Every seed, cutting, or plant that arrives at the Arnold Arboretum makes its first stop at our Dana Greenhouses, to be treated with care until grown to adequate size to face the seasonal challenges of our landscape in Boston. Other than the sounds of staff—who busily work year-round to fortify and expand the living collections through plant propagation—the greenhouses are quiet. Yet at a nearly microscopic level they are bustling with activity, as some of the Arboretum’s smallest helpers move from plant to plant to keep new accessions pest-free.
To give our developing plants the best start, Horticultural Technologist Chris Copeland implements an integrated pest management (IPM) plan, with support from all greenhouse staff. This strategy reduces our reliance on chemical pesticides by closely monitoring pest populations and using cultural and biological controls to protect our plants. The greenhouse rove beetle, Dalotia coriara, is just one species of beneficial arthropod that plays a role in our IPM plan.
The greenhouse rove beetle is a member of the Staphylindae family of the order Coleoptera. Members of this family are characterized by their long, segmented abdomens and short wing coverings, called elytra. They have retained this form for millions of years—fossil impressions and amber inclusions of rove beetles date back to the Late Triassic. At about 3mm long they are small yet visible to the naked eye, yet their presence is not usually evident to the casual observer. Close inspections of the humid environments they prefer—forest leaf litter, sedge mounds in bogs, intertidal areas, and even within ants’ nests—are likely to yield views of these busy creatures.
At the Dana Greenhouses, adult beetles and larvae scavenge the soil for protein. They are generalist predators and will eat anything that moves more slowly than they do, grabbing passersby with their strong mandibles to make a quick meal. Their main food sources in the greenhouse are fungus gnat larvae. Adults will eat shore flies and thrips, which are also greenhouse pests.
Fungus gnats, flies in the family Diptera, are unwelcome guests at the Greenhouses. Their larvae feed on roots, stunting seedlings and newly rooted cuttings. They are also a vector for numerous plant pathogens. At the Dana, numbers of these pests rise in the spring, as seedlings emerge and outdoor temperatures warm. Weekly checks are done to determine threshold numbers of the gnats and to guide courses of action to protect our plants.
Many effective treatments can be used to manage a fungus gnat problem. Beneficial nematodes, Steinernema feltiae, can be applied to the soil in a water drench. These are highly effective at controlling fungus gnats, if the application is done frequently enough to break the cycle of the soil life stage. Because tight control of moisture levels is needed for seedlings and cuttings, this treatment is not regularly used at the Dana Greenhouses. Bacillus thruingiensis subspecies israelensis is a bacterium that specifically targets fungus gnat larvae. Because this treatment is also applied by watering in, it has similar feasibility issues in the greenhouse setting. The roving rove beetle is an excellent fit for the plant production program because the beetles themselves can be released directly on the soil surface and will disperse from there to find food and new homes in the soil. The beetles arrive in ventilated cannisters and are sprinkled onto pots throughout the growing season.
More recently, the beetles have been given roomy accommodations so that their numbers remain steady in the greenhouse between releases, even when the population of their prey is at an ebb. This gives the advantage of breaking the boom-bust cycle of fungus gnat infestation by having beetles at the ready as soon as gnat numbers climb. Beetles are raised and fed a high-protein poultry feed in buckets distributed throughout the greenhouses. Holes high on the buckets’ sides allow the adults to disperse as the population within the buckets climbs. The soil mixture is sieved on a biweekly basis and the beetles are distributed to areas with high concentrations of gnats. In this new home, a miniature Arnold Arboretum, they play an outsized role in protecting our collections.