Manitoba Insect & Disease Update

 

 

Diamondback moth continues to be a concern in some areas, although it is the pupa stage that is now dominant in some fields.

Soybean aphids surpassed economic threshold in some fields. In some soybean fields natural enemy populations seem to be building in response to the soybean aphids.

Bertha armyworms are being monitored, and high levels of larvae have been reported from the Holland and Austin areas.

 

Pathogens:

 

 

Reports of root rot, suspected be Phytophthora, have continued to come in from across the province. In severe cases, up to 50% of plants have shown disease symptoms. Plants infected with Phytophthora root rot (PRR) will typically completely senesce and not produce yield. The spores of the causal pathogen are long-lived in the soil, so rotation alone will not be enough to manage the disease in future years. Seed treatments can be used to manage disease early in the season, and resistant varieties are key to managing PRR. Provincial surveys have indicated what races of PRR are present in Manitoba and resistant varieties are indicated in Seed MB with what resistance gene they carry.

 

Table 1. Current races of PRR detected in Manitoba and the resistance genes that are effective against those races.

 

 *Indicates resistance genes currently available in Manitoba (see Seed MB)

 

 

The provincial survey for Fusarium head blight (FHB) in spring wheat is underway and almost complete across the province. In most fields, infection is extremely low. Symptoms are more easily observed in fields that received a rainfall in early July. FHB ratings are also performed in MCVET trials across the province which do not receive a fungicide application. Disease levels in these trials in 2017 have also been low overall.

 

 

 

Some insecticide applications have occurred for diamondback moth. The economic threshold once we get into the late-flowering and podding stages is if larvae exceed 20-30 per ft². Make sure economic populations of larvae are present before deciding to apply insecticides. In some of the fields in the Carman area the majority of the population is now in the pupae stage, with very few larvae remaining. So even if pod feeding is noticed, make sure economic populations of larvae are present. They likely will be another generation of larvae, but this may be too late to be of concern for most fields, but something to watch in late seeded fields. Sometimes populations remain high into the later generations,  other times populations will decline due to natural enemies or weather.

 

 If insecticide applications are necessary for diamondback moth in canola, remember to consider the preharvest interval (which is the time until swathing) of insecticides. Most are 7 days or less, but chlorpyrifos has a 21 day preharvest interval, and may not be an option for some fields.   

Fig. 1. Diamondback moth pupae on canola pods.

 Soybean Aphids

 

Soybean aphids have reached economic threshold, and insecticides have been applied in some fields. In many fields, natural enemy levels are starting to build, which is making the decision making tricky. The Aphid Advisor app can be a useful tool to help factor natural enemies into the decision making process: http://www.aphidapp.com/.

The economic threshold for aphids is 250 per plant on average and the population is increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages. This threshold provides about a 7-day lead time before aphid populations are expected to exceed the economic injury level (670 aphids per plant), where cost of control is equal to yield loss.

 

Estimating yield loss from soybean aphids:  Regarding what amount of yield loss to expect from high populations of soybean aphids, the studies that were the basis for the economic threshold provide information on this. The unit used to measure aphid feeding is the study is called aphid-days; an aphid-day being one aphid feeding for one day. Ten aphid-days could be 10 aphids feeding for one day, or 1 aphid feeding for 10 days. The studies found that yield was reduced by 6.88% for every 10,000 aphid-days accumulated (Journal of Economic Entomology. 2007: 1258-1267). So if there were 500 aphids per plant on average, feeding for 20 days, a yield loss of roughly 7% could be expected.