Tamarillos, aphids and IPM

As you probably know, in addition to medicinal plants, there are fiber, dye, ceremonial and food plants in the Medicinal Herb Garden. In recent years I’ve been growing a bed of South American food plants, including oca (Oxalis tuberosa), yacon (Polymnia sonchifolia), jaborosa (Jaborosa sativa), tarwi (Lupinus mutabilis), anyu (Tropaeolum tuberosum), and most recently, tamarillo (Solanum betacea). Early last year, I started some tamarillos from seeds. Only one survived but it was a keeper. It grew to 4 feet in the garden before the cold weather set in, so I potted it up and put it on a bench in the greenhouse. It soon grew another 2 feet and started putting out flowers…then the aphids (I’m not sure which of the many genera of aphids, let alone what species we have) hit. Not just a few aphids but thousands. Every leaf was covered top and bottom. Even the flowers were covered.

But just as I was considering jetting them off with a high pressure water hose, I started to notice the aphids were turning silver and when I looked closely, I could see little holes on top of them. Some creatures were parasitizing them. They were taking care of the problem quietly and efficiently and doing a much better job than I could have with my hose.

In the Botany Greenhouse, we have at least two aphid predators. One is a species of gall-midge known as Aphidoletes aphidimyza. The larvae of these gall-midges inject poison into aphids that dissolves their insides. They then suck the aphids dry, leaving empty aphid mummies. I didn’t look closely enough to get a good idea of what percentage of aphids had been killed by gall-midges.

What I first noticed were the holes on the backs of the dead aphids and the change in color from green to silvery, obviously the work of different predators, larvae of a tiny wasp.  The adult wasps, a species of Aphidius, probably either matricariae or colemani or both, are so small  that you have to look very closely to see them. By the way, you have nothing to fear from them…unless you’re an aphid. If you’ve seen the movie, Alien, you know that a human was the host for the alien. Pretty gross. That’s the way that aphids are hosts for Aphidius larvae. Better them than us!

Look at the aphid mummies. The holes on their backs are the escape hatches for the wasp that has eaten its way out of the aphid. It started its life cycle as an egg laid by its mother on the back of an aphid. After the egg hatches, the larva feeds inside the aphid, ultimately killing it. Then it chews its way out and flies away to deal more death and destruction to aphids. If you click on the image and look closely, you can see a larva of a gall-midge.

It’s a tribute to the efforts of Doug Ewing, the former Botany Greenhouse manager, and his amazing staff, that so many counterbalancing predators have been introduced to the greenhouse to keep the vast array of plant pests in check. This strategy of using biological controls is part of an overall system known as Integrated Pest Management or IPM. Everyone employing IPM has a different threshold for acceptable levels of herbivory. In the greenhouse where research is conducted, the threshold is often quite low, so it’s also sometimes necessary to spray insecticides in the research rooms when pest populations get out of control. It’s difficult to keep a state of  perfectly balanced equilibrium at all times. In fact, it’s impossible.

In the Medicinal Herb Garden I have a lot more leeway to let things go. I use no pesticides except, rarely, in spring, when seedlings are first transplanted into the garden, an organically certified pyrethrum product to kill destructive ants. My tolerance for pests is high and populations are low due to the wide variety of plants that attract beneficial insects and the open air that allows freedom of travel for all sorts of organisms. The constant warmth, closed environment and close spacing in the greenhouse is much more conducive to sudden explosions of pests. In the garden, it isn’t long after the aphids find their favorite plants that lady beetles (Coccinella spp.) arrive. The larvae of lady beetles are voracious predators of aphids. They look a little like miniature, crinkly gila monsters with extra legs. But very small birds, known as bushtits (Psaltriparus minimus), are also quite effective at aphid control, and they often come through the garden in flocks. They love to take water baths when the irrigation sprinklers are turned on and are, in all ways, a joyous presence in the garden. Thank you, bushtits.

Other than aphids and pavement ants (Tetramorium caespitum) that will girdle the stems and kill certain seedlings, and slugs (usually Arion hortensis) that can be a problem, especially in cool weather when young plants are growing slowly, the Medicinal Herb Garden is largely free of noticeable pests. If they’re not noticeable, who cares? I can usually hand pick anything that is causing serious problems, at least until the beneficial predators show up. The wider the variety of plant families, the more diverse the fauna, and diverse ecosystems, managed or natural, have, in my limited observations, proven to be the most resilient and balanced.

Thanks to a nearly invisible wasp, we will be able to finally try the fruit of the tamarillo at the Botany Greenhouse. When we do, I’ll let you know how they taste.

Stuck in the southeast corner of the greenhouse, the tamarillo is doing well.

Immature tamarillos that better ripen if they know what’s good for them. I want to try a tamarillo.

Still green, two weeks later but they will ripen…they must ripen.

keeping a balance

not as easy as it sounds

but it all works out

See you in the garden.