Hemlock Woolly Adelgid control

The Hemlock Woolly Adelgid is a sucking insect that is like an aphid. It is introduced and therefore has no natural enemies and left untreated will kill Hemlocks. You can find Hemlock Woolly Adelgid by looking for small cottony masses at the crotch of the needles. The good news is that Hemlock Woolly Adelgid is easily controlled in the landscape and treatment can be applied any time of year. Late winter or very early spring treatments are most effective in New Hampshire and New England. The bad news is that Hemlock Woolly Adelgid is almost impossible to control in a forest situation.

The most common control treatments for Hemlock Woolly Adelgid are Horticultural Oil and Merit (Imidacloprid). Here I discuss the benefits and problems of each.

Horticultural Oil is an ultra refined Oil that is almost non toxic to other organisms and is not hard on insect predators.

Benefits
Horticultural Oil will give you quick results.
Is relatively cheap.
Is available at most stores that sell gardening supplies.
Can be easily applied with hand or backpack sprayer if the plant is not too large.

Problems
As oil works by smothering, any insect not covered with oil will not be killed.
Oil has no residual and must be re-applied if new insects reach the plant.
Can burn if it is used when the weather is hot and sunny, or if sprayed before new growth has hardened.
Can be hard to apply on tall trees, or plants that are not accessible to application equipment.

Imidacloprid is a neonicotinoid, which is a class of neuro-active insecticides modeled after nicotine and acts on the nicotinic acetylcholine receptor. Imidacloprid is notable for its relatively low toxicity to most animals other than insects due to its specificity for this type of receptor.

Benefits
Imidacloprid Is a systemic (a chemical that is taken up by the plant) and will travel within the plants vascular system to all parts of the plant where it will kill all the Hemlock Woolly Adelgid.
Will stay in the plant and kill the pest for two years. (some tests have shown that this can be a lot longer.)
Is usually injected into the soil, or sometimes directly into the tree trunk, reducing the chance of drift to non target areas.
Can be applied by digging small holes around the plant and placing the product in the hole, thus allowing plants to be treated that can not be reached by an oil spray.
Has a low mammalian toxicity, compared to some other common pesticides.
Breaks down quickly when exposed to sunlight

Problems
Imidacloprid is expensive.
Takes time to travel from the roots to the branches, thus Hemlock Woolly Adelgid can feed for several weeks and in a heavy infestation may have time to kill the plant before the chemical can control them.
Under certain conditions has been known to travel through the soil to water sources. ( Before applying, read the label and pay attention to situations where this may happen. )
Will kill benaficial insects including predators of Mites, another common problem on Hemlocks, though these pests rarely will kill Hemlocks.
Breaks down quickly when exposed to sunlight

Conclusion
Wherever possible Oil is my first choice for control of Hemlock Woolly Adelgid because of its ease of use, quick kill and low toxicity to other organisams. I try to reserve the use of Imidacloprid for use where the plant is is hard to treat with oil. One other reason that I tend to restrict my use of imidacloprid is because it is so effective on many hard to kill insects. Indeed for some insects, such as those have become resistant to other chemicals it is about the only product that will affect control. Overuse and or incorrect use of Imidacloprid could lead to resistance, or use restrictions, and this would remove a valuable weapon for the careful applicator. Imidacloprid is also the ingredient in most white grub control products for lawns and is used a lot by home owners.



Xavier
IPM OF New Hampshire