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.

 

Trichogramma catalog section

Trichogramma bulletin

Trichogramma release techniques