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Copyright 2005, Board of Trustees, University
SOIL INSECT PESTS IN PRODUCTION OF TROPICAL FOLIAGE
R. A. Hamlen
University of Florida, IFAS
Apopka, FL 32703
ARC-Apopka Research Report RH-78-1
Important soil inhabiting insects that frequently are detected in the
production of ornamental tropical foliage plants are fungus gnats, root
mealybugs and springtails. These are the pests that too often escape
detection during production and then become an enormous problem for the
consumer after sale. This report contains information on their biology
and discusses practices useful in the prevention of these insect problems.
An additional insect, the sugarcane rootstalk borer weevil, is discussed
along with the importance of this pest to the Florida tropical foliage
FUNGUS GNATS Foliage growers frequently become concerned about the tiny
'black flies' that can become quite numerous under greenhouse conditions.
These flies are often the adult stage of the fungus gnat (Sciarids), a pest
that when present in quantity is a real threat to foliage plant production.
The adult is approximately 1/8 inch (3 mm) long and has a delicate pair of
wings, many-segmented, thread-like antennae and long dangling legs. Adults
are weak fliers and are most visible near or running over the soil surface
or even under leaves. The soil inhabiting, immature, legless larva is slender,
white with a black head capsule and is about 1/4 inch (6 mm) long during final
stages of larval development. The larvae are often visible on the soil surface
in the early morning and will rapidly move into the soil if disturbed. In many
species, larvae are saprophagous, feeding on fungi and decaying plant tissues;
however, a few species are primary pests of foliage plants and are capable of
feeding on uninjured plant roots, root hairs and lower stem tissues. Organic
soils of high moisture content or conditions of excessive irrigation appear
to enhance infestations, especially in the presence of decaying plant tissue.
Feeding can be particularly injurious to seedlings, rooted cuttings, or young
plants. Feeding injury also can predispose seedling plants to pathogenic
fungal attack. Peperomia and Schlumbergera appear quite susceptible to larval
feeding injury and severe losses can occur in infested propagation beds.
Injury usually begins at the soil surface and is characterized by destruction
of internal root, stem or leaf tissues with severely injured plants toppling
over onto the soil surface.
The common sciarids which are problems in foliage production are species
of the genus Bradysia. Adult female gnats live for approximately one week
and produce in the crevices of the soil surface 75 to 200 microscopic eggs
which hatch in 4-6 days. At 72F (220C) larval development continued for 2-3
weeks followed by a 4-6 day pupal stage in the soil or on the soil surface.
With increased temperatures, the time of the life cycle will decrease and
several generations will overlap.
Controls are usually directed against the larval stages by applications
of chemical drenches, soil surface sprays or granules to the infested soils
of bench or potted plants. Treatment of walk areas and the soil beneath
benches often will aid in control by reducing the potential for reinfestation
of crop areas due to adult migration. Aerosol applications also have been
effective when directed against adults providing applications are made every
few days over a several week period to eliminate overlapping adult stages.
There are, however, several preventative measures which if followed will
avoid in many cases the need for applications of toxic chemicals. These
preventative measures include: media pasteurization, the avoidance of
accumulating piles of plant debris or refuse, avoidance of over-watering,
careful inspection of plant material and soil media, and very importantly,
avoiding plant disease development.
ROOT MEALYBUGS In addition to the foliar mealybugs which are well known to
most nursery personnel, several small species (Rhizoecus spp. and Geococcus
sp.) approximately 1/16 inch (1-2 mm) long are found below the soil surface
and feed on root and root hair tissues. Infestations are often over-looked
until they are severe and widespread, causing reduced plant vigor, foliar
chlorosis and reduced growth. Careful examination of infested roots will
reveal white, cotton-like masses which contain both eggs and adult females.
Nymphs or immatures are active and may crawl from pot to pot via drainage
holes or are passively disseminated in irrigation water. Populations are
slow to develop and 3 to 6 months may be required before infestations are
easily detected. Populations of the root mealybug may become prevalent in
soil beneath infested plants thereby establishing a source for infestation
of future plantings. Numerous foliage species may become infested but critical
major hosts are Araucaria, Asparagus, Chrysalidocarpus, Chamaedorea, Peperomia,
Nephrolepis, Dieffenbachia, Philodendron, Syngonium, Hedera, Epipremnum, Ficus,
Schlumbergera, Dizygotheca, Hoya, Pilea, Cordyline, Chlorophytum and various
Again, control is largely dependent upon the use of chemicals, usually
applied as drenches or granules to infested soil. Applications are frequently
repeated so that both adults and newly hatched individuals are eliminated. It
must be remembered, however, that it is extremely difficult to free infested
plants of this pest. Careful planning and thorough execution of preventative
procedures is once again the most efficient approach to control. Effective
cultural methods that are essential in preventing infestation include: media
pasteurization; thorough inspection of the roots of newly acquired plants
prior to incorporation into your nursery; sanitation, i.e., no refuse piles
or maintaining of plants which have not responded to previous chemical
treatment; and the use of raised benches for all stock, propagation and
finishing operations. Placing of containers on clean or unused plastic
sheeting will aid in preventing infestations.
SPRINGTAILS Springtails or collembola are small insects usually less than
1/16 inch (1-2 mm) in length. All species are wingless, although certain
species possess a springing organ which enables individuals to jump when dis-
turbed. Springtails are rarely pests, although high populations are frequently
detected in highly organic potting soils, especially on the soil surface
following a thorough watering. The presence of springtails in potting media
frequently causes concern that this pest is injurious. Feeding of springtails,
however, is on organic material in soil and the occurrence of this pest is in
general an aesthetic problem.
SUGARCANE ROOTSTALK BORER WEEVIL Diaprepes abbreviatus (L.), the sugarcane
rootstalk borer weevil, is an important pest of sugarcane, citrus and ornamentals
in the West Indies. In 1964 D. abbreviatus was first detected in the United
States in central Florida. By 1977 ca. 49,000 acres (20,000 ha) of citrus as
well as several ornamental nurseries in central and south Florida were infested.
Adult weevils feed on newly developed foliage whereas the subterranean larvae
feed on root tissue and are the primary cause of plant injury and decline.
Infestations of nurseries result in quarantine restrictions with shipment of
container grown plant material outside of quarantine areas prohibited.
Previous studies for control of D. abbreviatus have included tests for suppres-
sion of adults by foliar insecticides, control of large (late instar) larvae
by insecticide dips, evaluations of ovicidal chemicals and laboratory screening
of insecticides against newly hatched neonatee) larvae. As neonate larvae
drop from oviposition sites within foliage to the soil surface, and then
migrate into soil, larvicidal chemicals contained within the upper levels
of the soil present the best method to prevent infestation. Preliminary
studies have indicated preventative insecticide applications are promising.
Efforts to eradicate late instar larvae from highly organic soils have not
been totally successful, a necessary requirement concerning this quarantin-
able pest. Chlordane is not an effective larvicide and currently heptachlor
is incorporated into soil media as a preventative to infestation. Acceptable
alternatives to the chlorinated hydrocarbons are needed as these chemicals
become limited in usability due to governmental restrictions. Because of the
potential economic impact of D. abbreviatus on the tropical foliage industry,
several cooperative studies between the USDA Weevil Laboratory and ARC-Apopka
have been underway since 1975 to determine alternative methods for certi-
fication of ornamental plant material free of D. abbreviatus infestations.
This public document was promulgated at an annual cost of $18.55 or
3.7 cents per copy to inform county and state extension personnel,
foliage growers, marketers and allied trades of research results
and improved practices in ornamental horticulture.