Efficient Rearing of Trichogramma
   
 

What is Trichogramma

 

Trichogramma (Trichogrammatidae) are beneficial wasps, used widely for the control of over 200 species of moth and caterpillars including borers, webworms, loopers, leaf worms, fruit worms, cutworms, codling moth, bolloworms, and army worms in a variety of crops including sugarcane, cotton, corn, tomatoes, alfalfa, apples, walnuts etc. in more than fifty countries.

Trichogramma are extremely small insects having a wingspread of about 1/50th of an inch. The wasp oviposits inside the moth eggs. The parasite larva hatches inside the host egg and feeds on it thus preventing its hatching. As the parasite develops, the host egg turns black due to deposition of melanin granules by the late instar parasitoid larvae. After about 7-10 days, the adult Trichogramma emerges out ofthe host egg and begins its search for fresh moth eggs. Due to rapid completion of life cycle, it can quickly gain high populations in the field.

   
 

Hosts used for rearing Trichogramma

 

Trichogramma is being commercially reared on different hosts including Angoumois grain moth, Sitotroga cerealella; Mediterranean flour moth, Ephestia kuehniella; Mediterranean rice moth, Corcyra cephalonica and silk worm moth, Bombyx mori. S. cerealella is one of the most common factitious hosts used for rearing Trichogramma and can economically be reared on wheat grains.

   
  Improving rearing efficiency of Sitotroga cerealella
 

Experiments conducted at the Nuclear Institute for Food and Agriculture (NIFA) revealed that preconditioning the wheat before using it for S. cerealella rearing improved the moth egg production. We found that if the wheat grains are soaked in water for 3 h andthen dried for 24 h at 25-30oC before using it, overall egg production of S. cereallella was increased by almost 50% than moths reared on unconditioned grains.

 

Major problem encountered during S. cerealella rearing is contamination of culture with other store grain pests and natural enemies of S. cerealella. Mites and psocopterans, which can reach very high population in a moth culture, can become a real nuisance. These pests enter the culture through the stored wheat grains. If the wheat is kept at freezing temperature for 3-4 days before using it for culture, the problem can greatly be minimized. Sometimes roasting of wheat grains is also done but that reduces the moisture contents of grains, which adversely affects the development of S. cerealella.
Parasitic mites of genus Pyemotes are the most serious problem of S. cerealella causing moth culture to crash within weeks. The mites also cause itching in human workers handling Sitotroga. Freezing the grains before use and restricting the entry offoreignorganisms (mice, cockroaches, other store grain pests) can prevent the accidental entry of mites in a moth culture. Using the eggs instead of adult mothsfor reinfesting the grains can also minimize the problem. The collected moth eggs must be examined under the microscope before infesting the new grains. The mites can be cleaned off the eggs by placing the eggs under a 200 W incandescent lamp and the mites escaping off the eggs can be manually killed or trapped in a water bowl placed below. Acarides are an effective way of controlling mites in a Sitotroga culture. The experiments conducted at NIFA showed that Nissoron (Hexythiazox, 10% w/w) @ 1 g/L water can be used to spray the eggs of S. cerealella or treat the rearing medium without any deleterious effect on moth development. The wheat grains can be dipped in the acaricide solution for 3-5 seconds and then dried at 25-30oC for 24 h before using for moth rearing.
 

 
   
  Factors affecting the quality of produced parasitoids
 

Age of Sitotroga eggs, used for rearing Trichogramma, is an important factor affecting the parasitoid quality. Our studies indicated that older eggs (>2 days old when kept at 25-30oC) of S. cerealella were not preferred by the ovipositing wasp. Upon forced parasitism, the wasp laid unfertilized eggs leading to a highly male biased progeny. Such male-skewed sex ratio in commercially produced parasitoids can lead to failure of a biological control program.

Storage of Trichogramma is another important aspect of rearing Trichogramma for inundative releases. Under optimum temperature, the parasite emergence would take place within 7-10 days. So storage of Trichogramma at lower temperature is required to delay the parasitoid emergence until their field application. Studies conducted at NIFA showed that T. chilonis could be stored as pupae at 7-10oC for upto two weeks without a considerable loss in parasitoid quality. Storage beyond two weeks lead to an increased pupal mortality of the parasitoid.

Trichogramma rearing can be challenging in terms of maintaining the genetic diversity of parasitoids when multiple generations of Trichogramma are reared on factitious hosts in artificial environment. It is important to start the colony with a pure culture of reasonable size, frequently rejuvenate it with the individuals from wild population and to maintain the colony under at least semi natural conditions to avoid loss in quality of the mass reared parasitoids.

   
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