8.4 Common Pests

Mites: Mites are a very common pest, affecting hundreds of plants. These small arthropods are very small, often measuring less than 1 mm in length, and have sucking mouthparts. Damage to plants by mites includes brown stippling on leaves, upturned leaf margins, stunted plant growth, and webbing between plant structures (spider mites). Symptoms can mimic those of viral infections, particularly those caused by the broad mite, so identification should be done under a microscope. Mites typically have a 10-to-14 day life cycle and thrive in dark, humid conditions. Treatment options include neem oil and predatory insects such as ladybird beetles, lacewings, pirate bugs, predatory thrips, mites, and big-eyed bugs. Common types include Spider mite, Broad mite, Russet mite, and Cyclamen mites.

Aphids: A primary nemesis of most vegetable gardeners and plants, aphids can be very destructive to plants. Aphids are typically pear-shaped with two tail-like protrusions at the bottom of their abdomen (Figure 19a). The life cycle is very short, ranging from 10 days to three weeks. Their reproduction capacity makes them a particularly hard insect to control. Aphids can reproduce sexually or asexually and can switch between the two depending on the environment (Van Emden and Harrington 2017). Most aphids are born pregnant. Females will either create daughter clones that produce both male and female offspring, leading to sexual reproduction and eventually egg deposition, or female aphids will simply create live birth clones of themselves without the help from males. Female clones can survive the winter and continue the cycle by creating more clones.

Aphids are commonly found in colony clusters on new growth, base of buds, and on the underside of leaves. Feeding occurs through rasping mouth parts that drain essential nutrient and glucose from the phloem. As a result, leaves of plants infested with aphids often look shriveled, discolored, or stunted. Aphids excrete a substance called honeydew, a sugar-rich, sticky liquid that attracts ants. The ants protect aphids from predators.

Luckily, ladybird beetles (ladybugs) are natural aphid predators. Other treatment options include avoiding high nitrogen levels, physically removing aphids with a strong spray of water, applying a soap-water solution to plants, and applying of neem oil (Flint 2013).

Caterpillars: Caterpillars, the larval stage of butterflies and moths, can demolish leafy crops within a short window (Figure 19b). Their voracious eating habits make them one of the most significant agriculture pests. Adults feed on pollen nectar and are not a danger to plants; however, if you see adults, you likely have caterpillars as well. A caterpillar causes leaf damage that appears as holes or large missing section.

Frass, or fecal deposits, appear as small brown/black pellets and are present near damaged tissue.

Common pests include cabbage looper and cabbage worms (Figure 19c) on Brassica sp., cutworms (Figure 19d), diamondback moths (Figure 19e), hornworms (Figure 19f), beet armyworm (Figure 19g), and inchworms (Figure 19h).

Common treatments include hand removal, B. thuringiensis (Bt), assassin bugs, and lacewings. Chemical application is not recommended, as it is often more damaging to beneficial insects than target pests and leads to chemical resistance.

White flies: White flies are sap-sucking insects that are significant pests in a wide variety of vegetable crops (Figure 19i).

There are three primary whitefly species that impact vegetable crops in the U.S.: the sweet potato, greenhouse, and the banded-winged whitefly (Natwick et al. 2016). Adults of these species are small (1.52 mm) with yellow bodies and wings covered in a white, waxy powder. Most life stages are found on the undersides of leaves, where the adults and nymphs feed. Commonly affected crops include beans, broccoli, cabbage, cauliflower, cucumber, eggplant, melon, peppers, squash, tomato, and watermelon.

Plants with heavy infestation levels may appear stunted, have yellowing or silvering of the leaves, and have defoliation resulting in reduced yields. Honeydew, excreted during feeding by whiteflies, can reduce the quality and marketability of vegetable crops. Perhaps the most damage caused by whiteflies is their role as a vector for more than 100 different plant viruses.

Natural enemies can be effective in reducing or controlling pest levels in greenhouses. Common biological controls are predators (lacewings, bigeyed bugs, lady beetles), parasites (specifically Encarsia formosa, a parasitic wasp), and fungal entomopathogens. Insecticidal soaps and oils can provide some control of whiteflies, but active compounds must cover the undersides of leaves where the insects hide.

Thrips: Thrips are tiny narrow insects that are a common and persistent pest of vegetable crops in both greenhouse and outdoor systems (Figure 19j). Of the hundreds of species affecting vegetable crops, the Western Flower thrip and the Onion thrip are the most pervasive. Thrips, like other insects mentioned here, are sucking insects that drain water and nutrients from the leaves, leaving them discolored with silvery feeding scars and wilting of plant components.

All life stages may be damaging, as eggs are commonly laid inside plant tissue, leaving a scar. Typically, the larval and adult life stages are going to be the most damaging due to plant feeding behavior and the risk of transmitting viruses to the plant. Thrips complete their lifecycle in 3-5 weeks.

Thrips can be hard to see directly on the plant, depending on the species. Shaking the leaf over a white piece of paper can help make them more visible. Treatment options vary according to species. Biological controls include lacewing larvae, pirate bugs, and predatory thrips.

Management of the culture environment and prevention is key to preventing thrips. Use of sticky traps placed at the base of plants or examination of the underside of leaves for feeding scars are ways to monitor for presence of thrips. Thrips can be prevented by using proper sanitation protocols for culture equipment, only using seedlings grown in-house, and preventing weedy areas or overgrown vegetation near the plants or greenhouse.

Chemical applications can be effective at treating thrips however most treatments do not kill them outright and instead prevent them from feeding and thus starving the insect. Due to their lifecycle stages that exist within the plant, multiple applications may be necessary to eliminate them from the system or control an outbreak.

A more comprehensive overview of vegetable pests can be found at: https://entomology.ca.uky.edu/ent60 http://www.uvm.edu/~entlab/Greenhouse%20IPM/pestsandbiocontrols.html https://content.ces.ncsu.edu/insect-and-related-pests-of-vegetables

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.