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COPING WITH
SPIDER MITES Author’s Update & Warnings:
April 10, 2008 Spider mites thrive in warm or hot, dry
weather. They survive over the
winter on a number of plants, including many evergreens. Carbamates
such as DDT and Sevin cause an explosion in
fertility of spider mites. Use
any Carbamate and you will have spider mites all
over. Some of the pyrethrium based insecticides also increase the activity
of mites -- even Talstar, which was originally
considered to be a miticide! If a
plant gets a serious case of spider mites, the best control is removing the
plant, placing it in a plastic bag, closing the bag tightly, and sending it
to a landfill far away. For some reason, the "green
interest" lobby has classified certain methods of control (such as
horticultural oil) as acceptable and other methods (such as chemicals that
target only specific pests) as anti-green. If you plan to use any of these
products, rotate the chemical class.
Mites quickly develop immunity to any product that one uses (except
horticultural oil, which kills by smothering). However, over use of horticultural oil
can cause fungus problems that can be as bad as mite problems.
Dahlia growers from 50 or 60 years ago
would be astonished to hear our concerns with mites, because these tiny
creatures were not a serious problem until after World War II. Interestingly,
DDT created the mite problem two ways. First, DDT killed many mite predators.
Moreover, DDT vastly increased the fertility of mites -- by greating increasing the number of eggs each female mite
would lay and by substantially enhancing the percentage of mite eggs that
hatched. A widely used miticide in the More recently, many dahlia growers use
synthetic pyrethroids, a chemical class that
emerged in the late 1970s. The most highly regarded synthetic pyrethroid in the Compared to insects, mites reproduce far
more quickly and mutate more rapidly. The process of mutation creates new
variants resistant to any chemical used repeatedly on mites. Anyone using
chemical control must therefore rotate chemical classes frequently to prevent
the mites from becoming resistant. A general rule is not to use any specific
miticide more than twice per season. Horticultural oil helps control mites by
smothering mite eggs and the early larval stages, and the pests do not build
up an immunity to suffication.
However, one should only use the oil two or three times per season. Repeated
use of horticultural oil destroys the natural oils of dahlias and leads to a
breakout of serious fungus problems. Mites have 4 stages: egg, larvae, nymph,
and adult. For the 2-spotted red spider mite, the most common dahlia
predator, the cycle is 4 days from egg to larvae; 4 days from larvae to
nymph; and 6 days from nymph to adult. To attack 2-spotted red spider mites
with oil, one would need to spray 2 c.c. of oil per liter of water and repeat
in 8 days and then 6 days later. (Figure 5 c.c. per teaspoon and about 3.8
liters per gallon of water.) Because of the potential for fungus problems,
one should not use the oil again the same growing season. Because oil does
not kill adults, one should combine the first spraying with an adult
miticide, such as Avid, Pylon, or Floramite. In
theory, this schedule should control an existing invasion until new mites
migrated from some other nearby crop. Selecting the proper product also requires
ability to diagnose a mite invasion. The typical sign of 2-spotted red spider
mites is yellowing of the most mature leaves, lowest on the plant. The back
of the leaves shows a red webbing. While other
conditions can turn old leaves yellow, normally the leaves turn yellow all
over. With spider mites, the yellowing is uneven but over large portions of
the leaves. If in doubt, have someone experienced come and show you the
signs. Cyclamen mites, which are a fraction the
size of 2-spotted red spider mites, are entirely different. With cyclamen
mites, the newest foliage looks like spinach. While chemical damage can give
the same symptoms, when you see a spinach effect, suspect cyclamen mites. Because mites stay only on the backs of
leaves, control requires thorough coverage of the backs of all leaves.
Because of the difficulty of spraying under all leaves, the most popular
miticide in BASF's Abamectin, sold in many areas
under the trade name Avid. Growers appreciate Avid, because it is partially
systemic. On young foliage, Avid penetrates and is effective through the
leaf. This chemical, which is synthesized from a natural bacterium in the
soil, cannot penetrate mature foliage or move from one leaf to another.
Unfortunately, because of over use, many mites are developing resistance to Abamectin (Avid). Avid has been most effective against
broad mites or cyclamen mites. This experience makes sense, because these
types of mites tend to attack the youngest foliage, and Avid penetrates the
young foliage (and thus kills on the back of the leaves) while it does not
penetrate mature foliage (which the 2-spotted spider mites attack). Because mites have 4 stages, a miticide
can potentially kill mites at any or all of these stages. Some miticides,
such as Avid or Floramite, kill adults. A few
products kill or neutralize eggs (and sometimes make adults sterile even if
they do not kill them). The most effective and longest-lasting control would
kill both eggs, adults, and larvae or nymph stages.
