Of gnats and nematodes: How we fight the good fight against pests

A month ago, we shared the story of Nikki Hanson, an Oxbow Fellow from the University of Idaho. Nikki is spending her summer at our Native Plant Nursery, studying how nurseries can best propagate milkweed in the ongoing effort to save ailing monarch butterfly populations. Her project has gone really well since then–the milkweed has grown to more than a foot and Nikki is starting to see some variation among plants treated with different combinations of fertilizer rate and container size.

Though the plants look strong to the naked eye (some are even budding already, a surprise to everyone), it is not ALL smooth sailing. To the frustration of our nursery staff, our native plants grow in potting media that are highly desirable habitats for a lot of insects. Over the past few weeks, Nikki has been taking some interesting steps to ward off the unwelcome insects making a home beneath her milkweed plants.

First, the intruders: Nikki’s milkweed is at risk from AT LEAST three insects: Fungus gnats, who lay their eggs in the soil and feed on milkweed roots as larva; shoreflies,who are in much the same boat as the gnats but with the added difficulty that one of the adults’ favorite foods, algae, grows on the surface of moist soil; and Western flower thrips, who rasp on (scratch and suck moisture from) milkweed leaves.

All of these insects present a threat to the healthy growth of the milkweed plants, and therefor to Nikki’s research. Seeking an alternative to pesticides, Nikki reached out to Alison Kutz at Sound Horticulture for advice on using beneficial organisms (one that performs a valued service) as a means of tipping the scales back in the milkweed’s favor.

Alison made a few recommendations, and Nikki has been spending the last few weeks applying several living remedies to her plants. First, to combat the fungus gnats, Nikki introduced beneficial nematodes (about 6 million of them), a microscopic worm that lives in the soil and preys on fungus gnat larva. The nematodes will hopefully wipe out this generation of gnat larva, but Nikki also introduced a beneficial mite–Stratiolaelaps scimitus–which she hopes will establish a stable population that will keep the gnats at bay for the rest of the season.

Nikki brought in both a second wave of nematodes of two different species which will settle in at two different soil depths. She also introduced a beetle–Atheta corioria–a general soil predator but hopefully, in this context, a good second measure against the shore fly larva and potentially other unwanted pests. Nikki applied one final predatory mite to take care of the flower thrisps.

Where a conventional nursery may turn to chemical pest management, ours takes an approach that is more holistic. Chemical spraying can be quite effective at wiping out entire populations instantly, but does little to prevent pests from coming back in the future. Building up populations of beneficial predators is a more sustainable solution for long-term management.

For now, Nikki’s milkweed plants are still growing strong and it’s looking like the treatments have been working! Stay tuned for more updates as the season progresses.

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