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Horticulture Digest

Date Last Edited:  08/24/2001

Hawaii Cooperative Extension Service

Horticulture Digest #106

Hawaii's potted-foliage growers are concerned about shipment rejection due to live insect pests. During the past year, growers have had problems controlling the palm mealybug (Palmicultor palmarum), root mealybug (Rhizocus hibisci), and two species of slugs--the brown slug (Vaginula plebeia) and the two-striped slug (Veronicella cubensis).

This report describes each pest and summarizes the results of efficacy tests conducted at the University of Hawaii at Manoa, Waiakea Experiment Station in Hilo, Hawaii.
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Palm mealybug on Rhapis excelsa:

White waxy material secreted by the mealybugs are easily detected in the rhapis terminals. Adults and nymphs are found between the stem and rhapis fibers. Fast moving adults are often seen crawling on the foliage. Infestations are usually found when plant s are neglected or have not been sprayed for awhile.

Foliar applied insecticides were very effective against the palm mealybug. Dursban 50W and Talstar 8.0F resulted in 94 and 67% mortality, respectively. Marathon 1G achieved only 17% mortality.

Three insecticides were evaluated for efficacy against the palm mealybug--Dursban 50W, 1.0 lb/l00 gal; Talstar 8.0F, 20 fl oz/100 gal; and the systemic granular Marathon 1G, 1.3 g/6" dia. pot. Dursban and Talstar were applied to the foliage twice at two-w eek interval. The spreader-sticker, Ad-here, was added to both treatments at 6.0 fl oz per 100 gal. Marathon was applied once by distributing it evenly on the media surface and watering it in immediately after treatment.
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Root infesting mealybugs:

Prior to 1990, the major mealybug species causing shipment rejection from Hawaii was the coffee root mealybug, Geococcus coffeae. Since 1990, Rhizoecus hibisci, new to Hawaii, has spread to major potted foliage production areas. In addition, R. caladii has been recently intercepted by California in Rhapis palms from Hawaii.

Root infesting mealybugs occur throughout the root mass; however, they are concentrated between the root ball and the pot. Slow growing plants or pots that are root bound are more likely to get infested.

Infestations of root infesting mealybugs are noticeable only if pots are removed. Insecticide penetration is impeded by the pot, media, root ball, and waxy secretion from their bodies. Plant species and growing media also affect the effectiveness of insec ticides.

Female mealybugs secrete a white waxy material and are usually visible between the pot and the root ball. Female mealybugs lay eggs or give birth to live young (crawlers). If eggs are laid, they usually hatch in less than 24 hours. Crawlers are the disper sal stage and are highly mobile. Once the crawlers find a suitable site, they settle down and begin to feed. The entire life cycle ranges from 2-4 months depending on species. Adults live from 27-57 days, depending on species.
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Preventing the spread and establishment of root infesting mealybugs:

Because the root mealybug is a difficult pest to control, every effort should be made to prevent establishment. The following practices are recommended to prevent establishment and spread:

  • Inspect roots of newly purchased and slow growing plants by removing the pot.
  • Avoid root-bound plants by repotting as needed.
  • Use clean pots and media.
  • Treat or remove alternate hosts outside of greenhouse.
  • Do not allow water from infested areas to drain into clean areas. Crawlers can be transported in this way.
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Chemical control of R. hibisci on Rhapis excelsa:

Unfortunately, we could not achieve complete control with any chemical treatment. Dursban 50W and Talstar 8.0F resulted in only 50 and 44% mortality, respectively. Removal of pots before drenching increased efficacy of Dursban to 93%, but not Talstar. Dif ferences in efficacy may be attributed to root density which may have affected insecticide penetration. Phytotoxicity was not observed with any of the treatments; however, caution should be taken when applying insecticide drenches. Previous research has s hown watering plants prior to drench application will significantly reduce phytotoxicity.

One application of Marathon 1G significantly reduced the mean number of mealybugs per pot. Marathon treated pots averaged 12 mealybugs per pot whereas the untreated pots averaged 70 mealybugs per pot 52 days after treatment.

Dursban 50W, Talstar 8.0F and Marathon 1G were evaluated for efficacy against R. hibisci. Dursban and Talstar were applied as insecticide drenches by completely submerging the root ball in 5-gal buckets containing 3 gal of insecticide solution for 15 sec and gently agitating in an up and down motion 5 times. Controls were submerged in water.

Insecticide drenches were evaluated for efficacy in two ways--with the pots on and with the pots removed. This was done to determine how much the pot impeded insecticide penetration. Efficacy was evaluated by dissecting the root ball, recovering all mealy bugs, and observing for movement under a dissecting microscope. Visual assessment (unaided eye) in the field is not sufficient to determine mealybug mortality.


