ADS Bulletin revised July 13, 1999
Summer Pesticide Guidelines
by Alan A. Fisher
With the warm weather come the pests. The dahlia pests come in several categories: mollusk (slugs), insects (numerous), and mites (spider family). Mammal pests (deer, ground hogs, and rabbits) attack some of our gardens. One must have an appropriate solution for each. The products that virtually eliminate insect problems for dahlias (and other flowering plants) are expensive per package but will last more than one growing season if one uses a hand-held sprayer. (My sprayer holds 2 gallons.) With a hose-end sprayer, expect to go through enormous amounts of expensive products. While one can purchase an inexpensive sprayer from Wal-Mart, Home Depot, or similar discounter, a serious gardener would probably prefer a top of the line sprayer (approximately $50) from a nursery supplier like E.C. Geiger.
Pesticide recommendations require frequent updating, because insects and mites quickly develop resistance to virtually any single chemical. To minimize resistance problems, growers must rotate among a few different chemical classes. The EPA has been forcing older chemicals off the market, and each year new chemicals become available.
This article does not discuss organic approaches to pest control, except in passing (such as for rabbits). For growers in some areas with severe winters and comparatively minor insect and mite problems, organic gardening may be a superior approach to chemical pesticides. For those who wish to take advantage of the benefits of modern chemistry, this article should be helpful. Actually, the trend in pest management is to use lessons from organic gardening to find environmentally friendly products for pest control. For example, several of the recommended chemical classes are synthesized from natural products. Organic gardeners identify a natural substance that helps control pests and chemists modify the natural product to improve its performance. In many instances, these natural-based chemical classes are among the safest pesticides for humans.
When using any insecticides, herbicides, or miticides, avoid any unnecessary contact with the chemicals. While this article only discusses chemicals with low human toxicity, no one can predict the effects of long-term exposure or possible synergistic effects. (Related chemicals may build up in the body and reinforce the effects on each other.) Wear a long-sleeve shirt, long pants, and glasses. Ideally also wear a face mask. Tie plastic bags over shoes. Wear disposable latex (plastic) gloves. After spraying, rinse all equipment carefully and wash off chemical bottles. Run clear water or water plus tank cleaning solution (approximately $3.40 per quart) through the sprayer. Then immediately throw away the plastic, wash all clothes in hot water with detergent, then run the clothes through a hot dryer. Bathe and wash your hair immediately before eating or even using the toilet.
Instructions for Each Type of Pest
Use slug pellets every two weeks both inside the dahlia plots (especially near the edges) and in a 2 to 3 foot swath around the perimeters to keep the slugs out. If not, slugs can kill young dahlia plants and destroy foliage later in the season. It is easier to keep slugs from ever entering the dahlia patch than to eliminate them once they gain control.
Insect pests are of two types: in-ground and above-ground. For insects in the soil, use Diazinon or Dursban granules approximately once a month. These products have low human toxicity and are quite effective against a wide range of insects once watered into the soil. For insects on the dahlias above ground, there are two types of products: systemics (which absorb into the plant material) and contacts (which kill only when the insects come into direct contact). The best systemic for dahlias is Marathon, a synthetic nicotine that costs approximately $110 for a 5-pound jar. However, a 5 pound jar goes a long way, because one uses from a quarter teaspoon (for a 4-inch pot) to 3/4 teaspoon (for an average-sized plant in the garden around July 1). Dress fully and wear plastic gloves to avoid any contact with the product, which absorbs rapidly through the skin. (Marathon has very low human toxicity, but why ingest it unnecessarily?) Apply with a teaspoon measuring spoon to the soil above the feeder roots (under the mulch after mulching), replace the mulch, and water gently for 5 days to work the product into contact with the roots. One treatment lasts for 8 to 12 weeks and kills most insect pests. One bottle used annually around July 1 treats 500 or more dahlia plants.
