Otherwise known as Canada Fleabane this plant is from the Composite or Daisy Family (Compositae)
Wasps and flies are the most common visitors of the flowers, where they seek nectar. Among the wasp visitors, this includes Sphecid wasps, Vespid wasps, and Ichneumonid wasps. All beneficial wasps.
Ichneumonid wasps, introduced into the United States to control the European corn borer during the 1930s, are a valuable ally in controlling many caterpillars and other destructive larvae. The dark-colored adult wasps vary in size from less than 1 inch to 1-1/2 inches, and they have long antennae and long egg-laying appendages — called ovipositors — that are easily mistaken for stingers. The adults need a steady source of nectar-bearing flowers to survive.
Among the flies visiting Horseweed are Syrphid flies, Thick-headed flies, Tachinid flies, Flesh flies, Blow flies, and Muscid flies. A few small bees may also visit the flowers.
Syrphid flies are an important natural control agent. They feed primarily on aphids and other soft-bodied insects, such as scale insects.
Although they look similar to house flies, Tachinid flies are very important enemies of cutworms, armyworms, tent caterpillars, cabbage loopers, gypsy moths, sawflies, Japanese beetles, squash bugs, and sowbugs. Growing pollen and nectar plants to attract them builds a balanced and easier to manage garden.
Horseweed also attracts parasitic nematodes. These tiny, worm-like creatures live in the soil and are effective against the scourge of many gardens — Japanese beetles. The nematodes prey on the grubs, the larval stage of the beetle, as well as on armyworms, cutworms, onion maggots, raspberry cane borers, and sod webworms.
One thing to consider is that it can be an invasive plant in some areas of the country. We have not found that to be a problem here in Central Virginia.
Here's a link to photos of horseweed.
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