Weed Killer Biological Control Agents

1. Zygogramma bicolorata (Parthenium beetle / Mexican beetle)

Description:

• Zygogramma bicolorata is a leaf beetle native to Mexico, belonging to the subfamily Chrysomelinae.
• The beetle is characterized by a small body with a brown head and yellow elytra marked with distinctive elongated brown stripes. The pattern on the elytra is highly variable, with 29 variations identified in a study of 478 beetles.

Economic Benefits of Biological Control:

• In Jabalpur, the beetle controlled Parthenium infestations over 200 hectares within three years of release.
• The herbicide cost for a single application of metribuzin for 200 hectares was approximately Rs 5,40,000. Two applications would increase the cost to around Rs 10,80,000 during the rainy season.
• By the fourth year of release, Zygogramma bicolorata managed 900 hectares, preventing Rs 2.43 million worth of herbicide expenses.
• If manual or mechanical removal were required, the cost would be three times higher than the herbicide cost.
• Biological control is economically advantageous, especially in regions with high rainfall, with added environmental and public health benefits.

Number of Beetles to Release:

• A sufficient number of beetles must be released to ensure breeding and establishment. It is estimated that 0.4 to 0.7 million insects may be required per hectare based on weed density.
• However, beetles multiply rapidly, so releases of 500-1000 beetles per hectare can be effective for establishment and control.
• Once the plants at the release site are defoliated, beetles will migrate to adjacent areas. Multiple release spots can be used in a region for faster establishment.
• The more releases, the quicker the population builds, which leads to more effective control.

Time of Release:

• The best time for releasing Zygogramma bicolorata is after the commencement of rains when Parthenium plants are most abundant and succulent.
• Releases should be avoided between November and May when the beetles do not breed.
• In some areas where sufficient moisture is present, releases can be made in the dry season to ensure continuous Parthenium germination, allowing the beetles to multiply.

2. Neochetina spp. (Water Hyacinth Weevil)

Description:

• Neochetina is a genus of weevils, including N. eichhorniae (mottled water hyacinth weevil) and N. bruchi (chevroned water hyacinth weevil), both native to South America.
• These weevils primarily feed on water hyacinth (Eichhornia spp.), a highly invasive aquatic weed that clogs waterways and lakes.

Impact on Water Hyacinth:

• Both Neochetina eichhorniae and Neochetina bruchi have been widely introduced for the biological control of water hyacinth, which threatens the health of aquatic ecosystems.
• These weevils feed on the leaves, roots, and stems of the plant, eventually reducing its growth and spread, thereby improving water quality and flow.

Use of Biocontrol Agents for Weed Control

Weeds and their Bioagents:

These biological control agents provide environmentally sustainable alternatives to chemical herbicides, reducing both costs and potential ecological impacts. They can be a key component in integrated weed management strategies.