Infection control in the Conservatory, from mundane to amazing

Volunteering at the University of Minnesota's Conservatory Greenhouse

As mentioned in prior blog posts, for the past three years I've been volunteering for a few hours a week at the Conservatory greenhouse at the St. Paul campus of the U of M.  This blog is about activities there, and the many fascinating and beautiful plants in the collection.

A little background about plant infections

Everyone who has cared for a plant, from houseplant lovers to professional nurserymen, has learned the hard lesson that plant pests and diseases are an inescapable fact of life. All plants, domestic or wild, are susceptible to the infectious agents and parasites. Plants in their native habitats are largely protected by the ecosystems surrounding them, where pests do not have the luxury unfettered consumption. Plants grown indoors have been removed from their ecosystem and they are sitting ducks for parasites to proliferate unchecked.  Indoor gardens rely entirely on pest control methods with varying degrees of success.

A view of a now-clean window sill in the desert room. In the foreground are the sharp spines of the plant Vachellia cornigera, bullhorn acacia (N American plant in the bean family).

The mundane part of infection control: good housekeeping

Today I chose the task of cleaning of window sills from a list of tasks in the workroom of the greenhouse. The walls of the greenhouse are made of framed glass windows sitting on a concrete skirt-wall. Over time the window's large sills become littered with dead leaves, pollen, and other organic bits blown up by the enormous fans which regularly aerate the rooms. 

The point of keeping the sills scrupulously clean, apart from the esthetics, is the organic debris is fertile soil for pathogens. Clinging to the leaf litter are the eggs of parasites, molds, bacteria, and viruses. The task of cleaning the sills involves moving plants away from the sills to provide access, sweeping off the litter, and wiping down the surfaces with soapy water (with a cap full of bleach per pail). Finally, the sills are rinsed with a sharp spray of tap water from a very long hose. The litter falls to the floor of the greenhouse where it is swept up and removed.

Moving plants and cleaning sills has its danger, for example it is easy to get stuck by the spiky cactus needles and euphorbia spines in the desert room. The plant in the photo is a bullhorn acacia species in the bean family (Vachellia cornigera), it has very sharp spines on the branches, one of which caught the skin of my arm causing a small bleed. Luckily, I pack extra bandages in my wallet (a habit I learned caring for my two young nephews, perpetually and fearlessly in motion).

An amazing example of biologic control

The second method of control is the introduction of predatory organisms to combat specific pests. As I moved plants away from the sills, one plant in particular stood out as an example of biologic control. The plant, Zamia nesophila, is a native of tropical parts of North and Central America. It is similar in shape to a small palm tree, with long fronds of strap-like leaves. At the base of the fronds is a large seed cone. The plant is critically endangered in the wild due to habitat loss.

Critically endangered Zamia plant

Many of the Zamia leaves in the greenhouse collection were a coated with black sooty mold. The mold grows in the sweet and sticky parasite poop called honeydew. Although the mold is mostly not harmful to the plant, the scale insects that produce the honeydew can be harmful. Left unchecked the scale insects will proliferated and overfeed on the leaves, stems, and roots eventually destroying the plant. 

Rather than using a lot of chemical poisons to kill the scale, the staff of the greenhouse have wisely introduced predators of the scale insects. In the photo below, fuzzy white critters can be seen feasting on the little brown bumps, these are the scale insects attached to the leaf surface. The fuzzy critters are the larval stage of the beetle Cryptolaemus montrouzieri, also known as the mealybug destroyer (they feed on scales insects too). Mealybugs are fluffy white parasites that, like scale, suck sap and excrete honeydew. 

Mealybug destroyers at work on a moldy Zamia leaf

This predator-prey situation is a bit like the old Mad Magazine's comic strip called Spy vs. Spy. In the comic series, there are two spies; one is dressed in all white and the other dressed in all black. They are perpetually trying to outsmart each other and gain the upperhand. In this case there are the black spies - mealybugs and scales, and the white spy - Cryptolaemus. In the wild, ants will feed on the sweet honeydew excreted by scale and mealybugs. To protect their source of honeydew, the ants have evolved to attack the predators of mealybugs and scale. In response, the Cryptolaemus larvae have evolved to look like their prey the mealybugs. This false appearance protects the larvae from the ants and allows the larvae to continue feeding unharassed by ants. In turn, some scale species evolved hard shells to resist their predators.

Although I haven't seen ants the greenhouse, it is entirely probable they are hiding somewhere in a pot or planting bed. In that case, the Cryptolaemus larvae may be well suited to evade them.

An interesting coincidence is that the spines of the bullhorn acacia, described above, are hollow. The plant has a symbiotic relationship with a species of ant that lives in the hollow (called a domatium, pleural domatia). The ants eat the sap secreted by the plant’s leaves, and in return protect the plant from predators. The next time I pass by the plant I will try to spot ants in the branches or spines.

Hollow spines of the bullhorn acacia tree

Much, much more can be said about pest control in greenhouses in general and biologic control specifically; there are textbooks full of information. The two methods I've mentioned are a superficial introduction.

Here is an illustration of the ant species associated with the bullhorn acacia. The artwork was produced by one of the Conservatory's students.