Economical pest control

Having already addressed bollworms, this week we discuss the control of aphid, cutworm and leaf miner in lettuce crops.

A lettuce plant infected with lettuce mosaic virus transmitted by aphids.
Photo: Bill Kerr

A number of newer crop chemicals are available that will control these pests without harming their natural predators. They also present no potential hazard to humans.

  • Cutworms: These are often overlooked because they are not always obvious. Never assume that cutworms are not present; take precautions by spraying the area with a suitable insecticide before or just after planting.
  • Aphids: The main culprit in lettuce is the green peach aphid (Myzus persicae). Fortunately, it is fairly easy to control with a number of systemic insecticides, some of which are predator-friendly. Predators include hoverfly larvae, ladybirds, lacewing and minute wasps. The latter lay their eggs in the aphids and the larvae consume the aphid from within. 

All of these beneficial bugs are likely to be found in an aphid-infected crop. Indeed, some farmers control aphids purely through predators. A major problem is that aphids are the vector for lettuce mosaic virus and their presence, even in relatively low numbers, can render a crop unmarketable.

Organic growers use products that can control aphids on contact, but these usually also kill the predators too. The best practice is to ensure your seed is virus-free and remove weeds that may harbour the virus. One of the advantages of beneficial insects is that they can clear out aphids not reached by the insecticide, and which can be a source of re-infestation.

Nasonovia ribisnigri is a species of aphid that heads straight into the developing head of a lettuce, and so is not easily seen unless the outer leaves are peeled away. This makes them far more difficult to control – all the more reason to look after beneficial insects.

These aphids are usually greenish, but all colour variations have conspicuous lateral stripes across the body and black markings on their legs. They were far more prevalent a few years ago and I have not seen them for a while. The increased use of predator-friendy crop chemicals has probably contributed to their virtual disappearance.

  • Leaf miner: This is also no longer the threat it once was, mainly because most farmers have switched to predator-friendly insecticides, allowing for natural control to occur as well. 

Let the predators do their work
In short, the integrated approach is the way to go as it makes pest control more economical. Some of the new insecticide products are expensive, but do such a good job that spraying can be done far less often and beneficial insects can then prey on the remaining pests. Spraying with a safe insecticide removes most of the food sources for predators, forcing them to search all the nooks and crannies where most chemicals don’t reach.