COASTAL GARDENER
Franklin Laemmlen, Ph.D.
Q:
I have a tree mallow plant. Recently, the growing tips of the branches became
covered with small, black insects. There are also many ants present that seem to be
attracted to the black insects. What can I do to get rid of this problem?
A:
Your description suggests that you have an infestation of aphids that are being
attended by ants. This is not an unusual occurrence. There are many species of
aphids, and they come in all colors from pale yellow to green to red to brown to
black. Most aphids are wingless, but there may be some winged forms also present.
Most aphids give birth to living young, which are miniature forms of their adult
mother. All aphids feed by piercing the plant with a hypodermic needle mouthpiece
and suck plant sap. The plant sap is high in carbohydrates, so the aphid must take in
way more liquid than it needs to get its nutrition. The excess carbohydrates are
excreted as honeydew. Ants like the honeydew, so will attend to aphids to get this
sugar solution. In the process the ants protect the aphids (their food source) from
parasites and predators. This same relationship applies to whiteflies, scale insects,
and mealy bugs. Therefore, whenever garden plants are infested with these pests,
attention should be paid to ant control as well as aphid, whitefly, scale or mealy bug
control. Use diazinon or chlorpyrifos granules or bait applied to the ant colony
mound to control the ants. Apply summer oil and/or insecticidal soap to control
whiteflies, scale, and mealy bugs. Aphids may be controlled with diazinon,
chlorpyrifos, orthene, summer oil, or insecticidal soap. I suggest you spray once to
suppress the existing population, then monitor the infested plants and retreat only if
needed.
Q:
My Scotch broom plant is being destroyed by caterpillars. They appear to be attracted
only to this plant as no other plants in the garden are infested. Please suggest a
control.
A:
Scotch broom was originally introduced into California as an ornamental.
Unfortunately, it reproduces prolifically by seed, and now has become an invasive
exotic pest species in some wildland areas. Scotch broom can now be found in the
wild in many areas on the Central Coast. To counter this “pest” plant, the genista
caterpillar was introduced to eat Scotch broom. Now we have an exotic worm to eat
an exotic plant. The result is that when Scotch broom is used in the landscape, it is
often attacked by large numbers of genista caterpillars which, if left uncontrolled,
will very quickly defoliate the plant. The worms do such a good job of defoliation
that plants are often severely stunted and sometimes killed by the caterpillars. This is
called biological control. To save the Scotch broom, spray the plant with a Bacillus
thuringiensis (Bt) product, diazinon, chlorpyrifos or orthene to control the
caterpillars. On the Central Coast, most infestations can be controlled with one spray
when the worms begin to feed. Use all plant protection products according to label
instructions for best results.
Send your landscape and garden questions to: The Coastal Gardener, 624-A West
Foster Road, Santa Maria, CA 93455.