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Trichogramma Update 2007: T. pretiosum as a
substitute for T. platneri
With the lack of supply of Trichogramma platneri beginning in
July 2007 from the production facility we have been using in Canada,
many of our customers have had to decide whether to use
non-biocontrol methods or release T. pretiosum, which became the
only Trichogramma species commercially available. The following is a
brief review of our understanding from a large number of scientific
studies that might help you make a cost-benefit analysis for your
situation.
Experts agree that generally irrespective of economic
considerations the best species and strain to release is one that is
found in the ecosystem. The local strain might not be the
best choice when an imported one performing less well in a one on
one comparison does the job more cost-effectively in larger numbers
that are cheaper to mass-produce. In the future of biocontrol,
hopefully regional farmer-supported insectaries will collect,
evaluate and mass-produce the most appropriate strains for local
crops. At this time entrepreneurial businesses try to
cost-effectively obtain and mass-produce one or a few species to
satisfy the greatest demand in the marketplace.
The demand for T. platneri in the US has been dominated
by almonds in the Sacramento Valley, then southern California
avocados followed by apple, pear, walnut, kiwi, pistachio and minor
fruits and nuts along the entire west coast. Decisions
about which strain to grow are based on relative demand in different
crops. The Canadian producer blended gene pools from more than one
crop and region. Quality problems can arise from in-breeding if the
insectary does not collect new “sting-stock” from the field each
season. The complexity and unknowns plus trying to keep costs down
have made these decisions an evolving challenge.
Suppliers of biocontrol agents have marketed Trichogramma
over the past 25 years in an oversimplified way. Our
industry has promulgated the idea that members of the T. minutum
complex that includes T. platneri are found in moth eggs in trees
and T. pretiosum is found in field crops. Unfortunately neither
statement is true. Some also have understood science to say that T.
pretiosum does not fly up which is also not true.
In fact, both species are found in both tree and field
ecosystems and both are attracted upward toward sunlight and both
tend to inhabit the upper areas of any crop, short or tall.
For example, T. minutum that was originally produced for spruce
trees is also the species found in cranberry bogs on both coasts of
the continental US. It is found in bananas, grapes, sugarcane,
tomatoes, and with other species. T. platneri that was first found
in and produced for avocados, then apples, pears, walnuts has a
relative in artichoke fields. Trichogrammatoidea bactrae is native
to both kiwifruit leafroller eggs and cotton pink bollworm eggs in
Australia. Mesilla Valley, New Mexico, T. minutum was the only
species in cotton and was found with T. pretiosum in alfalfa and
corn.
What sheds light most significantly on our problem of not
having T. platneri available is that T. pretiosum Riley, the same
species (held to be the choice for field crops) is found native in
the tops of very tall pecan trees in Georgia and in pine trees in
Chile. In Brazil T. pretiosum is also reported in more than
one study to be the native species parasitizing moth eggs in
avocados (mostly a perforator rather than looper or amorbia). The T.
pretiosum we have available that is produced in the Southern US is a
combination of strains found in cotton, corn and pecans.
Trichogramma is a generalist on moth eggs
and is the most common Trichogramma in North America
parasitizing a very wide range of moth eggs in varied
habitats. It is capable of laying eggs in many things
besides moth eggs. It will try to lay an egg in a glass bead if that
is all that is present. Mansfield and Mills used Trichogramma
platneri to successfully parasitize species from five lepidopteran
families (Gelechiidae, Noctuidae, Pyralidae, Sphingidae,
Tortricidae) and the green lacewing. Female wasps spent more time on
heavier host eggs and the probability of successful parasitism was
related to the structural integrity of the chorion of the host egg.
We observed oviposition attempts on all other lepidopteran hosts
offered and on eggs of Geocoris punctipes (Hemiptera: Lygaeidae) and
Nezara viridula (Hemiptera: Pentatomidae). Our experience with
strains of T. platneri we have grown is that they need fresher eggs
and were less heat-tolerant than T. pretiosum. This suggests a very
wide potential host range for all species, especially T.
pretiosum.
Physical barriers to eggs such as scales, webbing,
layering or very hard “eggshell” are obviously going to make
Trichogramma ineffective. However, if one species of
Trichogramma can oviposit in a species of host egg, there is some
chance that others will, especially in a lab test where there the
variables of the host plant and environmental conditions are
controlled. Wuhrer and Hasan (1993) compared 49 species and strains
of Trichogramma, all reared in the substitute grain moth host,
Sitotroga. Five including T. pretiosum and T. bactrae had the
largest egg laying capacity (54 eggs per female for T. pretiosum).
Two others were most attracted to diamondback moth, and three,
including T. bactrae were the best searchers.
Numerous research projects minimize the importance of
host preference. Those on diamondback moth by Richard Pluke
and by Luis Vasquez Pluke’s dissertation (University of FL 2004)
found that T. minutum and T. bactrae grown on Sitotroga eggs
performed the same as T. pretiosum grown on Ephestia eggs. Vasquez
(Cornell University,1998) studied five species and found differences
in aspects of performance, but overall performance was not
statistically different. He found that T. minutum, T. pretiosum and
T. bactrae all caused between 71-87% mortality (T. bactrae was the
highest due to killing eggs without parasitizing them). T. pretiosum
and T. minutum were better at parasitizing diamondback moth eggs
(40-42%). He chose T. pretiosum for field trials because based on
all comparative studies it was the most promising candidate.
