Yes, you can use Criterion to treat a hemlock for the woolly adelgid.  You should follow the soil drench instructions on the product label.  You should apply 1 pouch or 1.6 oz of criterion for every 24 to 48 inches of trunk diameter.  You should apply the pouch in at least 10 gallons of water to apply the drench.  It really does not matter how much water you use, it just matters how much product you are applying over the area.  The more water you apply it with the deeper the ground will soak it up.  Slightly less than 1 pouch should be used for an 18" wide hemlock.

Mallet 75 WSP is no longer available as it has been discontinued by the manufacturer. We recommend Criterion 75 WSP which has the same active ingredient at the same concentration and pretty much the same label.   You can apply Criterion as a soil drench to control aphids but it will take time to see the results as the tree has to trans-locate the product throughout the foliage which can take some time. Immediately before or just after the soil drench you can simply use a strong stream of water from a garden hose to dislodge the aphids from the tree. Using Criterion on this tree will be safe for the children and pets as long as they are out of the area while the soil drench is being applied.

A soil drench will provide a longer residual as it will soak into the plants inner system and last for typically an entire growing season up to 1 year. Foliar application can be somewhat systemic as well but would need to be reapplied after the weather wears away at it on the surface. Per the product label: For Aphid treatment on Shrubs: Soil Drench: Uniformly apply the dosage in no less than 10 gallons of water per 1000 sq/ft as a drench around the base of the shrub, directed to the root zone. Use 1.6oz (1 packet) Criterion 75 WSP per 24-48 ft of cumulative shrub height. There is not a way to mix up less than what the packet states at a time so unless you have a large tank (at least 100 gallons or so) you would be far better off using a liquid Imidacloprid product rather than the extremely hard to use Criterion. We would recommend Adonis 2F instead for the soil treament. To Apply: Most people use a 5 gallon bucket or a large watering can to apply Adonis as a soil drench. You will need 0.1 to 0.2 fl. oz. (3 to 6 mL) of Adonis 2F per foot of shrub height or 0.1 to 0.2 fl oz per inch of trunk diameter for trees. The amount of water that is used will vary depending on soil conditions and volumes that are easy for applicators to use. The minimum amount of water that should be used is 1 pint per foot of shrub height (or inch of trunk diameter for trees) but you may use 1 quart -1 gallon of water per foot of height (or inch of trunk diameter for trees) as well. Denser soils such as clay do not accept water as readily, so the lower volume of water is suggested.  Since sandy soils can accept more water, using the higher volume of water is preferred. After the soil drench has been applied you should water it in with at east 1/2 inch of water. Note: Water simply acts as a carrier and it is important that even distribution of the product is achieved. Thus, more water may be used if the applicator feels runoff will not occur and proper penetration of the root zone is achieved. Insecticides applied to the soil are taken up by the roots and translocated throughout the tree or shrub. Root/soil drenches offer the advantage of requiring no special equipment to apply (other than a bucket or watering can). However, surface layers of organic matter, such as mulch or leaf litter, can bind the insecticide and reduce uptake. Before applying drenches, it is important to remove or pull back any mulch or dead leaves so the insecticide solution is poured directly on the mineral soil. It is also important to keep the soil around the tree or shrub moist for the next 7-10 days after applications to encourage uptake.