Recent research from North Carolina has suggested that there are times where tank mixing an insecticide with your nitrogen can be cheaper than using scouting and thresholds for cereal leaf beetle. The biggest reason for this is because there is a yield penalty for driving over wheat after jointing and because scouting costs money.
These recommendations were also made when wheat was $6 a bushel. However, when wheat is $4 a bushel, you lose $2 an acre using the tank mix approach. This is one big reason why scouting and spraying at threshold is the Extension recommendation (threshold listed below).
Here are some other things to watch out for:
- You need to start scouting right when peak egg lay occurs. However, once the peak occurs many fields will need only a single scouting for eggs and larvae. If the proportion of eggs in the sample is 50% or greater then sample again in 5-7 days. In NC, peak cereal leaf beetle egg lay generally peaks in mid-March around the southeastern coastal region, late March to early April in the Coastal Plain and northeast, and mid-April in the Piedmont.
- Insecticides are effective only on the larvae, not the eggs. Larvae tend to peak 2-3 weeks after egg lay. Furthermore, rain and other weather events can kill eggs. It’s better to time your spray when a few small larvae have hatched. The threshold is 25 eggs plus larvae total per 100 tillers (this is an average of one per each four tillers or 0.25 eggs plus larvae per tiller).
- Any insecticide sprayed prior to March 15 does not have an effect on cereal leaf beetle. Insecticide sprayed after this date might only slow the rate of infestation down. So it’s a good idea to scout your fields even if you’ve sprayed. Remember that cereal leaf beetle can still overwhelm your field if they invade in high densities. Insecticide residual for this insect runs out after about a month.
- Cereal leaf beetle tend to be worse in thin stands. Contrary to common opinion, this is not because cereal leaf beetle prefer thin wheat (they actually prefer thick and healthy wheat), but because there are simply more beetles per tiller in thin stands compared to thick ones.