By Raymond A. Cloyd

Insecticidal soaps may provide control of a variety of insect and mite pests of roses including aphids, thrips, scales and the twospotted spider mite (Tetranychus urticae). A soap is a substance derived from the synthesis of an alkali such as sodium (hard soap) or potassium (soft soap) hydroxide on a fat. Fats are generally a blend of particular fatty acid chain lengths. Soap is a general term for the salts of fatty acids. Fatty acids are the primary components of the fats and oils present in plants and animals.

Soaps may be combined with fish, whale, vegetable, coconut, corn, linseed or soybean oil. For example, “Green Soap” is a potassium/coconut oil soap that was used widely as a liquid hand soap in public restrooms. It is currently available as a hand soap, shampoo and/or treatment for skin disorders. However, it has also been shown to be effective, as an unlabeled insecticide, in controlling soft-bodied insects including aphids.

Soft-bodied pests such as aphids, scale crawlers, thrips, whiteflies and mites including the twospotted spider mite are, in general, susceptible to soap applications. Soaps typically have minimal activity on beetles and other hard-bodied insects due to the insect’s thickened cuticle, which is more resistant to penetration. However, this is not always the case since soaps have been shown to kill hard-bodied insects such as cockroaches. Soaps are effective only when insect or mite pests come into direct contact with the wet spray. Dried residues on plant surfaces have minimal activity on insect or mite pests because soap residues degrade rapidly — especially under sunlight. Insecticidal soaps may also be harmful to natural enemies including parasitoids and predators. For example, ladybird beetles and green lacewing larvae, are killed by wet sprays when present on treated plants.

The mode of action of soaps is still not well-understood although there are four ways by which soaps may kill insect and mite pests. First, soaps may penetrate through the fatty acids present in the insect’s outer covering (cuticle) thus dissolving or disrupting cell membranes. This impairs cell integrity causing cells to leak and collapse, destroying respiratory functions, and resulting in dehydration and death of the insect or mite pests. Second, soaps may act as insect growth regulators interfering with cellular metabolism and the production of growth hormones during metamorphosis (equals change in form). Third, soaps may block the spiracles (breathing pores), interfering with respiration. Fourth, soaps may uncouple oxidative phosphorylation or inhibit the production of adenosine tri-phosphate (ATP), which reduces energy production.

There are a variety of fatty acids; however, only certain fatty acids have insecticidal properties. This is solely based on the length of the carbon-based fatty acid chains. Most soaps with insect and mite pest activity are composed of long chain fatty acids (10 or 18-carbon chains) whereas shorter chain fatty acids (9-carbon chains or less) have herbicidal properties, so using materials that have short chain fatty acids can kill rose plants. For example, oleic acid, an 18-chain carbon fatty acid, which is present in olive oil and other vegetable oils, is very effective as an insecticidal soap. In fact, most commercially available insecticidal soaps contain potassium oleate (potassium salt of oleic acid).

There is a misconception that any soap or detergent can be used as a pesticide (insecticide or miticide). Although, as already discussed, only a few select soaps have insecticidal or miticidal properties; many common household soaps and detergents including Palmolive, Dawn, Ivory, Joy, Tide and Dove, which are unlabeled pesticides, may have activity on some soft-bodied insect and mite pests when applied to plants as a one percent or two percent aqueous solution including the sweet potato whitefly (Bemisia tabaci), green peach aphid (Myzus persicae), cabbage aphid (Brevicoryne brassicae), mites, psyllids and thrips. However, reliability is less predictable than soaps formulated as pesticides.

Despite this, dishwashing liquids and laundry detergents are primarily designed to dissolve grease from dishes and clean clothes; not kill insect and mite pests. These materials may cause plant injury (phytotoxicity) by dissolving the waxy cuticle on leaf surfaces. Although the leaves of roses tend to have a thickened cuticle and the flowers are waxy there is still a risk of phytotoxicity. Registered, commercially available insecticidal soaps are less likely to dissolve plant waxes compared to household cleaning products. Dishwashing liquids and laundry detergents, like insecticidal soaps, lack any residual activity and thus more frequent applications are needed. However, too many applications may damage the leaves or flowers of roses. In addition, detergents are chemically different from soaps. In fact, many hand soaps are not necessarily pure fatty acids. Most importantly, these solutions are not registered insecticides or miticides. Soap companies don’t intend for their products to be used against insect or mite pests because they have not gone through the Environmental Protection Agency (EPA) registration process.

The type of fatty acid, length of the carbon-based fatty acid chain, and concentration in many laundry and dish soaps is not known. In addition, the insecticidal effectiveness of these products may be compromised by the presence of coloring agents or perfumes. This often times leads to inconsistent results. Certain laundry and dish soaps will precipitate in hard water thus reducing their effectiveness.

Despite the activity of some dishwashing liquids and laundry soaps on insect and mite pests, their use should be avoided on roses primarily because they are not registered pesticides; they don’t have an EPA Registration Number. Even more important is that a pesticide company will generally stand behind a product when there is a problem. If a dish or laundry soap is used and roses are injured — there is no recourse.

If anyone has questions or comments regarding this article they may contact the author via email (rcloyd[at]ksu[dot]edu) or phone (785-532-4750).

Author:

Raymond A. Cloyd

Professor and Extension Specialist in Horticultural Entomology/Integrated Pest

Management

Department of Entomology

Kansas State University

123 Waters Hall

Manhattan, KS 66506-4004

The mysterious ‘rose replant disease’ or ‘rose sickness’ has puzzled rose growers for years. Nothing specific has been identified as the cause of this phenomenon. When new roses are planted where old roses used to be, they may not grow as well as they would if they were planted in soil never planted with roses. Many suspect that key nutrients may be depleted in soils where roses have been grown for a long time, and as a result, the new roses do not get all the nutrition they need. Additionally, where roses have been grown for years without attention to good soil health, the soil’s structure and general physical properties may have declined — due to compaction and reduced organic matter. The soil may not effectively deliver oxygen, nutrition and water to the roots to support the vigorous growth of new roses.