Insect learning and adaptation has a major role to play
in this question. T. pretiosum may not do equally well at
first on some host eggs, but the greater amount of eggs being laid,
the faster a species may “learn” to use what is available. T.
pretiosum may learn quicker than other species since it will accept
older moth eggs (up to 4 days old) whereas in the lab at least T.
platneri refuses a 4 day old egg. T. pretiosum has more opportunity
to recognize and accept a new host putting it in a learning mode to
increasingly accept the new moth egg. Successful foraging and
adaptation is also affected by the plants the eggs are on—their
size, heterogeneity and complexity, according to Pluke. In his study
of T. pretiosum on five different crop plants in cages, it
parasitized soybean looper eggs equally across all plants, large and
small. Prior ovipositional experience with one or the other of the
two host species had no effect on the subsequent parasitism levels.
The preference for the target host eggs was also not affected by
prior ovipositional experience. He concluded that while learning
seems to help improve the wasps use of whatever egg is in greatest
abundance, a Trichogramma is still flexible to use whatever new host
eggs become available.
How does Trichogramma find moth eggs? The
literature cites chemical cues, but we also think that the moths are
radiating low-energy signals picked up by relatively large, bushy
antennae that guide them to those areas in the ecosystem where moths
are laying eggs. Smell sensing equipment shows that Trichogramma
notice moth pheromones that work, not to “attract” Trichogramma, but
to intensify the foraging and parasitizing activity of the wasps.
Once the wasp is closer to egg-laying moths, short-range olfactory
cues of frass and wing scales help it find moths laying eggs. Once
the wasp is very close, then physical cues, such as shape and color,
play a role in how the wasp locates and accepts a host egg. Pinto
and Stouthammer (1994) showed Trichogramma to have a greater
“fidelity” to microhabitat than to the taxonomy of the host
moth.
Fournier and Boivin (2000) studied sensitivity to climate
during dispersal. Some Trichogramma need a certain amount
of solar radiation to be able to function. We know that the near T.
platneri found in California artichoke plume moth does nothing when
the coastal farms are foggy. They report that T. pretiosum is one of
the most versatile species in that it is least affected by
variations in solar radiation and wind. A prevailing wind of eight
hours per day will have no negative impact on parasitism
performance. But of all the factors for predicting the success of a
species the best is its fecundity or egg-laying capacity on a
factitious host (such as Ephestia or Sitotroga on which it is
mass-produced). There are some experts who say that Trichogramma
grown on Ephestia is bigger and better, but Bigler et al (1993)
concludes that while the wasps are slightly bigger when grown on
Ephestia, there was no perceived difference in quality.
It can be said that anytime Trichogramma is released a
new strain is started. A strain is mostly evolving in
response to environmental conditions. Temperature is the most
important factor. Trichogramma need enough heat to be active and
reproduce, yet releasing in very hot weather can be a problem even
for heat-tolerant species like T. pretiosum. Hot temperatures can
cause failure of the release for the heat-sensitive strains of T.
platneri. Cold storage en route from factory to field has a big
effect on fecundity, flight activity, longevity and emergence rate.
All insectaries growing Trichogramma have challenges maintaining
consistent quality in these areas due to the necessity of holding
cold until time to ship. In other similar beneficial insects, like
Encarsia, we know that if we do not chill to below 55’F, the
searching ability of the wasps can be as much as ten times greater.
To foster faster adaptation of a strain in a new system, higher
numbers, greatest possible distribution, and starting releases as
early as possible in the moth flight should all make a difference.
The minimum to consider in most situations is 50,000 per acre and
the maximum that would make economic sense in high-value crops might
be around 200,000 per acre. Studies on this question are unlikely to
be pursued.
Conclusion: In situations when T. platneri is not
available, it could make economic sense to experiment with T.
pretiosum for the rest of this year while the industry works out
what to produce next year. There might be some research we
do not know about to indicate otherwise and each situation depends
on the cost of the alternatives (chemical or biological spray
program).
The following excerpt from PAN Germany is a useful
overview: “Trichogramma species differ in their searching
behavior, host preferences, response to environmental conditions,
and suitability in biological control uses. The timing of
Trichogramma releases in the field is important. Non-parasitism
could be due to the use of less suitable Trichogramma strains to the
host pests, environmental conditions, and untimely release... It is
best to release parasitoids at the beginning of a pest infestation
(when moths are first seen in the field), followed by regular
releases until a natural breeding population of Trichogramma is
established. Modify cropping practices by practicing crop rotation
and by planting cultivars, which are favorable to Trichogramma
population build-up such as wild carrots, dill, golden rod,
leguminous plants, and flowering vegetables. Adults feed only on
nectar, pollen, and honeydew. Many of these [plant] species are
found naturally occurring… Many adult parasitoids and predators
benefit from sources of nectar and the protection provided by
refuges such as hedgerows, cover crops, and weedy borders.” Any
broad-scale use of toxic insecticide is normally incompatible with a
successful biocontrol program.
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