Potted Lisianthus:

Secrets of Success – Protection from Arthropods
by  Jim P. Price,  Brent K. Harbaugh, and Robert J. McGovern
University of Florida, Gulf Coast Research and Education Center – Bradenton, FL
Published in Greenhouse Grower, February 1998

Part 1 - Plug Production

Part 2 - Bedding and Potted Plant Production

Part 3 - Protecting Lisianthus from Fungus Gnats and other Arthropod Pests

Part 4 - Control Diseases affecting this Crop

Although it may not be the easiest to produce, lisianthus has been gaining in popularity as a potted plant each year.  Our third installment in a four-part series focuses on protecting lisianthus from fungus gnats and other arthropod pests.

            The first two articles in this four-part series emphasized general horticultural methods of plug and potted lisianthus production.  The remaining two articles will feature the protection of lisianthus form arthropods (insects and mites) and microbial pests in the greenhouse.

            Growers sufficiently experienced in ornamentals production to tackle lisianthus are likely to posses the pest management skills to control most of its insect pests.  For the most part, the arthropod pests of this crop are much like those of other floricultural crops:  various caterpillars, whiteflies, thrips, and aphids.  Lisianthus is no more sensitive to these general greenhouse insect pests than are other commonly grown bedding or flowering potted plants.

            However, there is one greenhouse pest that can be vexing to unwary lisianthus growers – fungus gnats.  This article will deal with management of this pest thoroughly, and briefly will discuss other important, although less prominent, insects.

            To escape losses from fungus gnats, growers should be able to identify their larval and adult stages, be familiar with their life cycle, have a knowledge of their activities, and apply the principles of management outlined here.

Identification is Critical

1.)        Appearance.  Fungus gnat adults are about 1/8-inch long, spindly, black flies with long legs and long, threadlike antennae (Figure 1).  They look more like tiny mosquitoes than common flies.  Larvae live in the soil and are difficult to find.  They are translucent gray to white, about 1/4-inch long, maggot- or worm-like with no legs and have well-formed shiny black heads (Figure 2).

Shore flies are the only insect in the greenhouse that can be mistaken for fungus gnats.  However, shore flies are distinctly different from fungus gnats when observed closely since shore fly adults have stout bodies, short legs and antennae, and look more like the common fruit fly than like mosquitoes (Figure 1).  Although shore fly and fungus gnat larvae are similar in size and color, shore fly larvae do not have prominent black heads that are common to fungus gnats.  Shore fly larvae do not feed on roots or leaves of lisianthus so they are only considered a nuisance pest.

2.)        Detection.  The best scouting for fungus gnat adults is performed with sticky traps, identical to those used for thrips, whitefly, and leafminer detection in greenhouses.  The threadlike antennae are a prominent feature of the adult fungus gnat stuck to stick traps.

            If larvae are present, they are most likely found in the very early morning (about sunrise).  They can be seen on the soil surface of a thoroughly wet pot or under leaves that are resting on the soil.  Often, larvae can be detected on the buried surface of a raw potato wedge pressed into the upper soil layer of an infested pot.

3.)        Biology.  These insects can infest a crop from soil or algae within the greenhouse, from contaminated potting soil or transplants, or by flying short distances into the production area.  Fungus gnats almost always are present in growing areas, at least at low densities.

            The development of fungus gnats depends largely upon the production temperature.  Females live about a week and lay 30-120 eggs singly or in batches of up to 30 in the soil; eggs hatch in 4-7 days.  Larval development requires about 8-20 days.  The resting pupal stage lasts about 3-5 days and is located near the soil surface.

4.)        Damage.  Fungus gnats can be unusually damaging to lisianthus because of the growth rate and structure of seedlings.  Since there is no stem elongation during development of the first two to four leaf pairs of lisianthus seedlings, all or part of these basal leaves may contact the soil surface.  Basal leaves cover a significant portion of the soil surface, protecting the soil from insecticide spray residues, and offering harbor and food to soil-borne larvae (Figure 3).

Lisianthus seedlings tend to remain small for an unusually long period, giving fungus gnat larvae the time they need to grow into big eaters and gnaw away at the undersides of basal leaves and roots.  If left unchecked, fungus gnats very often can eliminate large numbers of germinating seedlings and weaken other seedlings and mature plants.

Although adult flies do not damage plants, their presence in even small numbers usually signals a heavy infestation of larvae.  In addition, adults are objectionable to consumers and cannot be tolerated on potted plants in hospitals, grocery stores, or florist shops.  Adults can emerge from immature forms after sale even when none were evident earlier.

