Watch out for whiteflies

Whiteflies are prolific pests for many greenhouse and nursery crops. They spread viral diseases and suck out plant juices, causing leaves to yellow and shrivel before potentially killing the plant. These insects reproduce rapidly and reside on the underside of leaves, making them difficult to control. A pest like whiteflies requires a comprehensive approach to stave off rising populations, pairing chemical controls such as Mainspring® GNL insecticide with proven cultural practices.

“Many operations use biological controls as part of their pest management program, but there are times when whitefly populations can become too high for a biological program to manage on its own,” says Nancy Rechcigl, technical field manager for ornamentals at Syngenta. “There are tools that can be used to reduce or control pests without having negative effects on beneficial insects. Mainspring GNL is a good product for that – it’s been tested across predatory mites and parasitoids and has good overall compatibility with biological control agents.”

Although there are numerous species of whiteflies in the U.S., the greenhouse whitefly (Trialeurodes vaporariorum) and the silverleaf whitefly (Bemisia tabaci) are the two most commonly seen in greenhouse and outdoor nursery spaces. The silverleaf whitefly has multiple biotypes, with B and Q being the most encountered genetic variations. These biotypes look identical, but they can react very differently to chemical treatments – adding to the complexity of control. While lab tests can help growers identify and understand which biotypes they are dealing with, “it’s not uncommon to have a mixed population at an ornamental facility,” Rechcigl says. “So, what’s really important is creating a good rotation program with insecticides that have activity on both biotypes.”

Monitor the signs

Scouting for whiteflies can be as simple as seeing what emerges after tapping a crop canopy.

Like many insects, whiteflies have immature and adult stages. Adults lay eggs on the underside of leaves, which hatch and release tiny scale-like crawlers that settle into a spot and begin feeding. Over time, they become larger in size, going through three additional nymphal stages before emerging as an adult.

“You want to monitor for adults because it’s important to know if and when they’re present in your growing area,” says Rechcigl. “The best way to do that is by placing yellow sticky cards just above the height of the plant canopy. This is a helpful tool, because it can be used to also monitor other flying pests such as thrips.” 

Scott Ferguson of Atlantic Turf & Ornamental Consulting, says whiteflies dine upon a wide range of host plants. Many crops are susceptible, including poinsettias, gardenias, lantana, verbena, mandevilla and hibiscus. The tiny pests may also be found on cotton, peanut crops, and other vegetable plants.

“Here in Florida, every acre of tomatoes is treated with insecticide to control whiteflies, but we still have to turn over leaves to look for adults,” says Ferguson, an industry veteran who previously spent more than two decades with Syngenta.

Whiteflies at both the adult and immature stage have piercing-sucking mouthparts, which they insert into the plant phloem to extract sap. Nymphs cause the majority of damage, including leaf loss, reduced expansion, chlorotic spots on the top side of leaves, and discoloration or slivering. If whitefly numbers per leaf are large enough, they can do enough damage to kill the plant.

Whitefly adults can also transmit viruses from diseased to healthy plants. Similar to aphids, whiteflies excrete honeydew, a sweet substance that forms a sticky coating on leaves. Honeydew is often colonized by sooty mold, giving plants a black and dirty appearance. An abundance of sooty mold fungus prevents light from reaching the leaf surface, causing plant stress and eventual death.

High populations of whiteflies also draw ants, which feed on the honeydew and even protect pests from their natural enemies.

“You’ll see a shiny gloss on leaves below where the whiteflies are feeding,” says Rechcigl. “That’s the kind of activity you have to watch out for.”

The best defense

Year-round pests, whiteflies can rapidly overwhelm unprepared nurseries and greenhouses because of how quickly they mature from the egg stage to adults (as few as 21 days). Understanding this lifecycle can help growers prevent or mitigate infestations. 

Cultural controls and good sanitation practices may moderate the severity of a whitefly invasion, but chemical products are necessary to eliminate the problem. Ferguson suggests rotating insecticides based on their activity and strengths, meaning growers must study labels closely to ensure they’re choosing the right product.

“Make sure you use a material that controls whiteflies and doesn’t just suppress them,” says Ferguson. “Suppression will give you less than 70% to 80% coverage. Products that say ‘control’ are 90% effective.”

Mainspring GNL, a diamide insecticide, provides whitefly control as a non-neonicotinoid alternative for growers, acting as ryanodine receptor modulators in the whitefly nervous system. With a unique active ingredient in Insecticide Resistance Action Committee (IRAC) Group 28, Mainspring GNL is ideal for integrated pest management programs that incorporate multiple modes of action. 

Rechcigl says, “Mainspring GNL works via ingestion and acts like a shield. When whiteflies ingest the active ingredient, they’re prevented from feeding. This is going to keep populations from building to high levels.”

As a foliar spray, Mainspring GNL works best when applied sequentially in two- to three-week intervals. Drench applications for rooted plants provide broad-spectrum control for up to 12 weeks, preventing damage from whiteflies as well as thrips, aphids, leafminers and more. Upon use, insect mortality occurs within two to seven days. Mainspring GNL can also be used in an integrated program with biologicals like Amblyseius swirskii, a predatory mite, or Eretmocerus or Encarsia spp., both parasitoids, often utilized to combat whitefly populations.

“For poinsettias, you want to use Mainspring GNL two to three weeks after transplanting once the crop has rooted in,” Ferguson says. “It can also be applied through drip irrigation where you’re injecting it into the water. Because Mainspring GNL gives you long residual control, it’s way easier to maintain control before whiteflies show up.”

That’s not to say chemicals are an end-all for managing whitefly infestations. Implementing proper cultural practices – be it keeping weed-free production areas, inspecting new plant shipments, or installing screens to stop insects from entering a nursery – can deliver additional critical protection. 

Ultimately, a preventive approach, will save growers time and resources compared to strictly curative treatments.

“Trying to control a high pest population costs more than preventing it from reaching damaging levels,” says Rechcigl. “If you wait until populations are at a critical stage, then more control applications will be needed, especially if you’re using sprays. So you’ll have cost of product plus labor to reduce out-of-control populations.” 

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