When an Integrated Pest Management (IPM) program is evaluated, the
use of beneficial insects and disease-resistant plants are usually
acknowledged as the main tools of such a program. Cultivation methods,
such as controlled watering, pruning, and mulching, are not covered very
well in texts about IPM (Yepsen 1985, Horn 1988, Olkowski et al 1991).
This article will attempt to show how cultivation techniques can aid in
pest and disease control.
Controlling the amount and quality of water a plant receives is
important not only from the aspect of plant growth but also to prevent
insect and fungal problems. Stress caused by wilting has the same effect
as senescence. Moisture deficiency accelerates leaf protein breakdown,
causing an increase in the nitrogen content of the phloem sap and in
starch hydrolysis which increases the sucrose content of the phloem
(White 1984, Smith 1989). These reactions have shown to be a critical
factor in the population growth of many insect herbivores (Holtzer et al
1988). Allowing plants to wilt has shown an increase in the development
and survival of plant feeding aphids (Smith 1989). Stress through summer
drought and excess soil water in winter was implicated in the increased
survival of larval looper caterpillars (White 1974). Fungal problems in
plant roots are often caused by excessive watering and poor drainage
which can be alleviated by a more porous soil medium. High amounts of
soluble salts can build up in the soil due to poor water quality
affecting plant health.
Specimen or bedding plants can be permanently grown in containers to
allow the horticulturist to change a display often without damaging the
plants. When there is insufficient light or cool temperatures causing
damage, containerization will facilitate the easy movement of the plant
to the nursery where it can be grown under optimum conditions. Because
commercial soil mixes with peat moss and pine bark tend to break down
and hold water within a year under tropical conditions, a special soil
mix must be prepared. This helps reduce fungal problems on the roots.
Also since many tropical plants come from acidic areas, these conditions
must be duplicated in a medium that will not readily break down.
Well-broken down soil produced from a compost pile, with some extra sand
for better drainage, will do the job. Although heavier than a commercial
mix, this will not break down after a couple of years. Allowing plants
to become potbound will improve drainage, further reducing fungal
problems. Many tropical plants including bromeliads, crotons, heliconias,
gingers, etc., will do very well under potbound conditions as long as
adequate water and fertilizer is supplied.
Growth and reproduction of herbivorous insects and mites is dependent
upon the state of growth, or level of soluble nitrogen, in their host
plant (Price 1984, White 1984, Dixon 1985). The onset of senescence
brings about a breakdown of protein to soluble amino acids in the
senescening tissue. This soluble nitrogen is translocated via the phloem
to sites of storage (seeds) or new growth (Hill 1980). Many insects and
mites tend to specialize in the plant parts upon which they feed (Horn
1988). Phytophagous mites normally colonize senescent tissue to feed on
the nutritious phloem sap (White 1971, Shimonski 1991). Homopterans, as
a group, seem to be more versatile. Scale insects are mostly found on
older or senescent foliage, whereas aphids, tended by ants, are found on
new growth of inflorescences (personal observation on tropical plants).
Selective pruning to control phytophagous mites and Homopterans is a
viable tool in the IPM program. It has been shown removal of
presenescent foliage on Musoid plants can control phytophagous mites
almost to the point of eliminating the use of chemical controls (Shimonski
1991). Controlling aphids and scale on palms, as well as the subsequent
sooty mold, can also be achieved by selective pruning. Aphids are often
found on inflorescences being tended by ants. If the ants cannot be
controlled, they will protect aphids from predators and increase their
rate of reproduction (Holldobler and Wilson 1990). Removal of the
inflorescence upon emergence will alleviate this problem. Older or
senescent fronds being colonized by scale or other Homopterans should be
cut off.
Removal of the green tissue from a plant obviously has its drawbacks.
Reduced photosynthetic surface and loss of stored nutrients will affect
the growth of the plant. In one study, complete removal of leaves had
the effect of suppressing reproduction in some tropical trees (Rockwood
1973). It has been shown, however, that prior wounding by insect or
mechanical means induces increased resistance (inducible defenses) of
many crop plants and trees to insect damage through production of
allelochemicals (Howe and Westley 1988), (Smith 1989). Effects of
complete leaf removal on transplanted Sabal palmetto palms has shown to
be the best method for transplanting this species (Broschat 1991).
Chamaedorea tepejilote palms responded to experimental defoliation by
producing two to three times more new leaves than did controls (Oyama
and Mendoza 1990). Selective pruning may not be the best method for
controlling herbivorous insects and mites on all plants but it seems to
work well on tropical monocots.
BIBLIOGRAPHY
Broschat, T. (1991). Effects of Leaf Removal on Survival of
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