Cultural Management
            Sound crop culture denies fungus gnats the conditions necessary for development, reduces need for pesticides, and promotes beneficial parasites.  Fungus gnat problems may result from overwet conditions and diseased roots and should alert growers to poor cultural conditions.

            Overwet conditions can develop quickly in a greenhouse during rainy, overcast weather, especially when automatic irrigation hasn’t been reduced accordingly.  Potting media should be stored dry, and pots and production areas must be well drained.

            Sometimes naturally occurring beneficial parasites can become established and regulate fungus gnat populations.  This frequently occurs when broad-spectrum pesticides are not used in the production area.  One fungus gnat parasite is a fragile appearing wasp, much smaller than the fungus gnats, which may be seen walking on pot surfaces.

Applied Biological Controls

            Introduced beneficials can be purchased for release in greenhouses to aid in fungus gnat management.  One is a predatory mite, Hypoaspis miles, and others are predatory nematodes in the genus of Steinernema.  Suppliers of these and other beneficials can be located in Suppliers of Beneficial Organisms in North America.  For more information, contact the California Environmental Protection Agency, Department of Pesticide Regulations, Environmental Monitoring and Pest Management, Sacramento, CA  95814-5604, 916-324-4100.

            Hypoaspis miles is a brown mite that lives on the soil surface.  Commercial insectaries produce this species for sale mixed with a moist, peat carrier in 1-quart containers sufficient to treat 500-1000 pots.  The recommended amount of mites and carrier is applied to the soil surface where the mite thrives on fungus gnat larvae and other tiny insects.  This predatory mite should be applied to the crop before fungus gnats begin to build up significantly.  One application may be sufficient for control in about 3-5 weeks.

            When fungus gnat populations are already high, up to three applications of the predatory Steinernema spp. Nematodes at up to 1-week intervals may be useful.  The nematodes are shipped ready to be mixed with water and applied as a soil drench.  The nematodes attach the larval fungus gnat in the soil, releasing a bacterium inside the fungus gnat, resulting in death. 

Table 1.  Insecticides permitted for use on lisianthus in greenhouse production for control of fungus gnats.  For resistance management purposes, insecticides are listed by chemical family grouping where more than one entry per family is listed.

Other Important Insect and Mite Pests

            Fungus gnats are the insects that most likely would limit a grower’s production, but there are other arthropod pests that can cause losses and for which growers should be prepared.  Several species of lepidopterous larvae attack lisianthus and these include any of a few species of noctuid larvae such as been armyworm, and other armyworms.  The larvae feed on foliage of all the vegetative stages of lisianthus and may completely devour small seedlings.

            In addition, been armyworms may eat a hole in the developing flower buds.  They remain undetected while consuming petal tissue so that when the bud opens, the petals already have been damaged (Figure 4).

Frankliniella spp. flower thrips and other thrips are generally of the greatest concern at the flower bud and flowering stage.  Thrips adults and larvae hide between the overlapping petals of developing flower buds and can be difficult to detect or control since insecticidal sprays can’t contact them.

            Their feeding caused discoloration of the petals and is only observed after the flowers open (Figure 5).  In addition to the damage done to the flowers, thrips are known to carry various viruses including Impatiens Necrotic Spot Virus.  Thus, extra precautions should be taken to scout for and control these pests.

            Sucking insect pets that can be troublesome include aphids, greenhouse whiteflies, and silverleaf whiteflies.  In addition to directly damaging lisianthus, some may vector viruses and thus require extra precautions and control measures.

Success through Sound Pest Management

            There are other occasional insect pests that attack lisianthus, and sometimes even spider mites can be a problem.  The point is, lisianthus requires the same strict attention to arthropod control as needed for other floricultural crops.  To reduce problems from these pests, growers should practice good crop culture and sanitation and should keep weekly scouting programs in place.

            Once regular scouting indicates that one of the other important insects or mites is likely to cause losses, remedial measures must be applied according to sound pest management practices.  Additional recommendations for specific pest management strategies for these common pests of floriculture crops can be obtained from local cooperative extension advisors.

            Lisianthus can be a tricky crop to produce successfully, but growers who follow these guidelines for control of fungus gnats and other arthropod pests, and follow the principles set forth throughout this series of articles, likely will achieve success.  It is rewarding to produce top quality lisianthus and the extra challenge associated with this crop increases that reward.

Mention of a product does not constitute a guarantee or warranty by the University of Florida or endorsement over other products.  Growers are advised to check registration and label information for all pesticides before the use of a product.