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Magazine: Growing Edge
Issue: Vol.5#4, Summer 1994
Title: Biological Controls for Specialty Collections Part II
text by Justina Marie Kelliher
photographs by Stephen Maize
Many gardeners assume that biological and botanical controls are always
the best choice. Some growers would go so far as to sacrifice an entire
crop or lose a choice ornamental before reaching for a chemical control
method. But when you are working with rare, endangered, long-lived or
especially valuable plants, the choice of control methods becomes more
complex. My greenhouse, filled with rare and/or difficult to grow
succulents, many grown from seed, represents a substantial investment in
time, effort and money. As a result, my goal is to maximize the use of
biological controls and reduce pesticide use without sacrificing the
health of my plants.
The high value of my collection also makes a difference in the effort
and expense IUm willing to accept to prevent insect damage. The higher
cost of some biological control agents is more than justified in this
situation. Cheaper measures, pulling and destroying diseased plants for
example, might make more sense in a vegetable garden or flower bed.
This bias in favor of biological controls has its practical side.
Resistance by pathogens is a problem encountered in chemical-based
control systems. Substituting or supplementing with biological controls
is a way to help preserve the usefulness of the chemicals we do have and
are willing to use. Biological controls do not select for resistance to
chemicals and if used in an integrated program can slow the development
of resistance in pest species.
Customized Control
As you plan and implement your pest control program, it is important to
remember that successful strategies are usually site-specific. My
greenhouse is located in Western Oregon, for example. What works well
here in terms of specific problems, control measures and timing may not
work in your greenhouse or grow room. Even in the same area, there can
be major differences from one greenhouse environment to another. As a
pest management consultant, IUve learned to adjust the timing and
methods to the situation at hand. One of my clients grows bamboo and I
often have to time the releases differently because he maintains a much
higher humidity in his greenhouse. Even my own propagation room needs a
different management system than the main greenhouse.
This article will focus on my succulent greenhouse as a working example.
It is always important to look at your plants and identify your own
problems rather than following a rigid set of guidelines. After all, you
know your plants and conditions better than anyone else could. Your
methods and timing will need to be adjusted to your climate and the
system that you already have set up for the health of your plants.
Planning
The first step of any project is defining your goals. Integrated pest
management is a step-by-step management system. Research is used as an
input at most steps of the process, but it can only tell you what could
be done. What should be done can only be determined in the context of
your goals. The next step is to identify problems and possible problems,
and look at possible solutions, including the no-treatment option.
Choose a solution that best serves your goals and implement it. The
final step is the most importantQevaluating your chosen solution. If
something does not work, a change needs to be made, which usually brings
you back to an earlier step.
All of this might seem obvious, but it is surprising how many gardeners
reach in a haphazard way for any solution that comes along during a
crisis and ignore preventative measures when problems subside, all for
the lack of a well-thought-out and consistent plan. The system (outlined
at ????) consists of several loops to account for the constantly
changing nature of natural systems. Use it.
There are several different pests that I expect to appear in the spring.
The monitoring method that I use is a yes/no system. If one pest is
found, I plan the insect releases. It generally takes about two weeks
for the biological control agents to arrive, so timing is crucial.
The pests that I plan for in the spring are mealybugs, scale, whiteflies
and mites. Fungus gnats are a persistent problem all year and I try to
maintain a low population. Slugs, snails and sow bugs are also year-
round problems. I try to limit their population by excluding them from
the greenhouse environment and handpicking (see RPest Control for
Greenhouse Growers,S Vol.4 #1, page 39 for more on excluding snails and
slugs from the greenhouse).
The spring insect releases consist of the generalist predators, crypts
and lacewings. I also add predatory nematodes and microbes to the soil.
If I find any spider mites, I release predatory mites, although that is
usually not a problem until early summer when things dry out.
Mealybugs
Mealybugs of several different species are a major problem for succulent
growers. They tend to hide in the cracks and cervices of the plants
where they are difficult to spot. Hitching a ride on new plants is a
common way for mealybugs to enter the greenhouse. I always repot and
inspect plants carefully before moving them into the collection.
The species of mealybugs that are dominant in your greenhouse may change
with the seasons depending on moisture and temperature. Monitor these
changes and evaluate the effectiveness of your control measures at
different times. Only adult male mealybugs fly, but the young nymphs
will walk to find suitable food. Ants will sometimes bring them back
into the greenhouse once you have eliminated them. Outside vegetation
can harbor mealybugs. I have found them living happily in ivy growing on
the side of the house in midwinter. I have been killing and removing the
ivy, both to eliminate an ideal mealybug nursery and to stop damage to
the house.