"Hawaii pesticide law prohibits the use of many insecticides as drenches. Pesticide labels which prohibit drench usage or state the amount of product to be used per acre such as Talstar 8.0F in this study cannot be legally used as drenches. Used drench solutions should be disposed by applying to approved crops or site in accordance with all label directions. Contact the Hawaii Department of Agriculture, Pesticide Branch if in doubt about legal use and proper disposal of insecticide drench solutions."
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Hot-water Treatment of R. hibisci on Rhapis excelsa:

Hot-water treatment (46°, 46.5°, 47°, 47.5°, and 48°C) shows great potential against R. hibisci. All temperature regimes tested resulted in 100% mortality of R. hibisci.

Additional phytotoxicity and efficacy research is needed to determine the thermal death requirement of R. hibisci and heat tolerance of various plant species. Preliminary studies indicate that Rhapis palms may tolerate heat treatments.

The potted part of the plant was submerged in a 250-gal commercial size tank designed by Dr. Marcel Tsang, Agricultural Engineer at the College of Agriculture, University of Hawaii at Hilo. Plants were held in the 49°C water until the internal temper ature of the root ball reached a specified temperature. Five temperature regimes were tested against the root mealybug, 46°, 46.5°, 47°, 47.5°, and 48°C.
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Slug management strategy:

The following suggestions may increase efficacy of slug control programs:

  • Monitor slug populations at night.
  • Locate the source of slugs. A barrier type of treatment may provide satisfactory control once slugs are eliminated from the field.
  • Sanitation is very important in successful management of slugs. Slugs feed on decaying organic matter and eliminating weeds and fallen leaves removes their food source and breeding site.
  • Apply molluscicides after it rains. Slug baits don't last as long; however, slugs are active after it rains.
  • A split application of slug baits is more effective than a single heavy dose.
  • Liquid baits last longer than bullets and granules but has less coverage and take longer to apply. They are best suited as a barrier type of treatment.
  • Do not concentrate slug baits in one spot. This results in less coverage and increases the chances of the bait being eaten by animals. Dogs, horses, rodents, etc. will eat slug baits out of the open bag; therefore, keep the bag out of reach.
  • Use molluscicides without attractants if pets are of concern.
  • Rodents (rats and mice) eat slug baits and seem to have a high tolerance to metaldehyde. Therefore, if rodents are also a problem, eliminate rodents first to increase your chances for successfully slug control.
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The brown slug, Vaginula plebeia Fischer, and two-striped slug, Veronicella cubensis (Pfeiffer), continue to increase in Hawaii, causing severe damage to flowers and foliage. The brown slug was first reported in Hawaii in 1976 on the island of Hawaii. Its color ranges from beige to dark brown. The two-striped slug, with two longitudinal stripes on its back, was first reported on Oahu in 1985. Its color may also vary from beige to dark brown, and its stripes may be solid or broken.

There are many methods to control slugs including:

  • bait molluscicides
  • molluscicide sprays
  • copper barriers and
  • slug traps

Slug baits are a popular and effective method to control slugs that forage on the ground. Most slug baits contain up to 4% metaldehyde. Metaldehyde acts as both a contact and stomach poison. In low doses metaldehyde causes slugs to oversecrete mucus, resu lting in desiccation. In high doses metaldehyde acts as a nerve poison. Methiocarb (Mesurol) is another active ingredient found in molluscicides belonging to the carbamate class of insecticides. Unfortunately, methiocarb will no longer be manufactured, re sulting in the loss of several effective slug products.

Molluscicide sprays are most effective when slugs live aboveground or do not actively forage on the ground. However, because metaldehyde rapidly degrades in sunlight, thorough coverage and evening applications are important. In addition, certain formulati ons of liquid metaldehyde contain tallow (fat) as a attractant. These products have a short shelf life and should be bought as a need arises.

The effectiveness of copper barriers is short-lived in Hawaii because of our high humidity. Once the copper tarnishes, the barrier is no longer effective.

Slug traps or hand picking of slugs may be economical in home gardens but not in commercial production. Slug populations in heavily infested areas number in the thousands per acre.

Slugs are hermaphroditic--they possess both male and female sex organs. A majority of species need to mate. Once slugs mate, both slugs may lay eggs. Depending on the species, 10-200 eggs are laid which hatch in 14 to 30 days. Juveniles reach sexual matur ity in 3-5 months and may take as long as 2 years to become full grown. Juveniles and slugs past the reproductive stage are the most difficult to control.
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Bioassay tests of molluscicides against the brown slug and the two-striped slug:

Efficacy (% mortality) of molluscicides against the brown slug were as follows:

Brown Slug, Vaginula plebeia

Deadline Granules       81%    Corry's Slug and Snail       42%

Deadline Bullets        76%    Deadline One Last Meal       41%

Hacco AG 3.5            62%    Corry's Slug & Snail Killer  38%

Metaldehyde/Methiocarb  60%    Ortho Bug-Geta Plus          35%

Ortho Bug-Geta          58%    Corry's Slug & Snail Death   26%

RCO Slug and Snail      55%    Ortho Slug-Geta*             20%

RCO Rain Resistant      48%    Deadline 40                  18%

Durham Metaldehyde 7.5G	48%    Corry's Liquid S/S Control   14%

Durham Metaldehyde 3.5G	47%    Corry's S/S Insect Killer     7%

Efficacy against the two-striped slug were as follows:

Two-striped slug, Veronicella cubensis

Deadline Granules       84%   Ortho Slug-Geta            36%

Deadline Bullets        66%   Metaldehyde/Methiocarb     33%

Durham Metaldehyde 7.5G 63%   Deadline OLM               30%

Hopkins AG 3.5          60%   Durham Metaldehyde 3.5G    28%

RCO Slug and Snail      45%   Corry's S/S Killer         27%

Corry's Slug and Snail  45%   Deadline 40                18%

Corry's S/S Death       43%   Corry's Liquid S/S Control 18%

Ortho Bug-Geta          41%   Corry's S/S Insect Killer  17%

RCO Rain Resistant      40%   Ortho Bug-Geta Plus        12%

Tests were conducted to evaluate the effectiveness of various molluscicides. Field collected slugs were held in a glass aquarium with a screen cover and fed 'Iceberg' lettuce daily until tests began. A 3-inch thick layer of peat covered the bottom of the aquarium.

Molluscicides were applied at their recommended label rates to 1 ft^2 plexiglass cages. Immediately after treatment, 10 slugs were added to each cage. Each cage contained a l-inch thick layer of peat moss, a leaf of 'Iceberg' lettuce, and half of a 3-inch pot cut longitudinally to provide shelter for the slugs during the day. A plexiglass cover with 1/2-inch holes (blocked with aluminum screen) provided air circulation. Cages were held in a fiberglass greenhouse.
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Longevity and weatherability of molluscicides:

Slug baits were more effective after they were moistened; however, efficacy greatly diminished thereafter. Because granules give better coverage than bullet and liquid formulations, initial kill was high. Unfortunately, because of their size, granules do not last very long.

Deadline Bullets, Hacco Slug and Snail AG3.5, and RCO Slug and Snail Pellet Rain Resistant Formula provided consistently high mortality throughout the test period. Liquids baits such as Deadline 40, Deadline One Last Meal, and Corry's Liquid Slug and Snai l Control provided excellent control under wet conditions. Liquids are best suited as a barrier type treatment. However, poor coverage and the time and labor involved for application are factors which should be considered for commercial application.

Top Molluscicides Tested

                                 % Mortality

                                1 day   7 days

Deadline 40 (Liquid)              42      60

Deadline Bullets                  55      40

Deadline Granules                 54      15

Deadline One Last Meal (liquid)   13      55

RCO Slug & Snail Pellets          45      48

Hopkins Slug and Snail AG 3.5     58      47

Tests were conducted to evaluate the weatherability of the various molluscicides. Molluscicides were applied at the recommended label rate to 1-ft^2 cages consisting of a wood frame, aluminum screen bottom, and a plexiglass cover. Rainfall was simulated f or 5 min. daily (600 ml per cage) using overhead irrigation.
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Do not handle slugs with bare hands. Slugs and snails are known carriers of Angiostrongyliasis cantonensis, a nematode which may cause ensinophilic meningitis in humans and animals.


Each grower has unique conditions. Time of application and growing conditions (i.e., field vs. greenhouse culture) have a considerable influence on the efficacy and phytotoxicity of insecticides. Other factors including tank mixtures, temperat ure, and adjuvants also influence efficacy and phytotoxicity. Under such conditions, pesticides that were reported safe in this report may be phytotoxic.

When using unfamiliar insecticides, the grower should first test the pesticide on a small group of plants before treating the entire crop and continuously check plants (leaves, roots, flowers, active growing points, etc.) for phytotoxicity 4-6 weeks after insecticide application. Systemic insecticides can damage growing tissue and injury may not be visible until new leaves or flowers have emerged. Plants injured by systemic insecticides may never recover or take longer to recover than plants injured from contact insecticides.

Use pesticides safely. Follow the manufacturer's instructions on the use and application as well as storage and disposal. This publication is for educational purposes only. Consult your Cooperative Extension Service or the Hawaii Department of Agriculture for authorized special need (SLN) registrations or additional information.


We gratefully acknowledge DowElanco, FMC Corp., United Horticultural Supply, Valent USA Corp., GemChem Inc., Miles Inc., Meerkat Environmental Industries, and Agridyne Technologies for their support. This research was supported in part by the State of Haw aii, Governor's Agriculture Coordinating Committee, by the USDA, Cooperative State Research Service under a Floriculture Research Grant.

Trent Y. Hata,
Benjamin K. S. Hu, and
Arnold H. Hara, arnold@hawaii.edu
Department of Entomology, CTAHR
University of Hawaii at Manoa

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