Spider mites

With the hot weather we recently had, you might be starting to notice diseases or insects in your garden. One common garden pest during hot dry conditions is the spider mite. The two-spotted spider mite is actually an arachnid (not an insect), and is quite tiny being ¼ mm or less in length. These tiny little arachnids have piercing mouthparts that puncture the cells of your plants as they suck out the juices. This creates the “flecking” appearance on leaves. They have a broad host range, and are capable of colonizing many common garden plants including: eggplants, peppers, tomatoes, squashes, cucumbers, corn, many houseplants, etc. Adult spider mites overwinter in the soil, and lay their eggs on suitable host plants in the spring and summer. When weather conditions are suitable, the eggs hatch and begin to feed on the plant. When conditions are just right spider mite populations can bloom in no time. One day there are a couple of leaves with spider mites and within a week you can have an entire plant covered. Spider mites, as their name suggests, can spin disorganized webbing over the plant when infestations are high. This is one way to easily identify them.   

The splotchy patches of yellowing leaves are a good symptom of spider mites.

Symptoms and signs

Spider mite infestations are very easy to diagnose. If you see leaves that are becoming flecked with patchy yellowing, or exhibit bronzing then your plant is a good spider mite candidate host. Spider mites live on the underside of the leaf, so to look for them you’ll need to flip the leaf over. It is possible to see them with the naked eye, but a hand lens with at least 10x magnification is best for a good view. If you don’t have a hand lens, and even if you do, you can take a piece of white paper and shake a leaf over it. If you have spider mites, they should fall onto the paper and you should be able to see them begin to slowly crawl.

Close up of infested leaf

Management options

Not all mites are bad though. One of the top beneficial insects for controlling spider mites is other mites! If you smear the mite on a piece of white paper and it is green, it’s likely the parasite. If it’s orange or yellow it is likely a beneficial.  You can buy beneficial insects from companies like Koppert that specialize in biological controls, encourage beneficials through native plants or successive flowering plants. Or you can raise your own mites using subset of the predatory mites and growing them in bran. This can take a bit of time, but it very convenient for controlling mites over multiple months in a large area. Other beneficial insects like lady bugs, minute pirate bugs, and lacewings can also be used to control spider mite infestations with varying degrees of effectiveness.

Turn an infested leaf over and you can see the grainy appearance of the spider mites, their dust, eggs and frass. If you look really closely you can see yellow spider mites with little red dots on their bums.

If the infestation is light you can try to control mites using insecticidal soaps, horticultural oils, miticide sprays, or common soaps (Ivory and Murphy’s oil soap work well). If you’re using soaps or oils, don’t apply while the weather is hot and sunny because these can damage your plants if you’re not careful. With soaps in particular, you can typically apply with a hand sprayer and rinse an hour after application. The soapy water creates a film around the spider mite, suffocating it according to entomologist Dan Hulbert. Adequate watering is important to help your plants deal with the added stress of the spider mites. Some sources cite spraying water on the plants to help control mild infestations of spider mites.

Sources contributing information to this post:

Clemson University Extension bulletin

The Ohio State University Extension bulletin

Virginia Tech Extension bulletin

Colorado State University Extension bulletin

Insects and Diseases in the Garden: Scouting 101

As the summer progresses pests are likely to become a part of your garden. You might have already seen those faint spots that foretell the powdery mildew that will soon be visible on most of your zucchini plants. I know I have. Managing garden insects and diseases is essential for a long bountiful harvest. Even the most prolific variety of tomato or zucchini can have low yields if their resources are spent fighting leaf spots and rots. Commercial growers have a wide range of chemical and cultural controls that they can employ to manage diseases. Home gardeners, on the other hand, have limited chemical options; and most prefer to abstain from chemical controls entirely. This limits us to cultural options, insecticidal soaps, and some sprays, combined into an integrated pest management or IPM approach. IPM uses a combination of multiple techniques to manage pests below a certain threshold. This means that you’ll still probably get disease and insects, but the emphasis will be on keeping it at low levels.

While the slug is no where to be seen, the damage is clear. Low levels can be tolerated, but too much damage and yield becomes reduced.

 While much can be done with plant spacing, attracting beneficials, and rotating crops, at this point in the season we can't change those things. So, we are in defense mode. The first step in defense-mode management is going to be scouting. If you can catch the disease or insect early on, you have a better chance of minimizing its effects. Scouting involves walking through your garden plot and looking for symptoms or signs of disease and insects. Symptoms could be wilting or yellowing leaves, a dark leaf spot, or maybe a discolored fruit. Just to be clear, not every spot you see is going to be a disease, not every insect is damaging, and not every time a leaf wilts is it going to mean you have a root rot. But if there has been plenty of rain and a plant is still wilting, that is a good indicator that you might have a disease. 

These chlorotic (yellowing) leaves could be a symptom of insects, disease or nutrient deficiency.

 Scouting should be done every week. Typically garden plots are small enough that you should be able to find the time to walk through them weekly. It could even be while you are pulling weeds or harvesting. When you scout try to look at both the tops and undersides of the leaves (many insects hide underneath the canopy), and if you see something suspicious make note of it. Diseases can progress very quickly when weather conditions are suitable, and some insects like aphids can have explosive growth in their populations. I went to one commercial grower’s field that was covered in rotting fruit, and he swore that he had been out scouting last week and everything looked healthy. Later that same week though, the weather conditions became perfect for the disease (and the pathogen was present in his field) and the entire field was lost. 

Just a week ago a single leaf from this plant was wilted. Now the whole vine is collapsing.

If properly and consistently done, scouting can help us to identify and start managing small problems before they become huge problems.

Disease guides are ready!

As you may, or may not, know I've been working on a small guide to help home gardeners identify common vegetable diseases. Well the wait is over, the printers have cooled, and they're finally ready. If you're in the Lansing area there is a disease workshop with the Lansing Garden Project at Foster Park on Thursday July 11, 2013 at the Garden Project Resource Center 2401 Marcus St. Lansing, MI 48912 from 6pm to 7:30pm. We'll be distributing these tiny pocket guides and diagnostic hand lenses to workshop participants. More workshops are being scheduled for the Detroit and Flint areas, so if you miss this one there are others. If you're not in the MI area, but would like one, email me and I can send you a PDF.