Effect of Temperatures, and Natural Enemies, on Soybean Aphids

Effect of temperature on soybean aphids: As with other insects, soybean aphids have a temperature range where development is optimum, with development being slower below and above the optimum range. For soybean aphid the temperature for optimum growth is around 27C. At lower temperatures development will be slower. At the other extreme, in experiments where soybean aphids were kept at high temperatures, nymphs did not complete development at 35C and died within 11 days (J. Econ. Entomol. 2004: 854-861).
Soybean aphid consumption by natural enemies: High levels of natural enemies have been reported by some monitoring levels of soybean aphid. Predaceous insects are the easiest natural enemies to recognize; levels of parasitism and insect pathogens are not often as obvious but with training can be estimated as well. Lady beetles, lacewings, and larvae of hover flies are some of the common and easily recognizable predators in soybean fields. When checking soybean fields, consider whether the aphid populations are increasing or decreasing, and the level of natural enemies present.
Common species of lady beetle that may be noted in soybean fields include the sevenspotted lady beetle and the multicoloured asian lady beetle (Figure 1).

Figure 1. Larva of multicoloured asian lady beetle

A study looking at predation rates by these two species of lady beetles on soybean aphids found that for sevenspotted lady beetle third instar larvae could each consume up to 105 aphids per day, adult females could each consume up to 115 soybean aphids per day, and each adult male up to 78 aphids per day when soybean aphids are high. (Environmental Entomology: 2009. 708-714). For multicoloured asian lady beetle, third instar larvae could each consume up to 112 aphids per day, adult females could each consume up to 95 soybean aphids per day, and each adult male up to 53 aphids per day when soybean aphids are high.
Some have been sending in photos of hover fly larvae, which may be found in many crops where aphids are present, wondering what they are. Although the adults, which are good bee mimics, are easy to recognize, the larvae may not be as well known. Note that hover fly larvae (Figure 2) are legless, and narrow towards the head. There are different species that can vary in colour, often brown or green.

Figure 2. Larvae of 2 species of hover flies and soybean aphids

The above information was submitted by John Gavloski, MAFRD Entomologist.  It is available in this week’s issue of the Manitoba Insect and Disease Update which is posted at: http://www.gov.mb.ca/agriculture/crops/seasonal-reports/insect-report-archive/insect-update-2015-08-17.html

Have a follow-up question?

White heads in wheat, could it be wheat stem maggot?

The following information is available in the Manitoba Insect and Disease Update for the week of July 27th & posted at: http://www.gov.mb.ca/agriculture/crops/seasonal-reports/insect-report-archive/insect-update-2015-07-28.html
White heads of wheat (Figure 1) are noticeable in some fields of wheat that should otherwise have green heads still. One possible cause of this is wheat stem maggot.  If wheat stem maggot is the cause, most of the heads will easily slide out of the stem when you pull on them. And if you slit the stems you may be able to see the pale green maggots inside (Figure 2). They also tend to leave frass in the stem that looks like sawdust. In some of the wheat around Carman the white heads are noticeable, but at such low levels that yield loss would be negligible.

Figure 1. White head from wheat stem maggot.



Figure 2. Larva of wheat stem maggot.


This is something that can be very noticeable, because of the way the white heads stand out, but is rarely of economic significance. The are no insecticides registered for wheat stem maggot, nor guidelines for timing an insecticide. So it is worth checking out what is the cause of the white heads, but for wheat stem maggot it is not something to get too alarmed over.
Keep in mind too that there are diseases and environmental factors that can also cause white heads in wheat, so it is not always immediately obvious what the cause is.  More on other causes can be found in the Crop Chatter post “What’s Causing the Bleached Heads in my Wheat Crop?” at http://cropchatter.com/whats-causing-the-bleached-heads-in-my-wheat-crop/
By:  John Gavloski, Entomologist, & Pam de Rocquigny, Provincial Cereal Crops Specialist, MAFRD
Visit the Insect Pages of our MAFRD website at: http://www.gov.mb.ca/agriculture/crops/insects/index.html


Have a follow-up question?

What’s Causing the Bleached Heads in My Wheat Crop?

Based on archived post from September 2014

In 2012 and 2014 bleached, white heads showed up in wheat, barley, and oat fields across Manitoba.  It appears the bleached heads are making a reappearance in 2015.


White heads caused by root rot. (Image: Kansas State University)

There are a number of things that could be the cause:

  1. Insect damage – One of the tests to see if a white head is potentially wheat stem maggot is to try to pull the head out of the stem. If the head pulls out easily, it could be because a larva of wheat stem maggot has severed the stem, resulting in the head turning white. Larvae may be present above the top node.  See Manitoba Agriculture’s website for more information on wheat stem maggot:  http://www.gov.mb.ca/agriculture/crops/insects/wheat-stem-maggot.html
  2. Root rot – infected plants will generally pull free from the soil without much resistance. In years where the plants are under stress due to either lack of moisture or excess moisture, a loss of root tissue from root rots will have a much larger impact.
  3. Fusarium head blight – if seed is produced they are smaller, chalky, and can be shrivelled. Pink or orange mycelium may be visible at the base of the glumes.
  4. Aster yellows – infected plants show white heads, but green stems and seemingly healthy root systems.
  5. Environmental stress – high temperatures, bright sunlight, and hot winds can results in white, empty heads.

Additional information can be found in the July 27, 2012 issue of Manitoba Agriculture’s Manitoba Insect & Disease Report available at: http://www.gov.mb.ca/agriculture/crops/miu/2012/2012-07-27/report.pdf

Submitted by: Pam de Rocquigny, Provincial Cereal Crops Specialist, Manitoba Agriculture

Have a follow-up question?

Question from Jeff: Underground Insect in Corn

Question: I found this in a corn field last week, thinking it is a white grub. Just wanting confirmation on identity.


Answer provided by John Gavloski, Entomologist, MAFRD:

From the photo, this larva can only be keyed to a family of beetles known as Scarab beetles. This family includes the white grubs (of which there are several species) and dung beetles.

It could be an early stage of a white grub. Larvae of white grubs feed on plant roots, but with the species we have in Manitoba they are seldom a pest in corn, and often in patches.

Larvae of dung beetles often look like white grubs, but don’t get as big. We sometimes find them in fields that have had manure applied.  Dung beetle larvae are decomposers, so beneficial.

Thank you for your submission to Crop Chatter Jeff!

For more information on insects, visit MAFRD’s webpage at http://www.gov.mb.ca/agriculture/crops/insects/index.html
Have a follow-up question?