Fig. 1. The screens placed around the vents in this greenhouse will help keep insect pests from entering the facility.
Photo: Christopher J. Currey

Insect and mite pests are one of the biggest challenges hydroponic greenhouses growers face. The greenhouse serves as an excellent growing environment for plants. We maintain ideal light intensities and temperatures and provide all the water and nutrients plants need to thrive. But these conditions also set the stage for pests to survive and thrive in a nice environment with lots of healthy plants to feed on. Though entire books are written on pests and pest management, this article aims to provide an introduction to the most common pests we encounter in hydroponic food crop production, as well as measures you can to take to prevent or control them.

Common pests

Looking at the list of common pests in hydroponic greenhouse crop production, it looks like the list you may see for ornamental or flowering greenhouse crops: whiteflies, spider mites, aphids and thrips. Greenhouse whitefly (Trialeurodes vaporariorum) damage plants with their piercing-sucking mouthparts. They also deposit honeydew, which, in turn, promotes sooty mold growth and development. Two-spotted spider mites (Tetranychus urticae) can result in a variety of damage to plants because of their feeding on host plants. Damage includes distorted growth, interveinal chlorosis, and leaf necrosis. There are several different aphids which can be found on greenhouse food crops. Regardless of the species, aphid damage includes distorted growth of new foliage and promotion of sooty mold with their excretion of honeydew, like the greenhouse whitefly. Western flower thrips (Frankliniella occidentalis) are often found in flowers and unfolding immature leaves. Their feeding can damage plants and cause new growth to be distorted. More importantly, they are also vectors of tomato spotted wilt virus, which can decimate crops.

Prevention

Prevention is the best way to manage pests and diseases in the greenhouse. Being proactive takes diligence and will require investing resources, including time and money. However, a proactive prevention can be much less costly than finding yourself in a place where you must react to outbreaks.

Fig. 2. Sticky cards are an essential tool for any pest monitoring program. They not only aid in identifying which pests are present, but weekly or biweekly counts of pests help quantify pest populations.
Photo: Christopher J. Currey

Keeping insect and mite pests from entering your greenhouse is the first step in preventing problems. If you are bringing in plant materials, such as young plants or seedlings that were grown off-site, quarantine and inspect them before bringing them into your facility. Placing insect screens around the greenhouse where air is entering the greenhouse excludes pests from infesting your crop (Fig. 1). Wearing some sort of outerwear like a lab coat will help keep insects that may be on your clothing from other greenhouses from spreading. When you are going to be entering multiple greenhouses or ranges, always start with the greenhouse that has the most pest-susceptible crop and has the strictest sanitation requirements, visiting less-vulnerable crops afterwards.

Another step towards preventing pest outbreaks in your greenhouse is sanitation. Weeding around the perimeter of your greenhouses can eliminate habitat where insects that may potentially enter your greenhouse can breed and overwinter. In the greenhouse, eliminating substrate, plant debris or other organic matter on benches, floors, and in aisles where pests may be is always a best management practice.

Diligent scouting will help you identify what pests you have in your greenhouse. Scouting also will help you track pest population. Scouting requires dedication and commitment on a regular basis to keep a watchful eye out for pests. Instead of haphazardly waiting to see pests in your crop, scouting requires deliberate searching. The two most reliable methods of scouting include sticky card and plant inspections. Sticky cards placed among the crop trap insects just like fly paper (Fig. 2). When inspected regularly, sticky cards allow growers to identify what insects are in their greenhouse, as well as track the size of the populations, as reflected by the counts on the sticky cards.

While sticky cards are very useful in identifying pests, they may not all be on the sticky card. Regularly inspecting plants — looking at the underside of leaves, in flower buds and leaf axils, and even root systems — is another key component of scouting for pests. Try to always sample the plants from different areas — not just those that are right next to aisles. And be sure to inspect all the different species you are producing and sampling from all throughout the greenhouse, as different plant species and growing locations may vary in their susceptibility to pests.

Control

Compared to ornamental or floriculture greenhouse crop producers, there are fewer traditional pesticides available for greenhouse food crop producers. The primary reason for this is simple: Unlike ornamental plants, food crops are meant for consumption. Additionally, one of the benefits of growing hydroponically in a controlled environment is the potential to prevent pest problems and reduce traditional pesticide use. This is reflected in the numerous hydroponic and greenhouse food crop producers who like to label their crops as spray-free.

Fig. 3. This sachet contains predatory mites (Amblyseius californicus) which prey on two-spotted spider mites.
Photo: Christopher J. Currey

When pesticides are needed, biorational pesticides or biopesticides are great choices for the hydroponic greenhouse grower. Biopesticides are favored for use on food crops because of their safety, both for the environment and humans. There are several different types of biorational pesticides, including botanicals, microbial, and some synthetic chemistries. Botanical compounds, as the name implies, are derived from plants such as neem oil and azadiractin (both from neem trees) and pyrethrin. Microbial biorationals include bacteria such as Bacillus thuringiensis (Bt) and entomopathogenic fungi such as Beavaria bassiana, as mentioned earlier. Insect growth regulators are compounds that inhibit young insects into maturing as adults by inhibiting the development of an exoskeleton, keeping juvenile insects from maturing into adults and reproducing. These softer chemicals can be especially useful when controlling small pest populations to prevent large outbreaks.

Biological control is the use of beneficial insects, mites, and pathogens to control pest populations in the greenhouse (Fig. 3.). As the name implies, predators seek out their prey and kill them. Parasitoids also kill insects, though the mode of action differs from predators. Instead of feeding on and killing pests, they lay their eggs inside pests. Once the larvae start to grow and develop, they kill their host from feeding on the inside. Pathogens, such as entomopathogenic fungi, parasitize their pest hosts and may be used as well. While there can be a learning curve associated with biological control, it is an effective method for controlling greenhouse pests and can be compatible with controlled-environment food crop production.

The take-home message

Pests can take an economic toll on food crops grown in controlled environments by lowering crop productivity and, therefore, yields. However, greenhouses and controlled environments also provide opportunities to intensively manage these populations. Starting with exclusion and sanitation and a strong monitoring program, you can minimize pest problems before they start. For controlling insect and mite pests, there is a range of options, from traditional to biorational pesticides and biological control.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu