mycelium and spores. (See photograph at the top of this page)
In a quantitative study of natural mortality in chinch bug conducted in New Jersey, Beauvaria bassiana (fungus), Eumicrosoma benefica (parasitic wasp) and Alara (a predatory beetle) were identified as important biotic agents of mortality. Other factors identified were desiccation, failure to hatch and wet conditions at eclosion (the final molting which produces an adult). [C1]
The big eyed bug, Geocaris bullatus is often cited as a major predator, but usually peaks after serious damage has occurred.[C2]
It takes decades to centuries to build up high levels of humus in the soils of natural grasslands and forests. It takes much less time to lose humus when these systems are converted to agriculture, unless special care is taken to prevent erosion and to return a high level of organic matter to the soil.
Humus and Organic Matter
What's the difference?
Humus is a stabilized, colloidal form of organic matter that coats soil particles. It is a chemically complex material that is formed during the decomposition of plants and soil organisms in a soil environment. Humus breaks down very slowly, and is an important reservoir of global carbon.
Only a portion of decomposing organic matter ends up as humus. Most of it breaks down rapidly (compared to humus), releasing nutrients in a form that can be taken up by plants.
Standard soil tests report the content of organic matter in soils, rather than humus per se. Typically about 80% of the organic matter is humus and the terms 'humus' and 'soil organic matter' are used interchangeably. The other 20% is made up of living organisms and the partially decomposed remains of plants and animals.
Organic matter added from manure, yard wastes and immature compost contains much less humic material than native soil organic matter, and breaks down more quickly than native soil organic matter.
Hence, in general, a certain level of organic matter in recently manufactured topsoil, e.g. 3%, is not as stable and long lasting as the same level in a field soil. How much difference there is depends on the nature of the added organic matter (Section VI).
In the past, humus rich topsoils were stripped from rural meadows and sold in urban regions for construction of lawns and gardens. Such soils can sustain a very healthy turf without any inputs (with the possible exception of lime), as long as the clippings are recycled in place.
Today, however, the stripping of topsoils has largely ceased because of concerns about the loss of topsoil for agricultural production. In their place, we use mostly 'manufactured topsoils'. Commonly these soils are prepared by mixing sand or sandy loam (e.g. from an excavated subsoil, or gravel sievings), organic matter, lime and fertilizer. The organic matter is typically manure or partially composted yardwastes or, more recently, compost produced in municipal composting systems such as we have in Halifax.
Currently, the organic matter content of standard grade manufactured topsoils in Halifax Regional Municipality is mostly in the 1 to 3% range as delivered.[L6, L7, L8] However, except when well matured and cured compost is used, the added organic matter is much less stable than native humus, and as much as half or more of this organic matter may be lost within a year,[L6] leaving newly established lawn topsoils with only 1 to 2% organic matter or even less. For comparison older lawn soils in the Halifax Regional Municipality can have soil organic matter values up to 6 to 7%.[L6, L8]
Low organic matter soils have low water-holding capacity
In relation to chinch bug , the main consequence of low organic matter in a topsoil is low water-holding capacity. A soil with 1% organic matter has about one-half the water holding capacity of a soil with 3% organic matter. [V7]
In addition, the sandy texture of most manufactured top soils makes them naturally prone to drying out, especially if well drained, and often only 10 cm or less of manufactured topsoil, after settling, are laid down when lawns are constructed. (Landscape Nova Scotia recommends 20 cm for most purposes.[L9]) A 10 cm soil with 1% organic matter will hold 1/4 of the amount of water held by a 20 cm soil with 3% organic matter!
Depth and Organic Content of Toposil Make a Difference
During droughty periods, a topsoil of 10 cm depth with 1% organic matter will require watering 4 times more frequently than a toposil of 20 cm depth and 3% organic matter.
There can be other types of soil problems in newly constructed turfs, for example due to compaction of the topsoil or soil base by heavy equipment, or the on-site, native soils being too heavy and poorly draining. Topsoil suppliers can usually provide different sorts of soil blends on request, and many contractors do prepare lawns with a good depth and quality of soil without being asked.
