Migration probably isn’t something you associate with gardening in Gilroy. But maybe you should.
When we talk about migration, we generally mean large groups, moving from one region to another, due to seasonal changes, depleted food supplies, safety or reproduction. To most people, migrations are left to caribou, salmon, swallows and monarch butterflies. And therein lies our clue. Insects migrate. And those insect migrations can have a huge impact on your garden.
According to the journal Science, more than three 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, such as ours. For example, 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. Monarch butterflies don’t harm our gardens, but other migrating insects can and will. And there are a lot of them.
Several different migrating insects impact our gardens. 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): almonds, pistachios, pomegranates and tomatoes
• 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 and tree leaves
We are rarely aware of these massive migrations. Pests seem to appear out of nowhere. We may not see it, but insects use low, slow layers of air, or significantly higher, faster air currents to move from place to place. They can sense polarized lights, and changes in wind speed and direction. Insects also have built-in clocks that keep them on schedule. The magnetic field theory, related to bird and mammal migration, appears to only impact short distance fliers. And insects are anything but.
The distances some of these insects travel truly is amazing. British painted ladies travel 9,000 miles over six 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-inch long, it would be the same thing as a six-foot-tall person traveling more than 153,000 miles—on foot. Since insects are relatively short-lived, it can take multiple generations to make the complete trip.
You can join the citizen science movement related to insect migrations by reporting your sightings to groups such as The Big Bug Hunt (bigbughunt.com). Your information will be added to countless other sightings to generate ever more reliable prediction models. This can help you protect your plants better, and with less effort, using row covers.
Kate Russell is a UCCE Master Gardener in Santa Clara County. For information, visit mgsantaclara.ucanr.edu or call 408.282.3105 between 9:30am-12:30pm, Monday through Friday.