Plant Disease and Insects

Picture 1 Duckweed Weevil: https://bugguide.net/node/view/555211/bgpage

Insects

When duckweed grows in nature it is the food source of many birds and fishes as well as insects and microorganisms. Nevertheless, some of the insects that feed on duckweed and are considered pests are:

• Duckweed fly, the fly’s larvae feed on the mesophyll tissue of the plant
•  Duckweed weevil, the eggs are laid by the female and once they hatch, they fed on the fronds (see picture 1
• Waterlily Aphid, a common pest in aquatic plants. The insects suck sap from the leaves and are also able to transmit plant viruses. These aphids are able to reproduce very quickly. (USDA, 2002) (See picture 2)

Fungi

Duckweed can be infected by different fungi such as the Pythium myriotylum:

• In presence of the fungi, duckweed is killed in just a few days
• The fungus spreads easily in damp climates and can form zoospores that attack aquatic plants
• The fungus is able to break down the pectin in the plant cell walls of duckweed
  (USDA, 2002)

Another type of fungus that attacks duckweed is Myrothecium. Research shows that the fungus can be treated with weekly applications of fungicide (Thane-45). The problem is that fungicide can be bioaccumulated in the cell membranes of the aquatic plants. This means that if the plant is fed to fish or other animals, the pesticide is also able to enter the food chain. (Iqbal, 1999)

Finding pest and weed control for duckweed is almost impossible. Every entry in the internet tells you how to kill duckweed (since the crop itself is an invasive crop and can be considered as an aquatic weed). 

Nevertheless, I searched for methods on how to kill waterlily aphid, since they are a very common pest in duckweed and many more other aquatic plants.

Normally, in order to kill the insects, one needs very harmful insecticides. The disadvantage is that strong chemicals can kill many other insects like ladybugs. (Wolf, 2019). An interesting way of reducing the insects can be the use of fishes in the water, who would eat the waterlily.

IPM

With the help of Integrated Pest Management (IPM), one is able to reduce the pest population, using fewer pesticides.  Common IPM systems for aquatic plant systems are drainage systems. In case any fungi are found the pond can be treated. The water quality is key when looking at the IPM of aquatic plants. In addition, using triploid grass carps can be an option to reduce the amount of herbicides used since the fish will feed on them. Another interesting fish is the minnow, which is able to reduce mosquito populations in pods. (Harris, 2016)

All in all, pesticides as well as fungicides are used to kill certain establishments in the crops that will negatively affect the plant. Pesticides can be applied in many different ways, directly to the plants or the water. They can be in liquid, powder or gas form. On the other hand, fungicides are there to control fungal problems, there are eradicant and protectant fungicides. 

Picture 2 Water Aphid https://www.mobot.org/jwcross/duckweed/duckweed-pests.html#Duckweed_fly

References

Harris, B. (2016). Integrated Pest Management (IPM) for Ponds. Retrieved October 23, 2019, from https://www.solitudelakemanagement.com/blog/integrated-pest-management-ipm-for-ponds

Iqbal, S. (1999). Duckweed Aquaculture Potentials, Possibilities and Limitations for Combined Wastewater Treatment and Animal Feed Production in Developing Countries.

USDA. (2002). Duckweed Pests. Retrieved October 23, 2019, from https://www.mobot.org/jwcross/duckweed/duckweed-pests.html#Duckweed_fly

Wolf, J. (2019). Small Black Bugs on My Water Lilies | Home Guides | SF Gate. Retrieved October 23, 2019, from https://homeguides.sfgate.com/small-black-bugs-water-lilies-97322.html