We are fortunate that Traud Winkelmann and her colleagues (Bunlong Yim and Kornelia Smalla) are working to better understand rose replant disease. Winkelmann spoke on their research at the International Symposium on Rose Research and Cultivation this past August (RHA members Jim Sproul, David Byrne and I attended). Her talk was titled “Rose Replant Problems — Detection by bio-tests and investigation of bulk soil and rhizosphere microbial communities using DGGE fingerprints.”

Winkelmann described the challenge commercial nurseries face when growing roses year after year on the same land, and notes that reduced growth in rose crops on that land is typical over time. Many commercial German nurseries proactively address this problem by rotating rose crops with other crops on their land. For instance, when we toured W. Kordes’ Söhne we learned they rent neighboring agricultural land to have enough space to grow their roses with multiple years between rose crops on any given piece of land. The reason for the decline in growth of new roses on land where other roses once grew is still unclear. Winkelmann suggested the decline has been thought to result from nematodes, nutritional deficiencies, toxins and/or microbes.

To better document the cause or causes of rose replant disease, Winkelmann and her colleagues worked with three different German nurseries to acquire soil samples. Soil of different types (sandy loam and others) was collected at a depth of 5- to 20-cm, and the samples were also chosen based on how recently roses had been grown in the soil, and if there had been any other crop grown in the soil since roses were grown there. The soil samples were then each divided into three treatment groups: untreated, heat-treated (one hour at 50°C) and gamma-irradiated. In 3-liter pots containing all these soil variations, seedlings of Rosa corymbifera ‘Laxa’ and apple rootstocks (apples are also in the rose family) were planted and their growth documented. It was hoped that different growth of seedlings across the three soil treatment groups (untreated, heat-treated and irradiated), would shed some light on the cause(s) of reduced growth associated with rose replant disease. Heat can neutralize or weaken some toxins, and both heat and radiation can negatively impact soil organisms.

Across most soil samples, seedlings grew better in the heat-treated or irradiated soil than in the untreated soil. This result supports the hypothesis that an inhibitory factor is present in the soil and may be destroyed or neutralized by these two treatments. Winkelmann and her colleagues conducted denaturing gradient gel electrophoresis (DGGE) on a specific gene (16S rRNA) to begin to understand microbial diversity and how it differed between soils. Winkelmann was excited that, by analyzing all their soil samples and data, it appeared that the crop grown before roses were replanted had a stronger positive impact on rose replant issues than was gained through the heat and irradiation treatments on soils where roses had been grown most recently.

It sounds very promising that crop rotation and careful selection of the right non-rose species as an intercrop can dramatically reduce rose replant issues for commercial nurseries and avoid costly and toxic soil sterilization treatments. Winkelmann and her colleagues plan to continue their work by characterizing microbial species present in the soil and trying to understand how they vary in abundance across rose nursery soils managed in different ways. Learning which microbes are contributing the most to rose replant issues may suggest some ways to decrease rose replant issues.

Crop rotation (using other species between crops of roses) is practiced in Germany by some rose nurseries, and is also used in some of the German rose trials. We toured the German Variety Protection Office in Hannover. There, new roses are compared with established cultivars as part of the documentation necessary for securing European plant breeders rights. That location also hosts one of the Allgemeine Deutsche Rosen — neuheitenprüfung (ADR) trial sites. In the space allocated for the ADR trials, a portion of the field was planted in tall growing marigolds (Tagetes erecta; commonly called Mexican or African marigold) to prepare the soil for future rose plantings.

Hearing Winkelmann’s presentation and observing how rose growers handled rose replant issues at different tour stops helped me to realize how important this issue is. I wonder now to what extent rose replant issues may be affecting the rose seedling evaluations we rose breeders make.

I have very limited space and quickly reclaim land where I had rose seedlings to make room for the new ones. Perhaps some very nice roses are being overlooked due to mediocre performance just because of being planted in a pocket of soil with extra-strong residual effects from roses previously grown in that location.

After returning from the rose research symposium, I looked closely at areas of my garden in light of rose replant challenges and noticed something very interesting. In 2012, I expanded part of my rose garden, taking several feet of sod out to have enough room for the crop of new rose seedlings. This past summer the nicest looking seedlings typically have been in the part of the bed that was most recently in sod. As I look how I planted my family rows, part of each family is in the new bed and part of each family row extends into the older bed that has had roses growing continually since 2008.

Perhaps having the nicest looking rose selections in the newest section of the rosebed is a fluke, but it might also result from rose replant challenges making some of the roses less competitive. Hopefully someday I can solve this challenge by just having more land to rotate rose seedlings and having enough space to grow all I ever hope to, but I’m not sure if and when that dream can come true. In the meantime, I may just be a little more willing to select and grow on those seedlings which are better than average and are in pockets of the garden where plants seem to be struggling a bit more.

Field of marigolds adjacent to the current ADR rose trials in Hannover that will be planted in roses again in the future.

David Zlesak (zlesak[at]rocketmail[dot]com), ‘Advances in Understanding Rose Replant Disease’, Winter 2014. Buckeye Rose Bulletin, Mark Miller (tmille3[at]columbus[dor]rr[dot]com), ed. Buckeye District Rose Society. Reprinted from the Fall 2013 Rose Hybridizer Association Newsletter.