While Marathon will keep insects under substantial control, it alone will not eliminate insect damage to foliage. Until recently, one could obtain virtually complete control by supplementing Marathon with a combination of a pyrethroid with Piperonyl butoxide (PBO, a synergist for pyrethroid insecticides that prevents the insects from building up an immunity). Unfortunately, the EPA has forced PBO concentrate off the market. (The cost of meeting updated EPA requirements probably exceeded what the manufacturer could afford given the size of the market.) Some insects have built up a tolerance to the pyrethroid class of pesticides, so prudent specialists will probably supplement pyrethroids with a second chemical class for more successful results. In short, the EPA has probably forced growers to use more chemicals, not less, by forcing PBO off the market. (One may be able to find a few bottles until retailers and wholesalers sell out current stocks.)
Talstar FL (flowable liquid) is currently the most effective and easiest to use pyrethroid. Combine Talstar with PBO (if available) or with a second chemical class every two weeks, and the combination should kill virtually all insects that come into contact with it (those that hit the tops of the leaves). With PBO, the insects will not build up an immunity to the pyrethroid. Use 2/5 oz. (just less than 1 tablespoon) of Talstar plus 2 teaspoons of PBO per gallon of water in a sprayer. Talstar costs approximately $110 per quart but has been on sale for a 20 percent discount the past 2 summers. Instead of PBO, one may wish to rotate various organic phosphates to use with the Talstar. The most effective organic phosphate still available is Duraguard (approximately $55 per quart). Duraguard is micro-encapsulated Dursban, and the dilution is 1 tablespoon per gallon. Micro-encapsulation puts small quantities of highly concentrated chemical in a solution that binds to the plants and hairs of the insects. The encapsulated chemicals then release. Because of the full concentration, very small amounts are sufficient to kill the pests. For insects (such as earwigs) that hide under enclosures, microencapsulated formulations are the most effective controls. Unfortunately, the EPA has forced Knox-Out, microencapsulated Diazinon, into a restricted class limited to licenced applicators (because excess use was polluting streams).
Other common organic phosphates include the readily available Orthene (also sold as Isotox) and Cygon 2E, all systemics. Orthene is now available in granules as a soil drench to last for a month (Pinpoint, about $40 for 8 pounds; use 0.75 pounds per 1000 sq. ft.). Experts warn that Orthene/Isotox can burn foliage if used more frequently than once a month. Cygon 2E, which is most effective in cool weather, is also effective against several types of mites. Rotating among organic phosphates does nothing to prevent insects (and mites) from building up an immunity to all of them. However, mixing an organic phosphate with Talstar at least provides a double punch for insects that are building up resistance to either type of chemical.
Sevin, a carbamate (related to DDT), is a broad spectrum contact pesticide with very low human toxicity, but it seems to make mite problems worse. For that reason, experts tend not to recommend using Sevin on dahlias.
Dow AgroSciences has recently released Conserve, the first product in a new chemical class, spinosyn. Conserve is derived from the fermentation of a naturally occurring organism and offers excellent control of leafminers, thrips, cutworms, and many other caterpillars and worm-type larvae. At higher doses, it is also effective against spruce spider mites and two-spotted spider mites. While Conserve has some leaf penetrating ability, it is not highly systemic. Expect a residual of up to two weeks. Conserve sells for around $90 to $95 per quart, but there are often substantial discounts for 2 to 4 bottles ordered at one time. Concentration is 1.2 teaspoons per gallon at the highest rate (to control leafminers and mites). Since Conserve shows no phytotoxicity on ornamentals, it is a good candidate for trying on blooms (half a teaspoon per gallon, a rate that will control thrips). (The literature does not mention using Conserve on blooms, and there is no experience yet for this new product.) Conserve stops target pests from feeding within minutes (but takes a few days to show substantial kill) from either direct contact or from pests ingesting the material by chewing or sucking the foliage or bloom. It is also one of the safest of all pesticides based on a wide range of toxicity studies. If new products in the spinosyn chemical class broaden the scope (add more insects to the kill list), then it will be a major advance in the war to control pests with natural products that are as safe as possible to humans and pets. Conserve is an excellent choice to use in rotation with Talstar and Duraguard to lessen the likelihood of insects building up resistance.
