Spider mites are an economically important crop pest, especially in tomatoes. Dr. James Walgenbach, professor and Extension specialist in entomology and plant pathology, North Carolina State University at the Mountain Horticultural Crops Research and Extension Center, and Dr. Tom Bilbo, a post-doctoral researcher in the Walgenbach Lab, discussed the biology and potential biological control of the two-spotted spider mite (TSSM).

Walgenbach said TSSM is a serious pest of tomatoes. Adults are usually found on the undersides of leaves and are visible to the naked eye. “They’re one of the mite species that can produce webbing on the leaves,” he said. “It isn’t until there’s a very high population that webbing appears, but if you get to where there’s webbing, it’s difficult to knock back populations regardless of what you do.”

TSSM can result in indirect damage to tomato plants through feeding. “It causes early plant senescence and reduced yield,” said Walgenbach. “Most yield reduction we see is due to fewer number of fruit rather than fruit size.” When populations are large, mites feed directly on tomatoes, resulting in gold fleck.

Tomatoes are an ideal host for TSSM for several reasons, one of which is that tomato production is often a monoculture crop with high fertility. “Even in organic systems, a fair amount of pesticides go into the system,” said Walgenbach. “Some pesticides can cause flare-ups of pest populations and a lot of them reduce natural enemy populations.”

The generation time for spider mites is short, which means high potential for reproduction. “They also have limited mobility,” said Walgenbach. “Populations stay in one area and sometimes there isn’t a lot of rotation of tomatoes in organic systems, and those populations become well-adapted to tomatoes. Even though spider mites have a wide host range, it still takes time to adapt to an alternative host.”

Walgenbach explained the method for TSSM sampling: “They prefer to feed on the upper portion of the plant,” he said. “We take the first fully expanded leaf from the top of the plant and sample the terminal leaflet on that leaf. We’re looking at 10 leaflets in three to five locations in the field.”

Pyrethroid and neonicotinoid insecticides can lead to increased mite populations as can some fungicides. Pesticides that can suppress populations include many acaracides and spinosad. Several generalist predators are effective against the TSSM including Coccinellids (lady beetles), Anthocorids, Geocorids, Nabids (damsel bugs) and Phytoseiids, which are somewhat effective native predatory mites.

Walgenbach said Neozgites floridana, a naturally occurring fungal pathogen of spider mites, is an effective control. “It can decimate a population during very humid conditions,” he said. “But once the weather dries, the pathogen is no longer active.” For conventionally grown tomatoes, fungicides have essentially eliminated this fungal pathogen.

A promising natural enemy, Phytoseiulus persimilis, may be the answer. This specialist feeds only on spider mites. “It’s a voracious feeder of mites,” said Walgenbach. “It can eat up to 20 TSSM eggs or 10 adults per day. It’s well-adapted to high temperatures and high humidity. It’s the most commonly sold natural enemy throughout the world.” He added that P. persimilis is used on strawberries and in greenhouses.

Bilbo explained research to improve the use of P. persimilis, including better economics and more reliability to appeal to growers. His research focus is determining where naturalized P. persimilis overwinters and whether natural populations can be enhanced to show up earlier and be more widespread.

One research goal is obtaining a better understanding of where P. persimilis overwinters between growing seasons. “They’re in the field, the season goes on, then fall comes, frost comes and vegetables are done,” said Bilbo. “P. persimilis need spider mites. They disperse, but where do they go?”

Since P. persimilis is an obligate specialist, its appearance is expected exclusively where spider mites are living. “They don’t have the capability to hunker down into a dormant state,” said Bilbo. “They need some degree of activity.”

Research questions include where P. persimilis goes after a crop, where they overwinter and if their habitat can be manipulated to enhance early season predators. Bilbo said research involved searching for P. persimilis hosts and habitat and collecting plant and leaf litter samples around the plants. “We know P. persimilis predators are out there,” he said, “but they’re dispersed and there’s low probability of finding them.”

Since strawberries overwinter and are a known host of spider mites, researchers looked among strawberries for P. persimilis. Researchers also wanted to determine whether P. persimilis would be attracted by mint sentinel plants among late-planted tomato infested with TSSM.

“The later tomatoes were there, and hence spider mites and P. persimilis, the more likely they would still be in that localized area,” said Bilbo. “They haven’t had a chance to disperse yet. Once they go through fields and eliminate spider mites, they need to find more spider mites.”

Bilbo put P. persimilis in one strawberry field and found it every month throughout winter, indicating that it can survive in strawberry fields through winter. This could potentially provide a means of survival between autumn and spring.

Mint plants attract P. persimilis, so Bilbo wondered if small plots of mint attract them, build numbers, then when early tomatoes or other vegetables are nearby, P. persimilis can move in.

One trial involved releasing 20,000 P. persimilis per acre in a small section of the field when TSSM first appeared. Bilbo said releasing it into the whole field isn’t economical and isn’t necessary. Following release, spider mites and P. persimilis were counted weekly. In 21 commercial tomato fields, P. persimilis was detected in four fields and suppressed TSSM in two of the fields. Bilbo said in some cases, P. persimilis showed up too late to be effective.

A different trial involved a three-acre tomato field divided by drive rows into 11 sections. Bilbo released 2,500 P. persimilis into one section, then sampled the sections weekly. He found that the P. persimilis population dispersed over the field, continued to grow and took out TSSM completely. However, the P. persimilis population was also decimated due to the elimination of its food supply.

Another project involved releasing purchased or lab-reared P. persimilis for earlier and more reliable spider mite control. Bilbo listed several questions to be answered when releasing predators: Where will they be released, how many, when will they be put out and what’s going on in surrounding fields?

Remaining research questions include how to improve the use of P. persimilis in larger fields and determining how far it will range. One of the biggest challenges is the economical factor: whether lab-reared or purchased, P. persimilis is expensive.

“There is an impact if there are surrounding crops with spider mites in them because it will draw P. persimilis to them to some extent,” said Bilbo. “Miticide applications might influence dispersal. Another big question is how to make this system more economical. We know they can be good, and we want to make them economical.”

by Sally Colby