Mosquitos and black flies and ticks! Oh, my!
You don't even have to ask how they are going to be this year. This is Minnesota. They ride in on the last snow storm of the year; some years even earlier. The first mosquitos are usually the large, slow ones and should really be considered a blessing. They give us some good swatting practice before the little, fast ones show up.
Rhizosphaera needle cast of spruce - If you have had damage in past years apply a registered preventative fungicide when the new needles are about half elongated and again when the needles are fully elongated. It also helps to keep the weeds and grass mowed around the trees to encourage air movement to dry moisture off the needles which prevents future infections.
Birch leafminer - Control is generally only necessary on ornamental trees that have suffered heavy damage in past years. If using a contact insecticide, application must be done at the time the adults are actively laying eggs on the leaves. The adults look like small black flies. If using a localized systemic insecticide like acephate, the insecticide application can be made as the first mines show up on the leaves. The leaves will probably be about 3/4 full size then. Maintaining trees in a healthy vigorous condition is important. For ornamental birch trees this includes watering, fertilization, mulching, avoiding soil compaction and disturbance, and not using lawn herbicides around your trees.
Yellow-headed spruce sawfly - Adults lay eggs about the time lilacs are in full bloom. Tiny larvae emerge about 7 to 14 days later. Trees defoliated last year should be checked for possible control needs during the first part of June. It's too late if they are larger than ½ inch long. Remember, Bt is not effective against sawflies.
Spruce budworm - Larvae begin mining needles and buds in early May. Larvae should be readily evident about the time the bud caps start falling off of white spruce trees. Insecticide treatment is generally done about the time the bud caps start to fall off white spruce trees and about the time the needles on new balsam fir shoots start to flare out from the shoot. Bt is recommended for spruce budworm.