Mealybugs have developed some resistance to common greenhouse chemicals,
so the use of biological controls is especially important. I used to
hand pick and kill mealy bugs with rubbing alcohol, but it often caused
scaring on the growing tips and could be fatal to seedlings. Biological
controls have so far been the safest and most effective approach in
dealing with mealybugs.
The root mealybug is a particularly difficult insect to control because
it lives in the soil, making it hard to reach with contact insecticides.
In my experience, it is the most difficult to identify and control of
the different mealybug species. If I see a plant that seems to be
declining for no apparent reason, especially an older plant, root
mealies are the first thing I check for. If it is impractical to repot
the plant, I add microbes to soften the old, caked soil. Once the soil
has softened, I add predatory nematodes and then release the generalist
predators of adult crypts and immature lacewings. The soil needs to be
soft so the predators have a chance to reach the root mealies. The
beneficial microbes also help in make nutrients available to the plants.
There are several biological control agents that prey on mealybugs. I
use an early spring release of green lacewings (Chrysoperla carnea),
predatory nematodes and mealybug destroyers (Cryptolaemus montrouzieri)
to get maximum control. I want to approach 100 percent control, so I
often release more predators than my population of mealybugs can
support. Given the value of my collection, IUm willing to let the
predators starve out between introductions to minimize plant damage. The
adults of both of these species consume honeydew, a sugary liquid
substance exuded by several major greenhouse pests. There is a food
available to feed the predators called RwheastS that will keep them
around longer and allow them to lay more eggs.
The crypts released in my greenhouse have given good control and have
been seen eating root mealies. I use a high-pumice soil mix that may
make it easier for the crypts to reach the mealies below the surface.
The major problem with the use of the crypts is that the immatures look
very much looks like mealybug nymphs. The adult crypts lays its eggs in
clusters of mealybug eggs and nymphs. Too often people assume the
control measure is not working because they see a mass of white
filaments, most noticeable about a week after release.
Be patient and let the control measure work, especially if the initial
release took place when there was a heavy infestation of pests. Behavior
is one of the keys to identifying the crypts. Once the mealybugs find a
crevice, they settle in and stay. The crypts, on the other hand, will
move from day to day. Observe a group of mealybugs daily to note the
movement of the immature crypts.
Crypts are a type of lady beetle and will shift to other soft-bodied
insects when their primary prey population is low. This enables crypts
to establish for longer periods in the greenhouse and provides some
additional control. Adult crypts will fly away when they run out of
food, but in a screened greenhouse they often eliminate the pest
population before they starve to death. I usually get a second adult
population of the crypts with each release before they run out of food.
Scale
Lacewings work to keep scale populations under control. Immature
lacewings can reach scale insects in places that crypts miss. The
difference in the life stages of lacewings and crypts when released
makes it less likely that they will eat each other, but they will when
other food runs out.
Scale insects are usually found on the undersides of leaves along the
veins, but they can be found on the stems of various stem succulents,
including cacti. There are two major groups of scale insects. The soft
scales exude honeydew and are often used for food by ants. Ants will
move them back into the greenhouses so, as with the mealybug, ant
control is important in controlling this pest. The armored scale does
not exude honeydew. There is a predatory wasp on the market for scale,
but I have not needed to try it.
Whiteflies
Whiteflies are another uninvited greenhouse guest that I expect every
year. There are sticky traps on the market that are good monitoring
tools for the adult whitefly. The eggs are laid on the undersides of
leaves and the nymphs live on the leaves producing honeydew. They hatch
into small, white, winged insects. Vacuuming the adult whiteflies will
provide some control; immature whiteflies are susceptible to both crypts
and the lacewings. I have not used Encarsia formosa, the wasp biological
control agent for whiteflies since my succulents require a temperature
that is unsuitable for them.
Thrips
Thrips are another problem that responds to lacewing releases. Since I
expect the biological control agents to die out in the summer, the
spring release that includes lacewings is valuable. The crypts are
mainly for mealybug control, but will also feed on thrips. The thrips
have never built up to the point where they have caused damage to my
succulents. My major control is to maintain healthy plants. Thrips seem
to prefer damaged or unhealthy specimens.
