Finding plant pests can be confusing at times. These two pests may look similar, but in the end they are quite different. Green peach aphid (left) and lettuce aphid (right) found on lettuce.
Photos by John C. Palumbo
With high quality vegetable crops, correct identification of insect pests is essential to make effective management decisions. We previously discussed in American Vegetable Grower how insect scouting is viewed as the foundation of Integrated Pest Management (IPM) programs, where the primary goal of scouting is to identify the types of pests present in the field and their frequency and harm to rate. Why is it so important to correctly identify insects in vegetable crops?
Pest or beneficial insect?
When examining insects in your plants, a PCA must first decide whether it is a friend or a foe.
In Brassica plants, for example, we often find big-eyed beetles (Geocoris spp.), Which, by ingesting aphids, help prevent significant damage to the seeds. However, this useful predator can be confused with the most harmful pest found on cabbage seeds, the false chinch bug, Nysius raphanus, which requires insecticide treatments to prevent loss in seed quality. In this situation, correct identification can save farmers money by avoiding unnecessary insecticide applications.
Correct identification can also help PCAs select the most cost effective control tactic or insecticide.
A good example are aphids in leafy vegetables which, if not controlled, can contaminate heads and make the products unsaleable. Six species of aphids are known in lettuce in Arizona and California, but not all species are killed by all classes of insecticidal chemistry. While an insecticide can effectively control the green peach aphid, Myzus pericae, the same insecticide is ineffective against the lettuce aphid, Nasonovia ribinigri.
It’s also good to be extra careful about unusual pests. They can emerge as an invasive pest that is new to your state.
We experienced this firsthand in the desert to the southwest in 2009 when Bagrada hilaris (Bagrada beetle) was first found on broccoli. Many PCAs had reported seeing the strange-looking insect that resembled a ladybug and considered it harmless.
It was only later, after the pest had been accurately identified, that they discovered that the harmless insect was actually an invasive stink beetle that did significant damage to their crops.
Internet searches can be dubious
PCAs and breeders have many resources to help identify insect pests. An internet search can bring up literally any known pest, but it can be frustrating.
When using a search engine to identify a pest, it is best to include the color and shape of the insect, and preferably the type of insect (e.g. beetle, fly, etc.). The culture in which the pest was found can also narrow down the search.
Numerous publications from the University’s Cooperative Extension can be found online to help identify common or unusual pests in local plants.
Enlargement is a reliable source
But perhaps the most accurate way to identify a pest in your vegetable crop is to have it examined by an extension specialist. Most land grant universities employ entomologists who are skilled in insect taxonomy and have identification resources readily available to them.
At the University of Arizona we have several skilled entomologists and a dedicated extension insect diagnostician whose primary role is to identify insects for breeders, PCAs, and the public. If they can’t identify your pest, it is surely something new or rare.
Collect samples
To ensure quick and accurate identification, it is always best to collect live specimens and send them directly to the extension specialist. The better the insects are in shape, the faster they can be identified.
I recommend that PCAs carry small zippered plastic bags in the truck to keep the insects alive until they reach the specialist. It is best to keep the sample cool as well. Dead insects are also useful when they are in one piece. Nothing is worse than having a pest sample delivered to your laboratory in a plastic soda bottle that has been on the dashboard of a truck all day. In these conditions, identification can be challenging.
Take a phone photo
With the ever-improving development of cell phone cameras, it is now more common to receive images of an unknown pest via SMS or email. This works well when the image is of good to excellent resolution. I can’t tell you the number of times I’ve received a picture that is either too small or too blurry. An exact identification is almost impossible under these conditions.
With small insects, using a lens attachment on a cell phone almost always leads to high-quality, identifiable images. These can be purchased at a reasonable price and are very useful for small insects like aphids or thrips.
For larger insects such as caterpillars or beetles, it is always good to put a nickel or a quarter next to the insect to create a certain perspective.
Finally, take more than one picture and from different angles. Sometimes it is necessary to see the bottom of an insect or its antenna in order to narrow down the identification.
Omnivorous leaf roller larvae feed on melon fruits. It looks a lot like pimple worm.
Photo by John C. Palumbo
The case of the unknown caterpillar
Sometimes you need to do a little more to get a correct ID of an unknown pest.
We recently obtained a high quality image of a Lepidopterous larva found in Arizona and California as a food for melon fruits. When first examined, the larvae looked very similar to a cucumber worm, Diaphania nitidalis, which is known to feed on cucurbits.
However, it was not a complete match with pictures of cucumber worms found on the internet. Perhaps more importantly, cucumber worms are not known in the American Southwest
So we collected more larvae, took more pictures, and sent them to our colleagues at the University of Florida who have experience with this pest. Rather, they agreed that it looked like cucumber worm, but something was wrong.
Ultimately, we raised the larvae and took pictures of the emerging butterfly, which we sent to a lepidopterous specialist in California and eventually obtained our identification.
Since we had a live specimen, we were able to raise the larvae to the moth stage. Only then did we finally identify it as the omnivorous leaf roller.
Photo by John C. Palumbo
It turned out to be an omnivorous leaf roller, Platynota stultana. This pest can be regularly found on alfalfa, cotton, and citrus fruits in Arizona and California. However, it had not yet been found on desert melons. Good thing we made extra efforts, or we may have falsely claimed that a new pumpkin plague (pickleworm) has invaded the southwest.
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Do you want proper identification of vegetable insect pests? How to get it
John C. Palumbo is an Associate Research Scientist on Vegetables at the Yuma Agricultural Extension Center, University of Arizona. You can reach him at [email protected]. See all author stories here.
