How to stop feeding those 'very hungry caterpillars'

Greenhouse environments are making things comfortable for several species of caterpillars. Part of this is related to the plants grown in greenhouses, but it may also be due to the quality of light. That is, greenhouse coverings block at least some of the UV radiation to which insects respond.

Caterpillars, of course, are the wormlike larva of a moth or butterfly. And moths, of course, have very sensitive night vision. (Science Daily at http://bit.ly/1m8R6sf notes that this characteristic is inspiring development of better night cameras for vehicles). Therefore, the greenhouse ‘landscape’ may appear to them very different from the outside world. However, with the growing use of high tunnels, I expect to see an expanded list with more complaints about this group.

The vast majority of caterpillars considered to be greenhouse pests are the offspring of moths. (One group of caterpillars become sawflies as adults.) Most moths are nocturnal and caterpillars are often well-camouflaged and secretive, which explains why damage is often substantial when a problem is detected.

Following are some caterpillars I have encountered as greenhouse pests over the past 25 years. Although not effective enough by themselves for control, pheromone traps (wing- or bucket-type) can be useful for detecting the moth stage. Contact your local extension office for suppliers. The websites on page 42 are helpful for identifying target-species traps. Don’t worry – the pheromones, which are specific to each type of insect, only lure males; any additional moths attracted into the range will not lay eggs to enhance the infestation.
 

Yellowstriped armyworm (YSAW). Greenhouse infestations of this insect seem to correlate with outdoor outbreaks, where it can be a pest on various crops like corn, sorghum, soybeans, and tomatoes. Indoors, I have found it on roses, dahlia, pelargonium, cyclamen, and some edible greens, but it will feed on many others.

The caterpillars are greenish when small but become brown to black with bright yellow lines along either side of the back when older. They often hide under foliage or near the soil but look for ‘windowpaning’ of leaves (underside is eaten leaving the transparent upper surface) or their droppings when monitoring. The adults are brownish moths with pale markings. A University of Florida webpage (http://entnemdept.ufl.edu/creatures/veg/leaf/yellowstriped_armyworm.htm) provides good photos and descriptions. Migrating north each year from the south to more northern areas of the central and eastern U.S., YSAW typically appears in the northeast as a pest after mid-summer. A related species, the western yellowstriped armyworm, is present in the western U.S. The pheromone for YSAW is available from suppliers but usually as a special-order item.
 

Diamondback moth (DBM), imported cabbageworm (ICW), and cabbage looper (CL). These are common and widespread pests of brassicas including ornamental cabbage and kale and every year growers here need to do some kind of preventive treatment. While DBM is notorious for developing resistance to insecticides, it has not been an issue in greenhouse ornamentals, though it is a consideration for greenhouse growers of vegetable transplants. DBM caterpillars reach 1/3-inch long and the adult is a small brown moth (1/3 inch) with diamond-shaped spots down the center. It appears to overwinter regularly in our area (USDA Zone 6). ICW is among the few butterfly pests among caterpillars. The adults are the common white butterflies often seen on flowers or fluttering over fields of mustard, cabbage, and related plants and it overwinters even in the northern U.S.. Caterpillars are dark velvety green, up to 1¼-inch long. Although reported on some other ornamental hosts like nasturtium, I have only encountered it on brassicas. CL is generally migratory in the northern U.S., appearing around late spring to mid-summer. The green caterpillars, sometimes with pale stripes, grow to 1.5 inch with a ‘looping’ movement. They feed on many other kinds of plants including salvia, pelargonium, and tomato. Adults are dark brown with silvery markings. Pheromone traps are available for detecting DBM and CL. Helpful factsheets for these three insects can be found at http://nysipm.cornell.edu/factsheets/vegetables/default.asp.

Florida fern caterpillar (FFC) is a tropical insect confined to true ferns. It is occasionally encountered in northern areas, hitchhiking on plant material brought up from the south. Green when young, older caterpillars may be green, brown, or black, sometimes with paler bands along the sides and grow to about 1.5 inch. Their coloring and the divided foliage of ferns make detecting caterpillars and damage sometimes difficult. Adults are small brown moths. Good images and information are found at http://bit.ly/1cuwt4g. No pheromone is available for FFC. FFC is known for developing a resistance to pyrethroid insecticides.
 

European pepper moth (EPM), an invasive pest from Europe, was found in California begonias in 2004 and again in Florida and California nurseries in 2010. It now appears to be established in southern areas of those states and reported from 12 others around the U.S. With an extremely wide host range, the ability to survive on organic matter, reputation for severe damage to stems, and habits that make detection and control difficult, there is concern EPM will eventually become a major pest in greenhouses and established outdoors along the west coast. Some growers have already seen serious damage to crops such as kalanchoe. A pheromone is available and helpful for determining whether the moths are present. Some research found traps suspended over a pan of water are even more attractive, as the insect is apparently fond of wet conditions. Check under leaves close to the media surface for webbing, leaf damage, and other signs of infestation. The caterpillars are pale tan to brown with pale gray spots, reaching a length of just over 1-inch long. For more see http://bit.ly/Ljoh1t.
 

Greenhouse leaftier, also known as the celery leaftier, has a fondness for plants in the family Compositae, which includes those with daisy-like flowers such as lettuce, mums, and cineraria, but will also feed on carnation, pelargonium, celery, begonia, and more. The caterpillars are pale green and grow to about ¾ inch, feeding among leaves webbed together with silk. Stems are also damaged. The moths are about ¾-inch long, usually red- to gray-brown with darker markings. The dense growth habit of mums makes detection difficult and injury can be extensive in mums and cineraria before noticed. No pheromone is available for this insect, so rely on visual observations for detection.

A few other caterpillars are occasionally seen as pests of greenhouse plants, including tomato pinworm known for mining the leaves and fruit of tomatoes, and tobacco budworm for devouring flower buds of nicotiana, geranium, calendula, and others; and and European corn borer for tunneling in stems of mums and other plants.

Amidst all this, there is good news: most caterpillars are susceptible to some type of specialty or systemic insecticide or IGR (see chart). Bt materials are usually best when caterpillars are small. And since many will feed under leaves or in dense canopies good coverage is important. Where multiple stages (adults, caterpillars, pupae, eggs) are present, repeat applications are usually needed to halt entrenched problems. For some, like European corn borer, pheromone traps can be helpful for timing controls when the moths start to appear and before damage is apparent.

If unsure of the caterpillar type, bring samples to your local extension office for identification. Knowing the culprit can help in determining a management strategy.

Daniel Gilrein is a frequent contributor to Greenhouse Management and an extension entomologist at Cornell University Cooperative Extension of Suffolk County.