Use tadpole shrimps to control weeds
in transplanted paddy rice

These tiny crustaceans occur naturally if you know how to promote the right conditions in your paddy fields..

By Pat Michalak

 

A study in 1976 revealed that weed control in 1 hectare of paddy field required about 500 man-hours of work in the traditional way, about 90 hours if herbicides were used, and only 20 hours to hand pull remaining weeds after the action of the shrimps.

 


Rice at left. Barnyard grass
at right.

 

Annual weeds have continued to be a problem for small-scale growers of paddy rice in Japan, and biological controls, such as tadpole shrimps, are just one of the alternative weed weapons that natural growers may find useful. The continued use of herbicides in conventional rice has effectively controlled many weeds, including barnyard grass (Echinochoa crus-galli), one of the most troublesome annual weeds in Asian paddy fields. However, rice farmers practicing natural agriculture must be creative in order to customize a wide array of alternative weed control tactics to suit their needs.

A tadpole shrimp: better than hand weeding!

Alternative weed control tactics include traditional hand pulling, cultivation, crop rotation, and manipulation of plant spacing. Less traditional control methods of annual weed control in paddy fields include "allelopathy" (the use of naturally produced plant or animal chemicals that affect other plants), and the use of biological controls including weed-eating ducks, grass carp (Ctenophryngodon idella) and red tilapia (Tilapiamossambica XT. Nilotica-anrea); various pathogens that cause weed diseases; and–believe it or not–several insects. A planthopper (Sagatodes pusanus), a stemboring moth (Enosima leucotaeniella), and a distant relative, the tadpole shrimp, have the potential to help farmers control barnyard grass and other annual weeds in transplanted paddy rice.

Some farmers in Japan have used tadpole shrimps (several species of tiny aquatic crustaceans sharing the name Triops) since the 1920s to control weeds in transplanted rice. In Japan and elsewhere, the tadpole shrimps occur naturally in newly flooded paddy fields and are referred to as "weed picking bugs in rice fields". Their story provides a good example of the delicate balance that exists among organisms in agricultural ecosystems, and the way that natural farmers can manage this balance in order to produce a high quality crop of natural rice.

Tadpole shrimps control weeds in paddy rice in three ways:

  1. By uprooting small weed seedlings;
  2. By agitating the soil surface so that the water becomes muddy, thereby inhibiting photosynthesis of small weed seedlings;
  3. By eating young buds and roots of plant seedlings.

The tadpole shrimp is simply a critter that is in the right place at the right time, when it comes to annual weed control. Their eggs, deposited at a depth of about 1-3 cm, can lie dormant in the soil for many years, hatching within a few days after paddy fields are flooded. A small portion of the eggs will remain resting and ready to hatch at a later date should the current paddy dry up. The young shrimps develop quickly and may begin laying eggs within ten days of hatching. Early in the season, shrimps eat decaying plants and animals in the water and soil, along with germinating weeds and other small seedlings. At the same time, they eat fungus off of rice plant stems. They are one of the millions of microorganisms that periodically work to change inorganic matter into organic matter to nourish plants. The shrimps may live for up to three months, but water quality in paddy fields often shortens their lives to only one-month. Natural enemies such as birds, fishes, frogs and insects take their shrimp share, too

Tadpole shrimps may be paddy pests where rice is direct seeded, since rice seedlings are just as tasty as paddy weeds. However, in transplanted paddy fields, scientists have found that from 25 — 80 shrimps/square meter are able to effectively control annual weeds such as barnyard grass. At greater densities, their population soon crashes due to a shortage of food and to their cannibalistic appetite. A study in 1976 revealed that weed control in 1 hectare of paddy field required about 500 man-hours of work in the traditional way, about 90 hours if herbicides were used, and only 20 hours to hand pull remaining weeds after the action of the shrimps. Although the shrimps die within one month, the weed control effect lasts until rice is harvested. The shrimps have little effect on perennial weeds.

Unfortunately, shrimp numbers are unpredictable with each flooding. A large number of shrimps are necessary for the early weed control that will last all season, but you would need an expert to tell you just how many shrimps you have. How can you encourage sufficient shrimps after flooding?

One answer is to rotate! To maintain stable shrimp numbers in paddy fields, the environment must be repeatedly disturbed. The shrimps do well where paddy fields have been converted from rice to upland crops for a few years, and then returned to shallow flooded rice cultivation. In this way, natural farmers may be able to use crop rotation to encourage sufficient numbers of shrimps for annual weed control.

Wait! There’s another twist to the story: Scientists found that shrimp numbers initially increased during the period when conventional fields were changed to natural farming. Further investigation revealed that tadpole shrimps could not live in the low soil pH (4.0 to 6.0) common to natural farms. By using lime to raise the soil pH, natural farmers can benefit from the weed control that tadpole shrimps provide.