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Most commonly misidentified insect and mite pests in the greenhouse


February 23, 2021
By Dr. Raymond A. Cloyd

Topics
Figure 1. Twospotted spider mites: Leaves appear stippled or speckled

Proper identification of insect and mite pests, along with the plant damage they cause, is an important component of plant protection and pest management. Some of the most commonly misidentified insect and mite pests in the greenhouse are mistaken for one another due to similarities in appearance of the pest or the damage they cause. Here are some key culprits. 

Damage
Let us first discuss damage. Because twospotted spider mite (Tetranychus urticae), western flower thrips (Frankliniella occidentalis), and leafhoppers all have piercing-sucking mouthparts, the damage they cause may appear similar, though not identical.

Twospotted spider mites use their stylet-like mouthparts to pierce and feed on individual plant cells, which results in damage to the spongy mesophyll, palisade parenchyma, and chloroplasts. Symptoms of feeding damage include leaves that are bleached and stippled with small silvery-gray to yellow speckles (Figure 1). In addition, the upper leaf surface will be mottled. Leaves heavily infested with twospotted spider mites will have webbing on the underside along the mid-rib, and molting (cast) skins will be present (Figure 2). Leaves will be bronzed, turn brown, and eventually fall off plants. 

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Figure 2 – TSSM damage with molting (cast) skins and webbing present, especially when infestations are extensive, on the leaf underside.

Western flower thrips feed on the mesophyll and epidermal cells of leaf tissues using a single stylet in the mouth that punctures plant cells. They then insert a set of paired stylets that lacerate and damage cell tissues, which allows western flower thrips to imbibe cellular fluids. Symptoms of feeding include a silvery appearing leaf (Figure 3), leaf scarring, distorted growth, sunken tissues on the undersides of leaves, and deformed flowers. Black fecal deposits (“thrips poop”) may be present on the undersides of leaves (Figure 4). 

 

Leafhoppers are another insect pest that can cause plant damage similar to twospotted spider mite and western flower thrips. Leafhoppers feed within the vascular tissues of plants including the phloem (food-conducting tissues) and xylem (water-conducting tissues). Their feeding results in leaves appearing speckled or bleached. Nymphs and molting or cast skins of the nymphs will be present on the leaf underside (Figure 5). In addition, no webbing or black fecal deposits (“thrips poop”) will be present on the underside of leaves.  

Figure 5. Leafhoppers: Nymphs and molting (cast) skins of nymphs will be present on leaf undersides, but no fecal deposits .

Although plant damage may appear somewhat similar among the three pests, you can use a 10x or 16x hand lens and look on the leaf underside for the presence of the actual culprit. 

Appearance
Now let us look at pests similar in appearance. Although fungus gnat and shore fly adults may look alike when they are flying around or are present on plants, they are very distinct in appearance. 

Fungus gnat (Bradysia spp.) adults are winged, about 3 to 4 mm (0.011 to 0.015 inches) in length, with long legs and antennae resembling a mosquito. In addition, each forewing has a ‘y-shape.’ 

Shore fly (Scatella spp.) adults, on the other hand, resemble a housefly, Musca domestica, except they are smaller, 3.1 mm (0.125 inch) long, black, with short legs and antennae. Moreover, each forewing has at least five light-coloured spots. 

For comparison, Figure 6 shows a fungus gnat and shore fly adult captured on a yellow sticky card.    

Figure 6: Fungus gnat (left) and shore fly (right) adult on yellow sticky card

Still stumped?
If you still require verification regarding proper identification of twospotted spider mite, western flower thrips, leafhoppers, fungus gnats, or shore flies, then you can submit samples or specimens (live preferred) to an extension agent or independent plant diagnostic clinic. Below is a listing of educational material, which will help you identify all the major insect and mite pests that are encountered in greenhouse production systems:

  • Identification of insects and related pests of horticultural plants (Lindquist, R. K., and R. A. Cloyd). 2005. 
  • Plant protection: managing greenhouse insect and mite pests (Cloyd, R. A). 2007. 
  • Greenhouse pest management (Cloyd, R. A). 2016. 
  • Integrated pest management for floriculture and nurseries (Dreistadt, S. H.). 2001.
  • Pests & diseases of herbaceous perennials: the biological approach (Gill, S., R. A. Cloyd, J. R. Baker, D. L. Clement, and E. Dutky). 2006.  

Raymond A. Cloyd is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. He can be reached at 785-532-4750 or rcloyd@ksu.edu