Mite Control
While insect resistance is bad enough, spider mites (most much smaller than most insects) tend to breed even faster and develop immunity to miticides even more quickly than do insects. Mites come every year with the hot, dry weather, and they can kill a large dahlia plant in a few days.
If one took a poll of serious dahlia growers, spider mites would probably win easily as the worst menace. Many growers in various parts of the country reported resistance to Avid (the only broad spectrum systemic miticide) during 1998. The new hope, introduced in 1998, was Sanmite, a $200 plus product packaged for 100 gallon tanks. Unfortunately, field tests in Florida reportedly are already demonstrating resistance to Sanmite among several types of mites.
More than any other pest, mite control demands rotating weapons. One form of control is readily available to anyone. Once typical spider mite symptoms (yellowing or blanching of leaves, especially older leaves) appear, especially when there are microscopic black dots and threads on the backs of leaves with these symptoms, assume a mite invasion. Strip off and throw away any leaves with these symptoms. If an entire plant is affected, rip it out and throw it away. Once the attack is this far advanced, drastic action is often necessary to prevent the mites from taking over the entire dahlia plot. Throw any stripped leaves or destroyed plants into a plastic trash bag, seal securely, and throw the bag into a secure trash can. Never compost any dahlia material that may have mite and insect eggs!
Experts recommend using miticides with ovicidal action (products that kill mite eggs and usually larvae) to interrupt the life cycle, treating immediately at the first sign of problems, and not using any product more than once a season. This advice quickly becomes expensive! The mite ovicides include Cinnamite, Hexygon, and Joust.
Hexygon and Sanmite both requite the use of a metric scale, because they are powders packaged for 100 gallon tanks. For Hexygon, mix 0.5 grams per gallon for 30 to 40 days of control. In the absence of a metric scale, pour the powder into a plastic container and measure the total volume in tablespoons. Hexygon, which costs about $175 for a 6 ounce package, weighs 27 grams per ounce. Figure 3 teaspoons per tablespoon and divide out to convert from grams to teaspoons. Round up to the nearest quarter teaspoon. Since the volume of Hexygon depends on humidity, each person should figure the correct dosage individually. Wear a dust mask while measuring, and place the Hexygon into an airtight container. In an airtight container, the shelf life is several years as long as one keeps it from freezing or from reaching 100 degrees F. (This same method should also work for Sanmite, except the methodology would be to divide the total powder by 100 to obtain the quantity to use per gallon of mixture.) Hexygon has by far the longest residual of any mite ovidice, at least 30 days.
Hexygon (approximately $175 for 6 ounces of powder) kills mite eggs and larvae but not adults. However, mature females lay sterile eggs after treatment, so the product does interrupt the life cycle. If adults are present, mix with Avid or Sanmite for quicker action.
For Hexygon, Sanmite, and all other miticides except Avid, thorough coverage of the backs of all leaves is essential, because these products are contact only (not systemic), and because mites stay on the backs of the leaves. For all miticides, half a teaspoon of Stirrup-M (about $85) per gallon helps attract the mites to the miticide and make the product more effective. For Hexygon, add a spreader (if unavailable, use a teaspoon of dish detergent per 2 gallon spray mixture) and avoid overhead irrigation for several days after applying. Use Hexygon only once per growing season to avoid resistance.
Cinnamite, a derivative of cinnamon, is another effective miticide with ovicidal effect. This product also kills aphids and possibly other insects. While it claims effect on powdery mildew, one should expect other products to be more effective for mildew. To kill mites, use 0.85 oz. of Cinnamite per gallon of mixture. Because of phytotoxicity problems with some plants at that concentration, use Cinnamite only after the plants are well adjusted to their environment, try to use it during a period of comparatively cool or cloudy weather, and add some half strength liquid fertilizer to give the dahlias a boost at the same time. Because Cinnamite does not work well in alkaline water, buffer to lower the pH below 7.5. (For example, add some Domboro ear drops or vinegar to lower the pH.) Mix well and frequently when applying Cinnamite. Thorough coverage, especially of the backs of the leaves, is essential for control. While Cinnamite has "little" residual effect (duration of kill from contact over time), by killing all stages of mites, it interrupts the life cycle and provides control for a longer period than the duration of its direct impact.