Soil Biodiversity
About 2 to 5% of soil organic matter consists of living organisms. So another consequence of low soil organic matter is a low level of soil organisms and soil biological activity. Soil organisms - bacteria, fungi, protozoa, nematodes, insects, mites, earthworms and many more groups - perform a wide range of important services, amongst them: [V10]
breaking down thatch and other organic matter to create humus and recycle nutrients;
maintaining a system of pores in the soil which facilitates aeration and movement of water into the soil;
keeping other organisms in check (so that they do not become pests or agents of disease).
Besides the total quantity of organisms, their "biodiversity" is important, i.e. exactly what groups and species are there and their numbers. As native soils are turned over and cultivated, levels of organic matter and the diversity and quantity of soil biota decline.[V2, V5, V10] Pesticides and excessive fertilization do additional damage to the soil fauna.[L3, L4, V10 ]
There appear to be no studies on biodiversity in recently manufactured topsoils. Based on studies in other systems of varying age and organic matter content,[V2, V5, V10] it's a pretty good guess that their biodiversity is very low. That could well be a factor in the readiness with which chinch bugseem to multiply in lawns established on these types of soils. For example, Beauvaria bassiana, the fungus that attacks chinch bug is probably absent or present in very small numbers in recently manufactured topsoils.[V5]
Thatch: See Turf 101: Thatch
(www-p.afsv.af.mil/Golf/MaintenanceDocs/TurfandThatch.doc) PURDUE TURFGRASS PROGRAM Turf Tips, TURF TIP 5/14/03. Z Reicher, Auth. (2003, May 14. Viewed 16 May 2007)
A photograph on this page shows the thatch layer being measured. Note that the thatch layer is the dark brown, peaty layer, NOT the light brown, dead grass on the surface of the soil.
Pest Alert Sept. 1, 1999, South Dakota Tate University, Spons. (Viewed 16 May 2007) Under DISEASES: PATCH DISEASE INJURY IN LAWNS, there is a brief discussion of thatch; the links (thatch buildupandmore than 1" in depth bring up excellent illustrations of thatchy turf.
Big eye bug versus chinch bug: See General Turf Insects
(http://www.entm.purdue.edu/Entomology/research/cs/notes295N1/Lecture_links/Lecture24_Turf2_files/frame.htm")The Ohio State University, Spons.; D.J. Shetlar, Auth. (Viewed 16 May 2007) View slide no. 23 Predacious vs Pest Bug. It's important to distinguish between the big-eye bug, a natural enemy of chinch bug, and the chinch bug itself. Superficially they may look similar, but they have quite different appearances if you view them with a magnifying glass.
About Water
Sustainable Turf Manual (http://www.nbhta.ca/english/sustainable_turf_manual.html) The Landscape New Brunswick Horticultural Trades Association, Spons.
Jack Wetmore and Ken Browne, Auths. (Viewed 16 May 2007). Individual chapters are available online as PDF documents. Chapter 6: Maintaining Turf contains a lot of practical information on water stress and the ins and outs of efficient use of water on lawns.
About Thatch
Thatch: A Hidden Lawn Concern (PDF document) (www.oznet.ksu.edu/library/hort2/MF2131.PDF)
Kansas State Research and Extension, Spons,, M. Fagerness, Auth. (2001, September. Viewed 16 May 2007) Causes, effects and treatment of thatch.
About Soil Biology, Pesticides and Biodiversity
Soil Quality Publications (http://soils.usda.gov/sqi/publications/publications.html)
US Dept. Agriculture Natural Resource Conservation Service, Spons. (Viewed 16 May 2007)
This page provides access to a large number of documents dealing with soil quality.
Pesticides and Biodiversity (http://dp.biology.dal.ca/reports/zoliver/oliverst.html) J Oliver, Auth.
(2001. Viewed 16 May 2007) An overview of how pesticides affect biodiversity..
About Turfs Soils and Sites
Know your soil(http:www.landscapemanagement.net/landscape/article/articleDetail.jsp?id=3338) Landscape Management, Spons. J. Fech, Auth. (2001, November 1. Viewed 16 May 2007) The subtitle for this article written for landscapers is 'Realizing the importance of healthy soil can help you diagnose and treat its problems'. Soil profile and compaction problems on construction sites are discussed.
Purchasing Topsoil (http://www.ladybug.uconn.edu/PUrchasingTopsoil.htm) University of Connecticut Home and Garden Education Center, Spons. D.Pettinelli and H.D. Luce, Auths. (Viewed 16 May 2007) What to look for when purchasing topsoils.
