Insect Pest - True Bugs, Thrips and Mites

In the past, field trials I have conducted on gold flecking were inconclusive as to its causes as they showed more problems caused by environmental concerns than by thrips or mites. However, upon several visits to growers’ fields over the last 4 years that were having mite or thrips outbreaks there were almost always greater rates of gold flecking in the most heavily infested areas of the tomato field.

The hotter temperatures we have had have caused thrips and to a lesser extent two spotted spider mite, TSSM (Tetranychus urticae) populations to rapidly increase in some vegetable fields. These pests feed by puncturing the outer layer of plant tissue and sucking out the cell contents, which results in stippling, discolored flecking, or silvering of the leaf surface (fig.1).

Over the past few weeks we have seen several greenhouse (GH) and high tunnel (HT) vegetable (basil and tomato mostly, but also lettuce, pepper and spinach) operations from around Maryland having problems with thrips.

High tunnels (HTs) are up and running now and we usually do not see many pest problems early on, but there are some potential problems that could develop later in the season that get their start now. One potential problem is when the HT was not cleaned up after the previous year’s crop was finished. Many of these are old tomato plantings, most of which had their plastic, stakes and drip-tape removed but the plants were left to rot in the HT over the winter.

It is unusual that I see or hear about broad mites (Polyphagotarsonemus latus) being a problem in our tomato high tunnels. A grower was having symptoms of twisted growth and browning/bronzing of their tomato leaves this spring and guessed they might have broad mites. They did, with some plants severely damaged while others were fine. The grower had a late fall crop of cherry tomatoes that they kept into December but did not clean up their high tunnel until 2 weeks before they planted their spring crop of tomatoes. Unfortunately, the grower had a small infestation of broad mites in the fall crop of tomatoes that was able to overwinter.

I visited some strawberry fields over the last few days in Maryland. Most of the strawberry fields were on plastic but some were matted row production and a few in high tunnels. With the cooler weather we have had of late I was surprised to still find low levels of mites in the fields, with a few hot spots of mites in some high tunnels. There was only one species of mite found: the two spotted spider mite, Tetranychus urticae.

While visiting some strawberry fields over the last few weeks, I ran into a few areas that had two spotted spider mite (TSSM) feeding. These were mostly in fields that had been using row cover. Spider mites Tetranychus urticae are well adapted to high-temperatures and can complete their life cycle in as little as 7 days when temperatures are > 80º F. The temperatures under row covers in the early part of our season were much above normal and rainfall was below normal leaving conditions hot and dry, which permitted spider mites (if present) to develop quickly. Warm dry conditions along with the nitrogen content of leaves greatly influence TSSM reproduction rate. Applying excessive nitrogen favors spider mite outbreaks.

The Red legged winter mite, Penthaleus dorsalis has been found in more high tunnels (HTs) growing leafy greens this past winter for the 3rd year in a row. Most of the reports I have gotten from HTs are from the Eastern seaboard, but now I am receiving reports from locations further inland, such as Baltimore and Howard counties. Red legged winter mites have dark purple/black bodies and bright red legs (fig. 1). They thrive in what we would normally consider conditions too cold for an arthropod to cause problems.

We have had onion and garlic bulb mites and now this same mite group is being found in spinach crowns where they are called--wait for it--spinach crown mites Rhizoglyphus sp. County educator Ben Beale in southern Maryland found crown mites after he inspected damaged spinach plants and mites also were found in northern Maryland spinach fields.

Over the last three months a few early season high tunnel operations on the Eastern Shore were having problems with some of their seedlings and leaf crops. Crops like spinach would have ‘whitening’ and then browning and eventually dead margins of their leaves while seedlings would collapse. The problem was found to be ‘red legged winter mites’ Penthaleus dorsalis, which is a new pest in vegetables and herbs for us (fig 1).