Migration

Migration isn’t something people usually associate with gardening. But they should.

When we talk about migration, we generally mean large groups of animals moving from one region to another, due to seasonal changes, depleted food supplies, safety, and/or reproduction. Obviously, plants do not migrate in the proper sense of the word. [Wouldn’t it be a sight, if they could?]

Monarch butterflies roosting on migration in Texas (Loadmaster) CC BY-SA 3.0

To most people, migrations are left to caribou and wildebeests, whales and salmon, swallows and robins, and monarch butterflies. And therein lies our clue: insects migrate. And those insect migrations can have a huge impact on your garden.

Insect navigation

Insects travel within a low, slow boundary layer, or significantly higher up, using fast-moving air currents. We are rarely aware of these massive migrations. Insects can sense polarized lights, and changes in wind speed and direction, helping them find their way. Insects also have built-in clocks that help them stay on schedule. The magnetic field theory related to bird and mammal migration appears to only impact short distance fliers.

Which insects migrate?

Several butterfly and moth, beetle, dragonfly, and African locust species migrate. [North American locust swarms do not occur seasonally, disqualifying them as migrations.] Even tiny aphids and lesion nematodes migrate, though their trips are significantly shorter.

African locust (Christiaan Kooyman) Public Domain

In some cases, insect migrations work much like bat, bird, animal, and fish migrations: adults get a genetically-initiated urge to travel to a better wintering or breeding area. After spending a predetermined amount of time in the new location, the urge to return strikes, and off they go. In other cases, one generation will take the outgoing flight, and the next generation handles the return trip. In many cases of insect migration, it takes multiple generations to make the complete trip.

Mind-blowing numbers

Between 33 million and a billion Monarch butterflies migrate each year, from Canada to Mexico and back again. Technically, since it takes four generations to complete the trip, these one-way excursions are called emigration, but we’ll ignore that detail. Monarch butterflies don’t harm our gardens, but other migrating insects can and will. And there are a lot of them.

According to the journal Science, over 3 trillion insects migrate over south-central England each year. England’s cold, damp weather makes it fair to assume that those numbers are profoundly higher in warmer areas.

Painted lady, or cosmopolitan (Jean-Pol Grandmont) CC BY 3.0

The distances some of these insects travel is truly amazing. British painted ladies, or cosmopolitans, travel 9,000 miles over 6 generations. Wandering gliders, a type of dragonfly, travel 11,200 miles, with individuals flying 3,730 miles. For an insect that is only 1-3/4” long, it would be the same thing as a 6’ person traveling over 153,000 miles - under their own power.

Wandering glider (Jee & Rani Nature Photography) CC BY-SA 4.0

A few insects that migrate

Our gardens are impacted by several different migrating insects. A few of these pests (and their favored foods) include:

• Western boxelder bugs (Boisea rubrolineata) - almond, apple, cherry, peach, pear, and plum tree fruit, grapes, and ash, elder, and maple trees
• False chinch bugs (Nysius raphanus) - tomatoes
• Leaf-footed bugs (Leptoglossus sp.) - almonds, pistachios, pomegranates, and tomatoes
• Lesion nematodes (Pratylenchus vulnus) - cucurbit, grape, olive, and onion roots
• Lygus bugs (Lygus hesperus) - peaches, pears, pistachio, strawberries; they also carry fireblight and cause catfacing on tomatoes
• Migratory grasshoppers (Melanoplus sanguinipes)
• Stink bugs - almonds, apples, peaches, pistachios, and tomatoes; they also eat seeds, grain, vegetables, ornamental plants, legumes, weeds, and tree leaves

You can join the citizen science movement related to insect migrations by reporting your sightings to The Big Bug Hunt. Your information will be added to countless other sightings to generate ever more reliable prediction models. This can help you protect your plants better, faster, and with less effort, using row covers.​