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.

Wednesday, October 5, 2011

A long winter's nap

This fall, Iowa’s temperatures have fluctuated between freezing and 80 degrees. I started thinking about how insects are adjusting to the temperature swings as they make preparations for winter. My biggest decision the last two weeks was to wear flip flops or fleece. But insects go through a surprisingly complicated process to switch gears from active in the summer to apparently “sleeping" in the winter.

Most insects in Iowa need to go through a period of inactivity or diapause to survive the harsh elements (aka, cold weather and a lack of food). Diapause can be described as having little or no development, greatly reduced activity and metabolism, and increased resistance to environmental stresses. Any life stage can go through diapause, but is usually fixed for a particular insect species. Examples include:
     Egg: grasshoppers, corn rootworm, stalk borer
     Larvae: European corn borer, Japanese beetle, colaspis beetles
     Pupae: seedcorn maggot, fall webworm, sod webworm
     Adults: bean leaf beetle, green stink bug, lady beetles, billbugs
Diapause begins with external stimuli that signal upcoming changes in the environment BEFORE they happen. Usually that means colder temperatures in temperate zones. But in other parts of the world, hot and dry weather may trigger diapause. Hormones inside the insect start to change and are not easily reversed. So once the body starts to enter diapause, that behavior is permanent even if the weather improves.

The most important external stimulus is decreasing photoperiod or day length. Shorter days cue insects to start the process of finding suitable shelter. A common trigger for insects is experiencing less than 13 hours of light a day for 1-3 weeks. Another important stimulus in Iowa is cold weather. Insects are motivated to move before it freezes. Usually photoperiod and temperatures go hand-in-hand in nature. And in the case of field crops, their food source is often eliminated and stimulates diapause behavior.

Insect diapause is huge obstacle to overcome! You should see all types of insects starting the overwintering process right now.

Bean leaf beetle adults overwinter in leaf litter. 
Photo by Purdue Cooperative Extension Service.