Scouting the greenhouse
by Sally Colby
Scouting for greenhouse pests is one of the most critical tasks for greenhouse operators. It can mean the difference between catching a problem in time or total crop loss.
Dr. Luis Canas, associate professor of entomology at Ohio State University, said it’s useful for growers to review the greenhouse layout and create a series of stops where scouters make routine checks.
“Visual inspection is still the best way to discover where pests are,” said Canas. “I understand this takes the longest because you have to be on location, see the plants and flip the leaf to take a look. But it’s still the best way to find problems early.”
Although knockdown devices are more commonly used in the field, Canas recommended using them in the greenhouse to assess pest presence. “Have a clipboard with a white piece of paper, get close to the plant, shake gently and insects will fall onto the paper,” he said. “You can see what’s there and take counts.”
Indicator plants attract specific pest species. For example, western flower thrips are highly attracted to Gerbera daisies and mites are attracted to impatiens. “If you have many other crops in your facility, it’s likely there are thrips present and they will find the Gerbera daisies,” said Canas. “The sample needs to be concentrated on these plants to make sure you find early infestations and take action. Sometimes you have to shift resources to concentrate efforts early on indicator plants. If you wait until flowers are produced to assess thrips’ presence, you’re late.”
If the goal is to detect insects, sticky cards are useful. “Keep in mind sticky cards are only useful to detect adults,” said Canas. “And only to detect adults of flying insects, so they are really good for detecting thrips adults, whitefly adults and fungus gnats.”
Sticky cards will capture aphids, but there may be early contamination of wingless aphids. “Aphids begin to produce wings only when the populations are really high,” Canas said. “My point is sticky cards cannot be the only way of assessing insect presence. They should be coupled with visual inspection. They help by showing trends from one time period to another.”
For example, finding five thrips on a sticky card one day, then 15 or 20 the following week indicates an increasing population. “If I spray or release a biological control agent and the next week I find one thrip, then I know the population is going down,” said Canas. “That’s how sticky cards are best used.” With sticky cards that have squares, it isn’t necessary to count the entire card – the scouter can count what’s on two squares each week. “The key here is from week to week, you have to be consistent,” he said. “If you select two squares this week, the next week select two squares.”
For whiteflies and thrips, Canas recommended one sticky card per 1,000 square feet. However, some recommendations suggest using one sticky card every 10,000 square feet if the grower isn’t looking for a specific infestation of whiteflies. Canas suggested placing clear plastic on the card in order to count insects without getting sticky fingers.
When scouting for western flower thrips, the challenge is that adult thrips lay eggs inside plant tissue. Once the eggs hatch, nymphs cause plant damage, grow to adults and the cycle continues. Western flower thrips are not strong fliers but hairs on their wings allow them to glide.
“Thrips use their rasping mouthparts to break down cells, then consume the contents,” said Canas. “As the cell dies, they leave a brown marking on the plant.” Thrips tend to feed in a characteristic line, sometimes on the edges and sometimes on the inside of a leaf. “The person who is paying attention to infestations of thrips needs to understand how to assess early damage,” he said. “Thrips hide very well.”
In some cases, flowers remain intact despite thrips, but it’s difficult to control thrips once they’re hiding inside flowers. “They really like pollen and will grow well there,” said Canas. “The goal is to control them before we get to the point where flowers are present. This is where the clipboard and white paper can be used. Gently shake the flower and some of those thrips will come out. You’ll be able to identify and count them.”
Thrips can be managed with the predatory mite Amblyseius swirskii. “The benefit is that they are hunters and will follow thrips to hiding spaces,” said Canas. “They capture and eat them. Biological control is a prevention tool – you cannot wait until you see an outbreak of thrips and start releasing then. It wouldn’t work. It’s important to do the releases before the problem starts.”
Aphid damage is primarily done by non-winged adults. Canas said in some species, if the population is large and they detect a decline in their food source, they’ll develop winged forms to fly to another feeding spot. “That’s when they are captured on sticky cards,” he said. “It is key to notice that if we find a winged aphid it means somewhere in the facility we have an outbreak and need to act quickly.”
Several species of whiteflies are often present in greenhouse crops. Adults arrive on infected plant material or from outside the greenhouse. “They lay eggs that look like a football,” said Canas. “They hatch and the first nymph instar of the whitefly is the only one that moves. They’re called crawlers. They move and settle somewhere on a leaf, usually on the underside. They turn into nymphs, then into pupae, then the adult emerges … The adult looks very different than the immature whitefly.”
The banded wing, greenhouse and sweet potato whitefly are the three most common whiteflies. “The adult banded wing whitefly is easy to identify due to distinct wing markings,” said Canas. “The greenhouse whitefly and sweet potato whitefly are easily confused. Both are white-winged adults. You can tell them apart if you see them together. The easiest way to differentiate is to look at the nymphs – they live on the underside of leaves, and that’s where sampling should occur.” Canas suggested flipping the leaves to see the undersides and using a hand lens to identify the species.
Another serious pest is the spider mite, and the two-spotted spider mite will often be the culprit. “Spider mites do not fly, so they will never be captured on sticky cards,” said Canas. “This makes it critical that whoever does the scouting knows how to detect early damage. This is complicated by the fact that the human eye only begins to recognize damage by spider mites after seven days. That means in the best case scenario, if you pick up damage by observation, you’re already a week late.”
Canas said many of the products used control mites will not kill eggs. If insecticides are used, multiple applications two or three weeks in a row are required for good control. “The key is to detect them early,” he said. “That means we have to pay attention to the leaves with visual inspection.”