Trichogramma is the most commonly used and important biocontrol agent worldwide. More exactly it is a group of species which, however, are similar to each other and are all very small in body size (about 0.5mm). Nevertheless, different species might show different preferences and two species of Trichogramma are common in the rice crop (T. japonicum and T. chilonis).
How Trichogramma works
Trichogramma is an egg parasitoid that is, female wasps lay their own very, very small eggs into the eggs of their hosts. The most important hosts for Trichogramma in the rice crop are moth’ such as stem borer, caseworm and leaf roller. The Trichogramma larva hatches inside of the host egg to feed on the host tissue from inside. Within about 10 days (at 25°C), the Trichogramma larva completes its development and emerges from the host eggs. That means, instead of a pest larva which would do damage to the crop, an adult Trichogramma emerges that tries to locate even more new host eggs for laying eggs. Due to the killing of the pest before the larvae starts feeding, the application of Trichogramma has the potential to prevent damage in a very early stage.
How Trichogramma is to be applied
Despite Trichogramma can reproduce and multiply on hosts present in the rice field, in most situations, several introductions need to be conducted within a cropping cycle, also due to the short longevity of the tiny wasps. The Trichogramma biological control agent is released into rice fields by small cards on which about 1,000 parasitized eggs are clued that harbor the wasps at a stage close to emergence. A good timing for the placement of these egg cards is crucial: if you place egg cards too late, the wasps might have emerged already before reaching the rice fields; if you place egg cards much earlier than the expected emergence date, damage to egg cards might occur due to weather conditions or predators feeding on the eggs exposed. Again due to the small size and relatively poor flying abilities, the Trichogramma wasps need to be released from several points to achieve good pest control. The recommended procedure is to place 100 cards at regular intervals (about 10 x 10 m) per ha of rice, resulting in 100,000 wasps per ha.
Advantages of using Trichogramma
Using Trichogramma to control lepidopteran rice pests
- controls major lepidopteran pests with biological means
- reduces the use of insecticides
- avoids insecticide resistance development in insect pests,
- reduces exposure of farmers to insecticides,
- poses no health risk for farmers and do not leave any residues in food,
- helps protecting other natural enemies in the fields, ecosystem balance and the environment,
- does not pose any thread to other organisms in the rice crop, and neither to the soil, water or atmosphere,
- …does not cause any resistance problems,
- …is a sustainable and cost-efficient approach.
In IPM, pesticides should only be used as a last resort, i.e. when preventive and biological control measures are not available or sufficient. If used, the most benign option should be selected, although it is acknowledged that only a limited amount of pesticides are available in Myanmar, making this a challenging task.
For rice, more and more studies show that less pesticides is better in general and particularly when you only have broad-spectrum, toxic pesticides available, it is wise to reconsider using them at all. This is particularly true for insecticides which are causing the most problems in rice production, both in terms of disrupting natural biological control and being harmful for the farmer community. Below is a classification of pesticides and some information on their toxicity and associated problems.
- Organophosphorous insecticide, e.g. chlorpyriphos
- Carbamate insecticide, e.g. aldicarb
- Pyrethroid insecticides, e.g. deltamethrin
- Organochlorine pesticides such as DDT, most products are banned
- Neurotoxin pesticides, e.g. bisultap and monosultap
- Neonicotinoid insecticides, e.g. imidacloprid, acetamiprid, nitenpyram etc.
- Diflubenzuron pesticides, e.g. diflubenzuron, chlorfluazuron etc.
- Phthalimide insecticides, e.g. coragen etc.
- Juvenile hormone and molting hormone insecticides, e.g. tebufenozide etc.
- Pyridine, pyrazole insecticides, e.g. fipronil, pymetrozine etc.
The World Health Organisation (WHO) classified pesticides according to their mammalian toxicity (the best indication for human toxicity, see table below) which is very valuable information for IPM implementation. Obviously, the less toxic products should be preferred in IPM programmes and the most toxic ones not permitted. Though the table does not show toxicity to natural enemies in the rice field, the information may be used as a fist indication for their general environmental profile as well.
Problems with chemicals
- Resistance development in pests may result in low efficacy of chemicals and thus over-use with associated increased problems and costs;
- Most chemicals have a broad spectrum activity and thus will kill natural enemies in the fields
- As a result of 2) secondary pests regularly build up populations to become major pests;
- Soil and water may be polluted, which results in decreased soil fertility, loss of bio-taxa in soil and water, and human health risks;
- Residues in rice grains and other agriculture products result in food safety problems;
- The application of most chemicals is compromising the use of any biological control agent; thus toxic pesticides should not be applied when e.g. Trichogramma is used.
In the following pages, information on the main pests and diseases is provided, focusing on identification and management based on cultural or biological control options, which are preferred under IPM. For application of a pesticide, if really required, it is referred to the Green & Yellow List, which is an Annex to the IPM Technical Guideline developed within the Rice IPM Project.