Trichogramma:
An effective biological control for corn borer


Corn is one of the most important staple food in the country, next to rice. But the corn industry is suffering from tremendous yield loss due to increasing incidence of diseases and infestation of insect pests, the most destructive of which is the corn borer which cause a yield loss of 20-80%.
Nowadays however, corn farmers are able to regulate population densities of an organism by “other organisms.” Thanks to trichogramma evanescens spp. – the biological control agent of corn borer.
The decentralization of the mass production and utilization of this parasite started as early as 1984. To date, all Regional Crop Protection Center offices have established their own rearing laboratories.

Advantages of using trichogramma:
They attack only insects belonging to order lepidoptera, most member of which are insect pests, unlike when using pesticides, where pests develop pesticide resistance. Hence there is no problem of developing pest resistance or resurgence. Rearing of trichogramma can be done using locally available materials. Field application and release of the parasite require less labor, with no hazards to humans and the environment.
Biological control or the use of natural enemies is more effective and economical than the use of pesticides. It creates a balance of pests and beneficial species.

Components of successful mass production of Trichogramma
1. Most effective species are determined.
2. To maintain parasite quality, rearing procedures include semi- field conditions and renewal of the colony after six generations.
3. Development of effective release and distribution methods for field application.
4. Strong relationship between rates of egg parasitism and reduction in densities of borer larvae, leading to commercially accepted levels of control.

Poor quality of mass reared Trichogramma can result in control failures. The artificial conditions of mass rearing can bring about changes that reduce the effectiveness of the Trichogramma in the field. Such rearing conditions include rearing multiple generations on unnatural host eggs, the absence of plants, crowding and interference, rapid generation time, and failure to rejuvenate genetic stock.


Methods for Releasing Trichogramma
Trichogramma are typically shipped and released as pupae inside the host egg. Unhatched trichogramma are glued to trichocards which are then attached/hung on the third or fourth leaf from the ground or at about mid-height of the corn plant. Each trichocard contains about 1,500-2,000 trichogramma. One hectare of corn plants require 70 pieces of trichocards.
Trichogramma are initially released 4 weeks after planting. The second release can be done 5 weeks after planting. A distance of 12 meter or 17 steps between each trichocard is recommended to distribute the trichocards in the entire field evenly. A high proportion of females in the released Trichogramma is also important as only the females parasitize host eggs. The insectary standard is at least 50 percent females.
For the year 2004, the Regional Biological Control Laboratory of the Department of Agriculture RFU 5 has distributed a total of 27,403 trichocards to 13 corn cluster areas and 2 non-corn cluster areas covering 346.6 hectares benefitting some 182 farmers in the region.
Evaluating Releases of Trichogramma
Field releases of Trichogramma are evaluated by measuring egg parasitism, larval densities, crop damage and economic return relative to similar fields treated with insecticides or not treated.
Egg parasitism measures pest mortality directly and is determined by collecting eggs from the field. Parasitism is evidenced by eggs turning black or by the emergence of a wasp.
Trichogramma wasps kill host eggs by feeding on them. The host egg is stung and the adult feeds on the drop of liquid appearing at the site of the sting, but no egg is laid. The host egg dies, leaving no evidence of parasitism.
The densities of larvae must also be assessed because increased egg parasitism and mortality may not reduce densities of damaging larvae. For some pests an increase in egg parasitism by Trichogramma may represent compensatory or replaceable mortality rather than additive mortality. Comparisons of crop damage, yield and quality are important in assessing the economic return on augmenting Trichogramma.
When evaluating Trichogramma releases it is important to remember the indirect benefits. Unlike many insecticides, Trichogramma have very little impact on other natural enemies which may be valuable in holding the target pest and secondary pests in check. Also, mass rearing do not pose risks to field workers or leave toxic residues on produce

Environmental Factors that Affect Field Performance
The ability of released Trichogramma to locate host eggs depends in part on the distribution pattern of the Trichogramma in the field, the release technique, the size and structure of the crop, and the location of the host eggs. The distribution pattern should bring Trichogramma as close as possible to host eggs to reduce searching time by the wasp.
Weather conditions such as low temperatures and rain can reduce searching and parasitism. Wasps are easily transported by wind and may be blown out of release fields. In corn, 40 to 60 percent of released Trichogramma daily move out of the field this way.
References:


The Trichogramma Technology by B. A. Zaragoza, T. T. Tormes, A. P. Mutya and F. D Laysa, Department of Agriculture RFU 5; The Trichogramma Manual By Allen Knutson, Texas A&M University System

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Trichogramma is available at the Regional Crop Protection Center (RCPC) of the Department of Agriculture RFU 5, San Agustin, Pili, Camarines Sur. Or you can seek assistance from the Agricultural technician in your locality.



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Source:
UMAsenso
Official publication of the Department of Agriculture RFU 5
Vol. 14 No. 1
January - March 2005