Biocontrol for whiteflies in poinsettias
by Sally Colby
The ultimate goal of the poinsettia grower is beautiful plants, free of insects and disease. However, that doesn’t happen without careful monitoring and planning.
Doug Barrow, biological control advisor with BioWorks, said that in addition to being highly effective, biological control agents (BCAs) eliminate the need for reentry intervals, can be applied by anyone and can help growers stave off pesticide resistance.
The path to attractive poinsettias begins with cleanliness. Cuttings from offshore facilities often come with unwanted guests and possible pesticide residue. Ideally, cuttings have been dipped as an early intervention technique that sets up subsequent bio programs for success. Dipping can reduce the initial pest load, allow time for the bios to establish populations and result in better under-leaf coverage than spraying.
Two BCAs are especially helpful during early propagation. The greenhouse rove beetle (Dalotia coriaria) is a soil-dweller that’s excellent for use early in the propagation cycle. It helps control fungus gnats and shore flies as well as thrips pupae. It completes its lifecycle in moist, dark media. “Be aware that Dalotia doesn’t do well if you’re rooting poinsettias in oasis material,” said Barrow. “Look at the rates and apply full rates after transplant.”
Stratiolaelaps scimitus, a fungus gnat predator, is a soil-dwelling mite that is also applied early in the propagation cycle. It consumes fungus gnat larvae as well as thrips pupae. It comes in a peat vermiculite carrier and can be applied by hand-sprinkling. Barrow said Stratiolaelaps can be combined with Dalotia to help save labor. “Typically it’s applied immediately after transplant, pot to pot prior to pinching and spacing,” he said. “Be aware that it doesn’t do well on oasis, so look at rates to finish.”
The top three poinsettia pests growers contend with are whiteflies, fungus gnats and Lewis mites. Also potentially challenging are two-spotted spider mites, thrips and aphids. Barrow focuses on whitefly management since it’s the most troublesome pest.
Two species of whitefly are attracted to poinsettias: greenhouse whitefly (Trialeurodes vaporiorum) and silver leaf whitefly (Bemisia tabaci). Although the banded wing whitefly may show up, it doesn’t reproduce on poinsettia so Barrow said they aren’t a problem. “Bemisia tabaci is the most difficult to deal with from a chemistry standpoint,” he said.
The greenhouse whitefly is larger than the silver leaf whitefly and has a more triangular shape when viewed from above. It may appear whiter and chalkier due to wax excrement on its wings. Eggs on the undersides of leaves appear white at first, then progress to brown or purple. Pupae are oval and white, and have waxy strings attached to the body. The silver leaf whitefly is smaller and has a tent-like appearance from above. It is creamy yellow due to less wax excrement. Eggs are light green-yellow, turning to light brown. Pupae are flat, transparent and yellowish in color. Adults are easily recognizable with visibly red eyes.
Whiteflies undergo seven stages of development, from egg to several larval stages, then to pupae and mature adult. “The egg-laying capacity of the whitefly depends on the host plant,” said Barrow. “With poinsettias it’s about 200 eggs per female. Look at other crops grown in the vicinity of your greenhouse – if you have eggplants and tomatoes in a field and a greenhouse next to it, whiteflies from those crops can be a problem.” Growers should note that Bemisia doesn’t survive frost.
Growers should be aware that the silver leaf whitefly has two biotypes: B and Q. Growers who have B biotype will have better luck with pesticides since the Q type has built up resistance.
BCAs for whiteflies include Encarsia formosa, Eretmocerus eremicus and Amblyseius swirskii.
“Amblyseius swirskii, a predatory mite, feeds on eggs and L1 larval stage of the whitefly,” said Barrow. “Eretmocerus eremicus feeds mainly on the L1 larvae then parasitizes the L2, L3 and L4 stages. Encarsia formosa does separate host feeding on L1 and L2 and parasitizes L3 and L4.”
E. formosa will parasitize the greenhouse whitefly larvae with some host feeding on both species, but it doesn’t significantly parasitize Bemisia. The parasitic wasp E. eremicus will parasitize both silver leaf and greenhouse whitefly. “It’s more aggressive as a host feeder on the larvae,” said Barrow. “They are active at higher temperatures (68º F) in summer and less active in low level winter light.”
Barrow cautioned growers to be aware that Encarsia is very sensitive to sulfur and traditional pesticides, and advised growers to ask their cutting supplier what chemistries have been applied. For optimum results, growers should plan for parasitic wasps to be active prior to the arrival of whiteflies.
“The predatory mite Amblyseius swirskii is excellent to add in a mix of products for whitefly control in summer months since it can consume whitefly eggs as well as broad mite,” said Barrow. “They can be applied via sachet or loose material. They can sustain on some crops with additional pollen or food source is available.” Many growers will apply this mite when plants are pot to pot or after pinching when they leaf out prior to final spacing. “That’s when it’s most cost-effective,” he said. “Plants are pot to pot, so you aren’t wasting anything on the floor.”
Parasitic wasps are available in either blister packs or cards. Barrow reminded growers that while E. formosa can hatch from either end of the pupae, E. eremicus can only hatch on the top side of pupae, so proper placement is critical. One advantage of blister packaging is that it protects pupae from ants and environmental conditions. Blister packs include sawdust and Encarsia and Erotmocerus pupae so there’s 100% chance pupae will hatch. Cards are stuck to the side of a pot, in the shade and out of direct sunlight.
Barrow said that when using BCAs to manage whiteflies, the B type will become the dominant strain over the Q type during the growing season if no chemistry is used. “At the end of the crop, if a pesticide spray is required to clean up the crop, you have a much greater chance of having B type on the crop and the ability to kill the whitefly,” he said. “It’s easier to go from BCAs to chemistry than from chemistry to BCAs. Start BCAs and biofungicides early. All products should be on the plant prior to the onset of disease or pests. Start as clean as possible – dipping plant material is one of the best moves you can make.”
BCAs can help growers avoid pesticide resistance, and any employee can apply them because no license is required and there’s no reentry interval. As a monocrop with a longer growing period, poinsettias are a good steppingstone for growers getting started with BCAs who then find it easy to use biologicals with spring ornamentals.