What damage do sugarcane aphids do to crops?

Looking at sugarcane aphid damage, we need to look a little bit about the biology of sugarcane aphids. Their biology goes hand-in-hand with the damage they do!

Aphids belong to the ‘true bug’ order of insects. One of the defining characteristics of a being true bug is mouthparts.  Sugarcane aphids have piercing-sucking mouthparts. This means their mouth is like a straw they can use to pierce into plant tissue and suck out water and sugars. Their mouthparts are their means for causing damage to crops, for example grain sorghum (which is one of their favorites).

Yellowing patches on a leaf due to sugarcane aphid feeding. The aphids suck water and sugar from the plant, causing stress that can lead to the plant dying. Credit: Michael Schmitt

Grain sorghum, like other plants, has a network of pipe-like structures through which water, nutrients, and sugars constantly flow. When sugarcane aphids use their mouths to puncture through the plant pipelines, they steal away water and sugars that are valuable and necessary to plant health and development. This causes the sorghum plant to be stressed, and can harm yields.

Water is a necessary ingredient for photosynthesis. Photosynthesis is the process of converting sunlight, water, and carbon dioxide into sugar, which is food for the plant, and oxygen. Leaves that are highly photosynthetic are dark green and lush. The areas on which sugarcane aphids feed gradually start to yellow. Since the aphids are sucking out the water the plant cells need for photosynthesis, the process is hindered. They also cause physical damage to cells when they pierce through plant tissue that disrupts photosynthesis. Eventually, the damaged areas can no longer photosynthesize and have been stripped of all water, sugars, and nutrients, so they die.

Healthy sorghum leaves and grain in a field. Healthy leaves are dark green and carry water, nutrients and sugar in pipe-like structures in their leaves. Sorghum can be susceptible to a pest called sugarcane aphid, which sucks the sugar and water from the plant, reducing yields or even killing the plant. Credit: KSU sorghum breeding program

When sugarcane aphids consume the sugars made through photosynthesis, they excrete a sugary, sticky substance called “honeydew” onto the plant. Honeydew is shiny, and it is a good indicator that there is a sugarcane aphid infestation. The sugary waste product attracts a new pest to the plant called sooty mold. Sooty mold is a black fungus that comes in and coats plant surfaces where honeydew is present. When looking back at the equation for photosynthesis, it is clear that sunlight is extremely important for this integral plant process. If sooty mold is covering the leaves, sunlight is not able to get through, and photosynthesis is inhibited.

The sugars produced through photosynthesis are a necessary part in plant processes that grow the plant. They help fill the grain in the head at the end of the growing season. If there is not enough sugar available, grains that do form will be of lower quality, and yield will decrease. So, the presence of sooty mold, and its inhibition of photosynthesis greatly affects the plants.

A sooty mold infection. Sooty mold is a black fungus that comes in and coats plant surfaces where honeydew produced by sugarcane aphids is present. Credit: Michael Schmitt

All that said, there is yet another way sugarcane aphids cause problems, even if it is a bit more indirect. As mentioned before, the honeydew the aphids expel is very sticky. When harvest comes around, sticky leaves are a big problem. Plants covered in honeydew have a hard time making it through the combine; they ball together and end up causing clogs. This puts a lot more stress and work on farmers during harvest.

Looking at the big picture, sugarcane aphids, though they are small, can cause quite a bit of damage. They disrupt photosynthesis directly, through feeding, and indirectly, by secondary sooty mold infections. They also present a mechanical issue when it comes to harvesting. It is important that these insect pests be detected and controlled before they can do their damage.

Answered by Sophie Filbert, Kansas State University. This blog is based on Ms. Filbert’s Natural Sciences Education paper, published in 2021. Ms. Filbert was awarded the Darrell S. Metcalfe award in student journalism for her paper from the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America.