Two-Spotted Spider Mites and Thrips Thrive in Hot Dry Weather

Hot, dry weather has led to problems with two-spotted spider mites (TSSM) and thrips in cucurbit crops

TSSM feed by piercing individual cells with their stylet and sucking out the contents.  This often causes a bronzing effect or a generalized yellowing that turns to bronze. TSSM often (but not always) avoid feeding on the veins, which remain green. Damaged plants become stressed and begin to abort young fruit or produce misshapen fruit.

TSSM damage can be confused with nutrient deficiencies like manganese or magnesium.  It is important to determine the actual cause of the damage because, in the case of TSSM, foliar fertilizers may actually make the problem worse.

Leaves with dust or sand on them provide a good environment for the mites. If you are seeing leaf symptoms, but can find the mites, look at the newer growth.  Mites often abandon older leaves for fresh feeding.

Spider mites are incredibly difficult to see and often require a hand lens or microscope.

Rainfall and overhead irrigation will help to decrease mites in many crops.  Unfortunately the shape and hairiness of cucurbits provides a lot of hiding places for the mites.

Although thrips are larger than TSSM, the younger stages are still very difficult to see with the naked eye.  They are yellowish green to brown in colour and long and narrow in shape.  The adults are darker and have wings. 

Thrips feed with rasping-sucking mouthparts.  They scape the surface of the leaf and then suck up the fluids. Like TSSM, they feed mainly on the lower leaf surface and avoid the veins.  The rasping damage is initially dark green and then becomes a tan or silvery colour.  Interestingly, some thrips species like the western flower thrips are omnivores and will feed on spider mite eggs. However, feeding on the leaves causes plant stress and leads to fruit abortion and reduced yields.

Ensure thorough coverage when applying a miticide or insecticide. Use a high water volume and nozzles that are oriented forward and back facing to achieve coverage of the lower leaf surface as best as possible.

Insecticides with TSSM and Thrips Activity

Product	Pre-harvest Interval	Activity	Pests Controlled	Additional Notes
Minecto Pro	7 day	rapid	Spider mites Cutworm (Thrips)	Apply before populations reach damaging levels following a resistance management program.   Toxic to bees*
Acramite 50 WS	3 day	rapid	Spider mites	Maximum 1 application per year.
Oberon Flowable	1 day	Slow; control may not be apparent for 2-3 weeks.	Spider mites	Toxic to bees**
Movento	1 day	Slow; reduction in numbers after 3-4 days	Aphids (Thrips)	Toxic to bees – do not apply this product during crop flowering.
Exirel	1 day	Suppression only	Aphids (Thrips)	If thrips populations are high, use a registered insecticide with a different mode of action to reduce thrips populations before applying EXIREL insecticide.   Toxic to bees*
Harvanta 50SL	1 day	Suppression only	Thrips Cucumber beetles	Toxic to bees*
Beleaf SG	0	Thrips stop feeding immediately but it may take several days to see a reduction in numbers.	Aphids (Thrips)	Apply before populations reach high levels.
Malathion	3 day	rapid	Thrips TSSM (resistant populations have been confirmed in Ontario)	Toxic to bees*
Matador/Labamba	1 day	rapid	Striped Cucumber Beetle (Thrips) Squash Bug	Toxic to bees*

*avoid application during bloom period.  If applications must be made during the crop blooming period, restrict applications to evening when most bees are not foraging.

** May be toxic to bee brood. Bee brood may be exposed to residues on pollen and nectar brought back to the hive by bees foraging on flowering crops and weeds. To minimize potential exposure to bees, avoid application if bees are visiting the treatment area

(Thrips) indicates that this is not a labelled pest for cucurbit crops, however it is registered for other pests in this crop group and will also provide control of thrips.