Thursday, October 20, 2011

Can bugs be good and bad?

Building on my last blog topic, I am still amazed by how insects adapt to cold weather. Most insects seek refuge in the soil or under leaf litter, but some are adapted to finding cracks in rocky hillsides. Well, Iowa isn’t exactly the most diverse landscape and so many insects have to adapt. Unfortunately, that means some insects try to seek shelter in homes and other buildings. When insects are found inside, most people are very frustrated because they are a nuisance. But most people can keep these accidental invaders under control by sealing crevices around the house and removing them with a vacuum. 

There are a couple of fall invaders in Iowa that are especially annoying. But the multicolored Asian lady beetle (Fig. 1) probably tops the list. I think of it as a beneficial predator – a good bug – to have in field crops. The multicolored Asian lady beetle is one of the most abundant predators in corn and soybean. The larvae eat about 25 aphids a day and the adults can eat 15-65 aphids a day. Larvae feed for about two weeks and adults can live up to 90 days. That is a lot of aphids consumed in a lifetime. Egads!

Figure 1. Multicolored Asian lady beetle forewings range from yellow-orange to red with zero to 19 black spots, or may be black with red spots. Photo by Hedwig Storch, Wikipedia.

One myth about this beetle is that it was intentionally released to control soybean aphid. Not true! It was released in California around 1916, and was kept localized until the early 1980s. Since then, it has spread throughout the United States and Canada, and was confirmed in Iowa in 1994. So it was here before soybean aphid was confirmed in North America. 

Ok, it’s not all sunshine and lollipops. Here is where a good bug can do bad. The multicolored Asian lady beetle is native to China and Japan, and normally overwinters in the south-facing rocks in the mountains. They are especially attracted to prominent, isolated areas on the horizon, and light-colored objects. Adults like to mass on these structures and enter diapause. In areas that lack mountains and rocky hillsides (aka, Iowa), these beetles will attempt to enter human structures.

 Multicolored Asian lady beetles like to mass inside structures to survive the winter. Photo by Robert Koch.

In addition to invading homes, the adults can secrete a defensive chemical from the leg joints that is very offensive (e.g., wet, dead leaves). The chemical can cause stains and become an allergen. It can also taint wine grapes with a rancid peanut butter flavor.

But the biggest problem I have with multicolored Asian lady beetles is that they are too competitive! They are more aggressive and tend to displace our native lady beetles. The lady beetles eat aphids but will also eat other lady beetles. 

Overall, the multicolored Asian lady beetle is a good thing to see in field crops. Even though some may see it as a pest, I think the benefits outweigh the negatives. For more information, Robert Koch wrote an excellent review of multicolored Asian lady beetle biology.

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