While the manufacturer states that one may use Cinnamite for two consecutive applications before rotating chemical classes, it would be more prudent to use Cinnamite only once before rotating to two different chemical classes before using it again. An advantage is that a 21.25 oz. bottle costs only approximately $30.
Three organic hydrocarbons are common miticides, and one of them, Joust, kills all stages of mites for 5 to 7 days. Since Joust is not systemic, thorough coverage of the backs of the leaves is necessary. Since the product is several years old, some mites are probably resistant to this product. The dilution is half a teaspoon per gallon, and the residual should be up to 30 days. Kelthane, a widely available organic hydrocarbon miticide, has a long residual, but some mites had been resistant to it. However, Kelthane had been unavailable for a few years. Recent studies are showing Kelthane to be quite effective in areas where it had not been used for several years. Again, thorough coverage of the backs of the leaves is necessary for any control. Cygon 2E, a broad spectrum insecticide, is effective against some mites and is also systemic. Use 3 teaspoons per gallon with a spreader for mite control.
Sanmite, introduced in 1998, is a contact killer of adult mites but not of mite eggs or larvae. If a mite invasion has progressed to the point where several leaves show visible damage, an adult miticide is necessary to supplement any ovicide, and Sanmite could be useful. Because of the danger of developing resistance, BASF (the manufacturer) warns to rotate at least two different miticides between each successive use of Sanmite and not to use it more frequently than once a month. Sanmite is expensive ($200 plus) and inconveniently packaged (4 packets of an ounce each, and one uses 1/50 ounce per 2 gallon spray can). In short, each package contains enough miticide for 200 refills of a 2 gallon sprayer, but one must find a way to subdivide an ounce of powder into 50 parts. (See the suggestion above for Hexygon and use the same procedure. Measure the total volume, divide by 100, round up to the nearest quarter teaspoon, and the result will be the appropriate amount to use per gallon.) Because studies are already showing some mite resistance to Sanmite, it may not be the best product currently available. However, four growers in a dahlia society could order a package and each take an ounce for approximately $50. Since one ounce is sufficient for 100 gallons, a $50 supply should last most growers for years. When dahlia tubers often cost $5 to $25 each, a package of Sanmite that saves four or five plants is a good investment.
Talstar, an advanced generation pyrethroid and leading insecticide, is effective on some mites. Piperonyl butoxide (PBO, a synergist for pyrethroid insecticides that prevents the insects from building up an immunity) reportedly enhances the effect of Talstar on mites as well as on insects. While Talstar and PBO alone are insufficient to control mites, they give another chemical class to add to the weapons. Talstar costs about $110 per quart, but PBO is difficult to find because it no longer has EPA registration for direct sale to the public (although it will still be available for manufacturers to use in formulating their products).
Any discussion of miticides must include Avid, for years the most effective miticide, largely because it is the only systemic product. Avid is systemic on new foliage but not on older leaves, the place where mite invasions typically start. In addition to the problem of some mites now showing resistance, Avid has the disadvantage of now being packaged only in quarts (rather than 8 ounces as in the past). A quart of Avid costs at least $225. The way to obtain it is to put together a group of four people, at least three of whom have 8 ounce bottles. By subdividing, the cost per member becomes reasonable. Use half a teaspoon per gallon to attack both mites and leafminers. Because Avid is only systemic on new foliage, carefully spray the backs of older leaves to control mites. Avid seems most effective if repeated after approximately 10 days.
Protecting Blooms
What about blooms? Systemics do not absorb into the blooms, and most of these chemicals are too harsh for use on blooms. The best products to use on blooms are Talstar (without PBO!!) or Mavrik. Use a quarter teaspoon of Talstar or a quarter teaspoon of Mavrik per gallon of water every 5 days, late in the day (the best time to spray for insects and mites). A very fine mist in this concentration will kill a wide variety of pests that attack blooms and will not mark as long as the spray dries before the sun hits it. Since Mavrik is no longer available in the 8 ounce size, and since Talstar is generally more effective on foliage, Talstar is the product of choice for blooms.
