Mites, They kill
Aloes, don’t they?
By
Michael J. Green
Well, at least one does, sort of. The
dreaded Aloe mite, aka Aloe gall mite, Aloe wart mite and Aloe cancer mite,
doesn’t exactly “kill” the Aloe, but it does destroy the aesthetics. The
technical name is either Aceria aloinis or Eriophyes aloinis, two names for the
same villain. In addition to aloes, Eriophyes aloinis is reported to infest
Haworthia species. Googling each name will return different sets of
information. About 45,000 species of mites have been
described, but this amounts to only about 5% of the number of species estimated
to be alive today.
Mites have successfully colonized nearly every known marine,
terrestrial, and fresh water habitat including polar and alpine extremes,
tropical lowlands and desert barrens, surface and mineral soils to depths of 10
meters, cold and thermal surface springs and subterranean waters with
temperatures as high as 50C, all types of streams, ponds and lakes, and sea
waters of continental shelves, deep sea trenches to depths of 5000 meters AND
some Aloes. They’re everywhere. Mammals, including us, and birds, are hosts to
innumerable species of parasitic mites (e.g. scabies and mange mites), as are
many reptiles and some amphibians. Many mites have complex symbiotic
relationships with the larger organisms on which they live. Some, like our
“Aloe mite”, are more parasitic. Mites found on agricultural crops are major
economic pests (e.g. spider mites) or useful bio-control agents (e.g.
phytoseiid mites) of those pests.
There are three major groups of mites
that attack cacti and succulent ornamental plants. These are the spider mites,
the false spider or flat mites, and the gall or eriophyid mites. Aloe mites are classified as eriophyid mites.
Specie specific information is difficult to find; thus much of the information
presented is derived from eriophyid mites in general. The eriophyids are a
group of plant-feeding mites that inject a plant growth regulator
similar to the weed-killer 2,4-D (2,4-Dichlorophenoxyacetic Acid)
into the plant cells as they feed. This induces formation of a gall, which surrounds the mite as it
feeds. Eggs are laid within the gall; nymphs mature within the gall and the
emerging adults infest new foliage. Galls themselves are abnormal plant
growths.
Since
they are mostly host specific, eriophyids can be identified by the plant and
damage seen. Various other
organisms such as insects, nematodes, fungi, bacteria, and viruses can also
cause galls, though not necessarily on aloes. After the induced change has
altered the behavior of the affected cell or cells, the mite does not have to
remain on the site (i.e. in the gall) to insure continuation of gall growth. Stems, leaves and flowers may be affected.
These tiny mites typically over winter on their host plant. They begin feeding
and initiate gall formation in spring as the leaf or flower buds open.
Eriophyid mites usually do not cause serious “injury”. Large populations can be tolerated by
plants, but the damage may / will be unsightly. The damage to the aloe plant is
irreversible. There is an Aloe bainsii compact form at the JMCC that has its
galls cut out almost annually and the Aloe still looks healthy.
Since
eriophyids are among the smallest of mites (less than 1/100 of an inch in length),
field identification of species on the basis of their morphology usually is
difficult or impossible. Eriophyid mites are more closely related to spiders
and ticks than to insects. Unlike spider mites, which have 8 legs (as do
spiders), and insects, which have 6 legs, eriophyids have only four legs. They
are located near the head so that the elongate posterior portion of the body
must be dragged along the plant surface. They are long, ringed (annulate), and
worm-like. Eriophyids are also the only important mite transmitters of plant
viruses.
They are poor crawlers, but their small
size facilitates travel between hosts by wind, water, insects, birds, and
people. Eriophyid mites reproduce rapidly. Fertilization occurs when females
come in contact with sperm sacs left on the host by males. Females can lay as
many as 80 eggs in one month under favorable conditions. The majority of
eriophyid mite species go through four stages of development - the egg, two
nymphal instars and the adult. The length of life cycle is variable depending
on the species, but it is usually approximately seven days to reach adulthood. Adults
live for about one month, and there are as many as six to eight generations per
year where seasons are long.
It
is fairly easy to control (but not destroy) eriophyid mites. Infected areas can be cut out to eliminate
adult mites and remove unattractive tissues. Burn, or (plastic) bag and
dispose, of infested plant tissues in the trash. Removing and destroying
galls is often necessary to stop the spread of mites on the current host plant
and neighboring plants. Exposed mites are easily controlled,
but most pesticides do not kill the mites living within galls.
Heavy
infestations can be “controlled” with insecticides. Spraying, or root drenching
plants, will not get rid of the galls (deformity) once they have been produced,
but BOTH are necessary for efficient control. Root drenching alone is
effective, but takes longer. Apply insecticides in early spring, and again just
after bud break for later blooming species. Dormant oil, carbaryl, dicofol
(commercial only, in California), horticultural oils, and insecticidal soaps
may be effective. Carbaryl (Sevin) is highly toxic to bees and should not be
applied when they are active (i.e. when the aloe is in bloom). Orthene, used
according to the directions, has been successful for some hobbyists but the
formulation does not contain miticides. Based on experimentation, dimethoate is
used by at least one aloe specialist, and does contain a miticide. Applying any
chemicals to control aloe mites may also kill beneficial insects.
References:
http://doacs.state.fl.us/pi/enpp/ento/entcirc/ent206.pdf
www.cals.arizona.edu/pubs/garden/az1399.pdf
www.puyallup.wsu.edu/plantclinic/resources/pdf/pls89eriophyidmites.pdf
www.unce.unr.edu/publications/files/nr/2004/FS0447.pdf
http://www.extension.umn.edu/distribution/horticulture/DG1009.html
http://
essig.berkeley.edu/CIS/cis02_1.pdf
