The aphid, Aphis gossypii, is a small sap-sucking insect causing severe damage to cotton, Gossypium species, and hundreds of other crop plants. Using pesticides for control makes aphids resistant over time and harms the environment.
Thankfully, there are natural predators of aphids. Cheilomenes sexmaculata, a ladybird beetle, feeds voraciously on aphids. Due to the characteristic zigzag marks on its back, the insect is nicknamed six spotted zigzag ladybird.
Using such natural predators for bio-control can be an eco-friendly approach. Often, the mere presence of a predator can have an impact on the prey population.
Can zigzag ladybirds affect the aphid population without even eating them?
Sachin S Suroshe and team, ICAR-Indian Agricultural Research Institute, set out to study the non-consumptive effect of ladybird beetles on aphid growth and reproduction. They exposed aphids to various situations for 24 hours.
In one scenario, aphids had direct contact with ladybird beetles. Ladybird beetles could eat the aphids, reducing the aphid population.
In another arrangement, aphids were separated from ladybird beetles by an iron mesh. The beetles could not feed on the aphids. But volatile or olfactory chemical signals could pass through the mesh.
Aphids with no ladybird beetles acted as control.
“The presence of predators and the risk involved is sensed by prey”, says Y Venkanna, ICAR-Indian Agricultural Research Institute, New Delhi.
The stress should produce oxidative damage in the aphids, reasoned the researchers. They checked the amount of superoxide dismutase and catalase in the aphids. These enzymes were significantly higher in active predation and also in aphids that sensed the risk.
“When ladybird beetles are present, aphids show several physical and biochemical changes”, says Sachin S. Suroshea, ICAR-IARI.
This should impact their lifecycle and perhaps even the next generation. So the researchers investigated mortality, fecundity and impact on progeny. Both active predation and the risk of predation resulted in lower aphid reproduction.
Mature aphids, after all the treatments, were kept for observation, to investigate any further changes. Generally aphids produce wingless offspring that feed on the same plant. But, when they sense stress, they produce progeny with wings to escape the stressful situation. After exposure to the threat of active predation as well as to the risk of predation, there was a three-fold increase in winged aphids.
“The zigzag ladybird is a double-edged weapon against aphids”, says Anil Dahuja, ICAR-IARI. “It feeds on aphids and scares the next generation away”.
These ladybirds are easy to rear. Farmers could let these beetles loose in their fields to keep aphid populations below damage levels. In fact, they don’t even have to be released into the crops. Just keep some ladybirds in mesh cages among the plants and next gen aphids will fly away. If we identify the volatile substance that causes this phenomenon, it would become even more easy to get rid of the aphids from crop fields.
Biocontrol Sci Techn, 19 (10): (2021);
Aradhana L Hans
Babasaheb Bhimrao Ambedkar University, Lucknow