New product information on Conserve reports no phytotoxicity on ornamentals. For this reason, Conserve is a good bet to try directly on blooms. To control thrips, try half a teaspoon per gallon every 7 days. Try Conserve on a few blooms and evaluate both its effectiveness and any phytotoxicity (damage to the blooms) before using it generally on blooms. Conserve may gives dahlia growers another option for effective control of insects on blooms. The late Jack Almand used Orthene or Isotox at half strength for the same purpose.
Regardless of which product, the technique is to add a quarter teaspoon of Talstar per gallon to my sprayer, or half a teaspoon of Conserve per gallon, or Orthene or Isotox at half strength, spray all blooms and buds that show color, then return, refill the sprayer with water, add the other chemicals, add a liquid fertilizer at half strength, and then spray the foliage. As with any other spray program, rotating product to spray on the blooms lessens the likelihood of insects building up an immunity. One should spray blooms every 5 days (for Talstar) to 7 days (for Conserve) but the foliage approximately every 10 days (depending on products used and their residuals).
For specific control of cucumber beetles, Adios is a wettable powder of a combination of a powerful beetle-specific attractant with micro-encapsulated Sevin. While the manufacturer had to discontinue Adios, because the market was too thin to support continuing the regulatory registrations, some stock is still available. Adios has very low human toxicity, because the beetle attractant is not harmful, and Sevin is one of the safest insecticides for humans (especially in the minute quantities needed with micro encapsulation). The problem is that this product is only available in cases of 4 bottles each, 5 pounds per bottle, for $160 per bottle. For most dahlia growers, a bottle of Adios would last a lifetime. Anyone interested in Adios should contact Bob Romano of the Pittsburgh Dahlia Society (see judges' list in the ADS Classification Handbook for address and phone).
During 1998, nothing seemed to protect blooms from the spider mites that crawl all over red and purple blooms, although this generalization predates the availability of Cinnamite and Sanmite. The best control for mites is to kill them before they take hold. By spraying at the first sign of mites, stripping off and destroying badly affected foliage and plants, and using ovidicides to interrupt the life cycles, one may have better success at keeping the worst mites from taking over the dahlia garden.
Ones head probably rolls from the investment required to purchase all these products. The cost of basic insect control (Marathon, Talstar, and Duraguard) is about $350, but if one grows 100 dahlias, the Marathon and Talstar will each last two or more seasons. Mite control is becoming much more expensive, especially now that Avid is only available in a quart size and growers are learning that rotating among several chemical classes is essential for successful control. Since none of the miticides is highly systemic, mite control requires careful attention to thorough coverage of the backs of all leaves. Regardless of how many of these chemicals one expects to use, a high quality hand sprayer is essential. Hose end sprayers are inexpensive, but they flood and waste tremendous amounts of the chemical. Moreover, one cannot obtain a fine enough mist from a hose end sprayer to hit the backs of the leaves where spider mites congregate. A high quality 2 gallon sprayer costs approximately $50 and lasts for several years. Normally one can replace parts when needed and avoid having to replace the entire sprayer.
Mammal Pests
Mammal pests require a different approach. Look elsewhere for an expert on deer or ground hogs. Bugs Bunny's relatives, however, consider dahlias a gourmet restaurant, and an organic approach works well for them. To keep rabbits away from dahlias, use a fence of chicken wire or wood (railroad ties, pressure treated wood, or cedar) around the dahlias. The fence must be firm against the ground, because the rabbits go under the fence rather than above it. A fence a foot high will deter baby rabbits, but one needs a higher fence for adults. Home Depot sells rabbit fence in 28 and 36 inch heights. A 28 inch fence is sufficient to convince most types of adult rabbits to patronize other gourmet restaurants. Use short stakes (bamboo rods about 3 feet tall will work) every 2 p feet along the fence to keep rabbit fence or chicken wire in place. Use twine to tie the chicken wire firmly to the rods near the top and bottom. The twine will last all season. No other approach, including Hinder (an expensive rabbit deterrent), hot pepper and other herbs in sprays (they clog virtually any sprayer), and blood meal has worked. Both rabbit fence and chicken wire work perfectly as long as one is careful about avoiding any gaps near the ground.