Fungus Gnats
Nematodes are my primary control for fungus gnats and other soil-borne
organisms. I water them into the soil every four months. Plants in a
high mineral soil have less of a problem since fungus live on the soilUs
organic material. In Oregon there are many species of scarid flies
(fungus gnats) in the forest environment that fly into my greenhouse.
With an unlimited supply just outside the door, I am certain that I will
be dealing with fungus gnats as long as I have my greenhouse. I use BtI
on occasion in the winter when the population has gotten out of control,
but through much of the year I tolerate a modest population. Fungus
gnats only cause real damage to succulent seedlings, so the spring
control is most important.
Mites and Aphids
Mite pests require a treatment system of their own as the generalist
predators donUt touch them. Spider mites prefer hot, dry conditions so
they are not a problem in the early spring. As the season progresses,
the chance of mite problems increases. Once I notice the first mite,
sometimes as early as late April, I place my first order of predator
mites. My chosen solution is to vacuum the pest if the population grows
while IUm waiting for the order to arrive. It usually takes three
releases about a month apart to maintain good control.
There are several different species of predator mite on the market that
do better in different environments and on different pests. Select the
one or combination that best suits your area. The releases are often
made in June, July and August. In Oregon we donUt get much rain at that
time of year so maintaining a irrigation system for the succulents is
important. Misting the plants can reduce this pest and some soap or oil
sprays could be used if they are not applied during hot, sunny weather.
Summer irrigation has to be done with caution since cold water on a hot
day can cause scaring and too much water can cause splitting and shape
deformity, especially in cacti. The late summer predator mite release
can be combined with the fall release of other predators.
Aphids are only an occasional problem for me. The generalist predators
that I release feed on aphids and do a good job of controlling them.
Once again, vacuuming and the use of light sprays of soap or oil can
help control the pest.
Monitoring and Implementation
Monitoring for pests should become automatic every time you enter the
greenhouse, but you should set up a system that includes a thorough
visual inspection of plants at least once a week. Visual inspection is
the best monitoring tool for pests that are attached to the plant like
scale, mealybugs, and immature whiteflies. Yellow sticky traps are a
useful tool in monitoring some pests like adult whiteflies and aphids. A
hand lens is useful in identification of pestsQa necessity with mites
because of their small size.
In monitoring you should also look at the ratio of beneficials to pests
and use that information to time later releases. Mite populations can
build up fast under the right conditions so monitoring the ratio of pest
to prey is critical. An experienced grower in the indoor environment can
see pests before they would be caught in a trap. Past records and
experience is invaluable at this point.
Always check your biological control agents when they arrive. A hand
lens is a necessary tool to check for the viability of the predators.
Timing of the release after arrival is important. Adult crypts are
hungry and thirsty and need to be released soon after delivery. I mist
the greenhouse before setting them free. Feed them with wheast to
encourage them to stay in the greenhouse, especially if you have a very
low pest population. Lacewings arrive as eggs and often hatch a few days
later. You want to release them as soon as they hatch. A hand lens can
help you determine when hatching has finished. If you wait too long,
they will eat each other.
Nematodes come in an agar base that needs to be dissolved in water. You
should be able to see them swimming around with a hand lens. They can be
applied with a watering can or a sprayer. Water the greenhouse well
before releasing nematodes in the morning, evening, or on a cloudy day.
You can keep the undissolved agar containing dormant nematodes in a
refrigerator until needed.
Predator mites arrive ready to be released and should be seen with a
hand lens walking on the top of the container. Cool temperatures slow
down the development of insects, so a refrigerator will give you a
little time if you need to delay release. Warm temperatures speed up
development, but high heat can kill. I have had an entire order of
predator mites die from being left in a mailbox on a hot day.
In addition to biological controls, there are several insecticides that
I have used in the greenhouse with success. They include BtI, soap, oil,
botanicals, including pyrethrum, and insect growth regulators. These,
combined with biological controls and preventative measures in a
coordinated system of integrated pest management, should give you all
the edge you need in dealing with pests in the greenhouse.
After every step in the program remember to evaluate the effectiveness
of the choices youUve made and see how close youUve gotten to your
goals. Sometimes you need to make changes in your system or perhaps your
goals are unreasonable. Just because you have a plan doesnUt mean that
you should be inflexible in your approach. It is, after all, your
greenhouse. The level of control and the means to achieve it are your
decisions. Others can tell you whatUs possible and how to do it, but
only you know what you want.
Justin Marie Kelliher is a Master Gardener and professional pest
management consultant.
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