Greenhouse Pest Identification

This page is to help Plant Growth Center users identify common insect pests and disease issues of greenhouse plants. To request treatment of a pest or disease issue, please fill out a pesticide treatment request form. For other questions or assistance, please contact the PGC manager at pgc@montana.edu.

Pest Monitoring

Correct and timely pest identification is vital to effective treatment. Ideally, users should be examining their plants on a weekly basis for insect presence or disease development. One of the most common scouting methods for insect pests is the use of yellow and/or blue sticky traps (pictured below) to monitor pest populations. If using sticky traps, traps should be replaced every one to two weeks; traps left in a greenhouse for months build up so many bugs that they no longer accurately represent the population density. Users can also tap plants against a sheet of white paper, which knocks insects onto the paper for easier detection and identification. Proper greenhouse scouting will only take a few minutes each week and is vital to proper pest and disease management. Pesticide treatments are most effective when pest populations are low to moderate, so catching infestations before they reach extreme levels is key.

Sticky cards used for monitoring pest population levels; different insects are attracted to different colors.

Insect Pests

Aphids

A cluster of small green insects covering a plant leaf

Small soft-bodied insects with rounded bodies and long antennae; usually green to tan in color, but can also be red, orange, or black depending on the species
Feed by piercing leaf tissue with their mouth parts and sucking the sap from the tissue
Typically found clustered together in groups, often hidden on the underside of leaves
Aphids secrete a sticky honeydew waste that can create small shiny specks on leaves; in larger infestations, leaves can become entirely shiny in appearance
When population levels rise, aphids will start to produce winged offspring to disperse to other plants

A group of small green insects A small dark-colored insect with wings

From left to right: An adult aphid with a group of immature aphids (called nymphs); and a winged adult aphid

Thrips

A small tan insect with a long skinny body A cluster of tiny tan insects on a leaf surrounded by black specks of frass (insect waste) Tiny insects stuck to a yellow card with a black grid

From left to right: close-up of an adult thrips; thrips on a leaf, with black frass visible; thrips on a sticky trap with a one-inch grid for size comparison

Tiny yellow, brown, or black insects with long thin bodies, usually just visible to the naked eye
Feed on the outer layer of plant tissue and sometimes consume pollen
Damage appears as silver to tan scrape-like or patchy marks on the leaf surface, often with tiny black flecks of frass (waste) visible
Large populations can scar or distort plant tissues and stunt plant growth

A leaf covered in thin silvery scrape marks Two leaves covered with blotchy tan scrape marks

Thrips damage on leaf surfaces

Fungus Gnats

A small black gnat perched on a leaf A cluster of small tan grubs with black heads Multiple small insects stuck to a yellow card with a black grid

From left to right: an adult fungus gnat; fungus gnat larvae; fungus gnats on a sticky trap with a one-inch grid for size comparison

Tiny black flying insects with long antennae and legs and a pointed abdomen
Adults do not feed on plants and are mostly a cosmetic annoyance, but can vector fungal pathogens
Larvae are small whitish or translucent grubs with visibly-distinct black heads that live in the soil and feed on fungi and decaying plant matter; can damage roots of smaller plants, and the damage can provide an entry point for fungal diseases
Thrive in consistently-moist conditions; to reduce infestations, let top few inches of soil dry out between waterings and keep greenhouse floors cleaned of loose soil and algae

Shore Flies

A small black fly with white spots on its wings A small tan grub Two small flies stuck to a yellow card with a black grid

From left to right: an adult shore fly; a shore fly larva; shore flies on a sticky trap with a one-inch grid for size comparison

Small stout flies with five white dots on their wings, very short antennae, and large eyes
Adults do not feed on plants and are mostly a cosmetic annoyance, but can vector fungal pathogens
Larvae are uniformly-tan grubs with no distinct head visible and live in the soil, but do not tend to damage plant roots
Thrive in consistently-moist conditions; to reduce infestations, let top few inches of soil dry out between waterings and keep greenhouse floors cleaned of loose soil and algae

Mite Pests

Spider Mites

A cluster of tiny yellow mites with black spots gathered on a leaf A small yellow mite with two black patches on its back

Very tiny mite the size of a grain of sand, usually tan with two black spots; often hard to notice until the infestation is advanced
Feed by piercing plant tissue with their mouth parts and sucking plant fluids
Damage appears as fine yellow speckling across the leaf surface; in heavy infestations, damage can increase into patchy yellowed blotches
Larger mite colonies create thin but visible webbing on and between plant leaves and stems, which in large infestations can fully envelop plants
Thrive in hot and dry conditions

A leaf covered in white webbing and tiny mites A leaf covered in fine yellow speckling

From left to right: spider mite webbing covering a plant, with mites visible in the webbing; and spider mite damage on leaf tissue

Fungal Diseases

Powdery Mildew

A pea leaf covered in fine white fungus Wheat plants covered in a dusty tan fungus

Tan to white fungus with slightly raised, fuzzy or dusty appearance; develops on the surfaces of plant leaves and stems
Severe infections can stunt, wilt, or distort plant growth and diminish photosynthesizing capabilities
Thrives when days are warm and dry and nights are cool and humid
Spores are easily spread by air currents to surrounding plants
To reduce risk of development:
- water early in the morning so that the greenhouse is dry by nightfall
- ensure adequate spacing and air movement between plants to reduce humidity in the plant canopy