Combining an ovicide (which kills the eggs) with a
product that kills adults greatly enhances the effectiveness of any spraying
program. If the spray program does not control all stages, re-spray on a
schedule that takes into account the timing of the stages in the mite life
cycle. Because mites often lay eggs on the tops of stakes (and the webs are
often visible), when spraying with an ovicide,
spray the tops of the stakes as well as the backs of the foliage. Any article on miticides depreciates
quickly, because manufacturers release new products annually, and because
mites develop resistance rapidly. Because of the expense of the EPA regulatory
process and because the ability of mites to develop resistance quickly means
that effective product lives are short, most miticides are very expensive. In
the National Capital Dahlia Society, some of us get together, purchase a
product together, and then subdivide the product. For a $200 product, with
three purchasers, we can normally have enough miticide for a few years for
less than $70 each. The most effective ovicides
are horticultural oil and Hexygon (from Gowan Chemical). Hexygon has a long
residual (up to 21 days). By interrupting the reproduction cycle, Hexygon used with an effective adult miticide, tends to
be very effective if used at the very first sign of any mite activity. Since Hexygon is only effective as a contact killer, one must
cover the backs of all the foliage to use it effectively. As with most
chemicals (other than horticultural oil), a good spreader-sticker enhances
the effectiveness of most miticides. One organic approach is very helpful in
controlling 2-spotted spider mites and other mites that attack the oldest
foliage. Strip off all old leaves, put them in plastic bags, and hide them in
household trash. Never leave old dahlia foliage in the garden. After topping
the dahlia plant, when replacement laterals develop a few sets of leaves,
strip off the foliage from the main stem. After cutting a bloom, remove and
destroy foliage lower on the lateral. If any plant shows extensive mite
attack, rip it out and throw the entire plant into the trash. By the time one
recognizes the common yellowing of foliage common from a mite attack, the
mites have moved up the plant and left nothing but eggs. If one sees evidence
of extensive mite damage, the plant in question is more a breeding factory
for mites than anything else, and it will not produce decent blooms, but it
will spread mites elsewhere in the garden. Careful attention to stripping old
foliage can minimize spread of mites and be more effective than virtually any
chemical miticide. There are several adult miticides, and
numerous others are undergoing EPA testing. Uniroyal introduced a new adult
miticide called Floramite in 2000. This product
seems to be very effective against 2-spotted spider mites and some other
types, but it is not effective against broad mites or cyclamen mites. Floramite claims that it kills some eggs, but it is not
reliable as an ovicide. Pylon, introduced in 2000
or 2001, has the major advantage of being partially systemic. Like Avid, it
penetrates young foliage. Pylon was tremendously effective when first
introduced, but there are now reports of resistance in some areas. Pylon does
not damage either blooms or foliage, and it kills the large mites that often
run all over dark blooms (primarily reds, dark reds, and purples).
Unfortunately, Pylon has a short shelf life, and one must discard it within
three years of purchase. Cinnamite, made from food
grade cinnamaldehyde, is effective but has a short
residual. Cinnamite can damage foliage in hot
weather, so be careful not to use it when high temperatures will be greater
than the high 80s. Cygon 2-E, an organic phosphate,
has been effective against mites. Cygon 2 E is
systemic and long lasting. One may use it as a soil drench early in the
season for broad spectrum control of insects and possibly help with mites.
However, because most mites do not reproduce rapidly until hot, humid
weather, the absence of symptoms early in the season could indicate that the
mites have not gotten started yet rather than any effectiveness of the Cygon 2-E against mites. Several other new products are
under regulatory review, including one or more refinements of Abamectin. A Practical Schedule for Attacking
Mites Early in the season, as long as the
temperatures remain relatively cool, a soil drench with Cygon
2-E is probably sufficient in most areas. However, at the first sign of
mites, one should begin a regular program. Because mites quickly develop
resistance to any chemical, any effective program requires rotating a minimum
of three chemical classes before repeating any chemical class, and discipline
not to use any chemical control more than twice in any growing season. If the first sign of mites is early enough
in the season that high temperatures are no higher than 85 degrees, consider
using horticultural oil plus Cinnamite for the
first treatment. An advantage of these products is that both are inexpensive.
(Cinnamite costs approximately $30.) The
horticultural oil will smother the eggs and early stages while the Cinnamite will kill the adults. Be careful to hit the
backs of all foliage, because neither product is systemic. Use the timetable
above to time a second treatment of the horticultural oil in 8 days and a
third treatment 6 days later. Do not use the horticultural oil again the same
season (because of the potential for fungus damage). Because horticultural
oil kills by smothering rather than by chemical action, the mites cannot
build up resistance to it. An excellent choice for a second chemical
class is to use Hexygon for long lasting control of
eggs plus an adult miticide such as Avid. For 2-spotted red spider mites, the
next two treatments should be Pylon and Floramite
(the order does not matter). In case of cyclamen or broad mites, the choices
are Avid or Pylon. I have normally found a single treatment of Avid to be highly
effective against cyclamen mites. After using a minimum of three different
chemical classes, it is safe to repeat a miticide and use it a second and
final time in a given season. Try to reserve Pylon to use it around show
time, in case of any mites attacking dark colored blooms. Because Pylon will
not damage the blooms and will kill the large mites that can ruin dark
blooms, it is especially useful during show season. However, if you have used
Pylon recently before a show, using it again before rotating two other
chemical classes is an invitation to developing chemical resistance and
losing the benefit of one of the most effective miticides we have ever had. |