Fungus and Powdery Mildew
Growers in some parts of the country have more problems than others with fungus. Cleary 3336, a combination product with a broad spectrum of control, is useful as a preventative in a regular spraying cycle. Cleary 3336 also makes a useful soak for tubers when digging in the autumn. A quart of the flowable (liquid) form of this fungicide costs about $55, and one uses a tablespoon per gallon of water. An alternative in a powder form is Consyst, which adds Daconil to Cleary's and costs only $27 for a 2 pound bag.
Our main problem with fungus is powdery mildew, the white powdery substance that starts on old leaves of some dahlia plants, usually from mid August to the end of the season. A second, perhaps more common, type of mildew turns old leaves brown and curls them up. Rubigan EC, which costs about $75 per pint, offers excellent systemic control of these mildew problems. Half a teaspoon of Rubigan EC per gallon added to the regular spray program starting a few weeks before powdery mildew normally hits should completely eliminate mildew problems.
Good luck with the pests. Hopefully this report covered all the important ones.

[Editors note]: Some recent quotes from the dahlia-net listserv

Cucumber beetles normally come out much later in the season than Japanese
Beetles. Here, they get worse as the season progresses, starting after mid
summer. Adios is the specific treatment, but it is not very available (ask
Bob Romano). I use Talstar at half strength on the blooms, but one can also
use Orthene at half strength on the blooms. I believe that Orthene has a
much shorter residual than Talstar, and you might need to reapply after 4 or
5 days. Spray on blooms only very late in the afternoon, right before dusk.
Japanese Beetles and many other insects go for white and yellow colors.
Alan Fisher

Sevin will kill most insects on blooms, but one needs to use a cheesecloth
or similar to float a fine dust with the wind. Too much will mark a bloom,
while neither Talstar nor Orthene (either at half strength) will mark the
bloom. Moreover, Sevin is a Carbamate, related to DDT. Carbamates greatly
increase both the fertility of mites and viability of mite eggs. In short,
Carbamates on dahlias virtually guarantee mite problems. Some pyrithrium
based insecticides may have a similar effect, but perhaps not to the same
extent. Alan Fisher

Re: horticultural oil and mites. Yes, horticultural oil is quite effective
on spider mites, if you hit the backs of the leaves. Oil on the fronts will
have no effect, because the mites are only on the backs. The problem is
that repeated use of oil will give a massive fungus problem by the end of
the season.

NCDS had one of the 3 world-class experts on mites as a speaker at our
meeting last week. He said that one should use horticultural oil at the
rate of 2 cc per liter, repeat in 8 days, and then repeat in 6 days. (I
believe that this information was correct; someone who was there perhaps can
verify from notes.) However, do not repeat the oil again, because
horticultural oil removes natural oils from plants and leaves them
vulnerable to fungus attacks. Rotate methods to treat mites!

When I have an opportunity, I'll try to transcribe my notes and write up the
result for the ADS Bulletin.

Re: Japanese beetles. When they come, they are everywhere. After several
years, they fade as a pest. I strongly recommend Milky Spore. This spore
causes a bacteria that only attacks Japanese Beetles and turns them into
more spore to attack more of their fellow. Milky Spore spreads over a
number of years until it provides a natural barrier to kill the grubs.

Japanese Beetles are very vulnerable to Merit/Marathon. One should use
Merit on the lawn each May. One treatment will kill all Japanese Beetle
grubs in the lawn for the entire season. Without this treatment, the grubs
will eat the roots of the lawn and ruin the lawn. Marathon on July 1 will
protect dahlia plants (but not blooms) from Japanese Beetles until frost.
Apply with a teaspoon measuring spoon under the mulch, water lightly each
day for 5 days, and the dahlia will have broad systemic protection for 3
months. (For more, download my article on pests from www.dahlia.org.)
Systemics do not go to the blooms. I would use Talstar at half strength
each week to protect blooms.

Alan Fisher