The Clone Wars: Aphids!

March 18, 2003 - 12:56

Find out how to fight the fast-growing pests.

There is much talk about how we as a society must stop
cloning at all costs. Just think about what would happen if humans were to
become like aphids. Sexual reproduction would no longer be necessary, and all
females would be born pregnant.

Well all right, that's going a little far, but that's what
happens to an aphid in the greenhouse. For example, the green peach aphid is
known to reproduce sexually outside in nature, but in the controlled
environment of the greenhouse, they reproduce asexually. They are all females,
and yes, they are all born pregnant. When aphids give birth, they give birth to
live young rather than eggs, and they are more or less born pregnant. They
cannot give birth until they become an adult, but the young inside of the
newborn aphid are already developing--their reproductive capacity is amazing.
It's exponential. Needless to say, pest populations can appear to develop
overnight. In fact, scientists of old used the sudden appearance of aphids on
plants as proof for spontaneous generation -- the theory that living organisms
can originate in nonliving matter.

According to the literature about aphids, every plant on the
planet can be fed upon by one or more aphid species. However, when one
considers that there are more than 4,000 species of aphids described worldwide,
there are relatively few that are ornamental pests.

Aphid Biology

Aphids are small, pear-shaped, soft-bodied insects varying
in color from light green to dark brown, and they are commonly found on the
undersides of leaves or collecting around the terminal buds. Aphids possess
cornicles that extend from the end of the abdomen, a key morphological feature
that separates them from other insects. Cornicles are tube-like structures, and
aphids use them to excrete alarm pheromones, warning other aphids when they are
attacked. The cornicles and the cauda (the tail more or less) are used for
aphid species identification.

Aphids have piercing, sucking mouthparts and feed from plant
phloem tissues. They process the proteins from the fluid, leaving the majority
of the carbohydrates (sugars) for excretion. Those carbohydrates are excreted
as a sweet, sticky juice, commonly referred to as honeydew. If left unattended,
the honeydew will allow the growth of sooty mold, turning the plant surface
black and unsightly. The presence of ants is a good indicator of an aphid
infestation because they readily feed on honeydew. If you can follow an ant
trail in the greenhouse, you can usually identify an infestation of aphids or
some other related family member such as whiteflies or mealybugs.

Aphid development is dependent on species and temperature,
but in general, they grow to maturity in 5-7 days. Adult aphids usually do not
have wings, but winged forms are more common in large populations. It is
important to control aphids before they disperse throughout the greenhouse.

Early detection

Plant damage from aphids includes the following: 1) simple
presence of the pest and cast skins; 2) honeydew and sootymold; 3) transmission
of phytopathogens; and 4) a general decline in health of the plant as exhibited
by yellowing, leaf distortion and stunting. If any of these symptoms are
evident, the populations are already quite high and are going to be difficult
to control.

Certain species of aphids have demonstrated very high levels
of resistance to pesticides. Therefore, it is critical to locate and eliminate
small populations of aphids so that frequent applications of chemicals are not
necessary. Monitoring for aphids may require a professional scout or a
well-informed team of growers and workers. It is most important to visually
inspect the plants and growing areas, and it may require a hand lens for visual
inspection. Experience and common sense tells us that the more intimate a
relationship the grower or worker has with the plants, the easier it is to
identify a potential problem before it gets out of hand.

Sticky traps can be used for monitoring adult alate (winged)
aphids though the presence of winged adults usually means the population is
already large. There is some debate in the scientific community over whether
alatae are formed because of population size or other biochemical processes;
regardless, you should be aware that they are lurking in your crop. Traps can
warn of aphid presence, hot spots and pest migration and can be used to give a
relative measure of the effectiveness of pesticide treatments. You should use
one trap per 1,000 sq.ft. Place the traps a few inches above the plant canopy
and move them up as the plants grow. Check the traps weekly, and keep a good
record of trap counts. These are the most important records you can keep. You
may be able to identify trends in pest pressure on different cultivars or immigration
from a specific location.

Exclusion

Make the growing area as pest-free as possible before
planting. This includes removing crop debris, old plants and weeds and
sterilizing the soil. Obviously, you should also start with pest-free stock
plants. There are other things, however, that can be employed to exclude
unwanted greenhouse pests. Aphids can be excluded with exclusion screening,
screening vents and entrances, and restricting entry into growing areas.
Growers and other workers should avoid Á wearing colors that attract
aphids because aphids can be hitchhikers. Weeds or even well-cared-for
landscape outside the greenhouse will harbor aphids, which can enter the
greenhouse through vents or open sides.

Control

Aphid reproduction occurs so rapidly that reapplication of
pesticides for control, if necessary, should occur in 5-7 days. In addition,
pesticide class should be rotated every 2-3 weeks.

The chloronicotinyls Marathon and Flagship continue to be
very good products against aphids. Two old standbys, the carbamate Mesurol and
the organophosphate Pinpoint, also prove to be very effective against aphids.
Of particular note are the soil-applied treatments in Figure 5, page 17. Soil
applications successfully make their way into the phloem where they are
accessible to piercing, sucking insects like aphids. Some newer products show
good promise for aphid control for the future. BSN-2060 from Bayer, Flonicamid
from FMC, Pedistal from Uniroyal and V-10112 from Valent cause significant
mortality of aphids compared to the control. Some resurgence is noted in a
couple of these products at lower rates, which means they may require repeated
applications for success. In theory, that means they may have a less likely
chance of causing pesticide resistance due to the lack of persistence of the
chemical.

So, yes, you as a grower may be caught up in a clone war, a
war against that persistent foe the aphid. However, never fear, they are still
susceptible to most registered pesticides, and there will be a number of backup
products available soon.

Note: Figure 3, page 17, lists the pesticides used in recent
trials at the University of California-Riverside, and Figures 3-5, page 17, are
summaries of recent trials. All rates in the figures are per 100 gallons unless
noted. Some of the pesticides in the figures are registered for use on the
intended target, and others are experimental because it is always good to know
there is hope in future products. We do not always use labeled rates in our
trials because these trials are for experimental purposes only. However, our
knowledge about the products and their capabilities grows from every trial.
Labels constantly change. Therefore, it is always the pesticide applicator's
responsibility, by law, to read and follow all current label directions for the
specific pesticide being used. No endorsement is intended for products
mentioned, nor is criticism meant for products not mentioned.

About The Author

James A. Bethke is a research associate and Richard A. Redak is a professor in the Department of Entomology at the University of California-Riverside. They may be reached by phone at (909) 787-4733 or E-mail at bethke@citrus.ucr.edu.

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