2008 Florida Citrus Pest Management Guide: Citrus Root Weevils¹

C.W. McCoy, M.E. Rogers, S.H. Futch, J.H. Graham, L.W. Duncan and H.N. Nigg²

Citrus root weevils represent a complex of species known to infest citrus trees and various alternate host plants in Florida. The most common species infesting citrus in order of greatest geographical distribution are the blue-green citrus root weevils, Pachnaeus litus and Pachnaeus opalus, the Diaprepes root weevil, Diaprepes abbreviatus, the little leaf notcher, Artipus floridanus, and the Fuller rose beetle, Asynonychus godmani. Other lesser species inhabit citrus on occasion.

All citrus root weevils have a similar life cycle. They have three immature stages: egg, larva, and pupa. Adult weevils emerge from the soil and lay eggs on host plants above ground, the larvae enter the soil to feed on roots, and the pupae and teneral adult stages are spent below ground. Adults emerge from the soil throughout the year. Peak emergence varies within species and by geographical region (ridge vs. coastal and interior flatwoods). Peak adult emergence for the blue-green root weevils and Fuller rose beetle is normally April and May. Diaprepes adult emergence from the soil peaks in late-May to early-July, while peak adult abundance on the tree canopy parallel adult emergence in May/June but can have a second peak in late-August to mid-October. The second peak is sporadic. Little leaf notcher has three generations per year. Although there is some overlap of generations, adults appear most abundant in a tree in April/May, July/August, and October/November. All adult weevils are attracted to the nonreflective silhouette of the citrus tree trunk. Little leaf notcher and Fuller rose beetle are flightless and must crawl up the trunk, other species will fly to the canopy.

The most visual plant damage resulting from adult feeding is notching of the margins of leaves of young, tender shoots. Notching patterns differ slightly among species and can be confused with grasshopper injury. Prolonged leaf feeding by adults appears to cause no economic effects in mature groves; however, on occasion, feeding will cause virtual defoliation of small replants.

With the exception of little leaf notchers that prefer a weed host, larval feeding injury to the roots by other root weevils, particularly Diaprepes root weevil, can have a devastating effect on citrus trees since all larval stages feed on the roots for most of the year. Tiny hatchlings feed on fibrous roots whereas larger larvae feed on the larger structural roots, forming deep grooves as they consume the outer bark, including the cambium layer. Roots may be girdled and killed in the process or the crown may be girdled causing tree death. Larval feeding sites predispose the root system to infection and girdling by Phytophthora spp., thereby exacerbating economic loss. The rootstocks, trifolate orange and a hybrid "Swingle" citrumelo, are resistant to the complex of P. nicotianae and Diaprepes root weevil while Cleopatra mandarin is susceptible to this complex. When P. palmivora is coincident with P. nicotianae in fine-textured, poorly-drained soils, Swingle citrumelo is more vulnerable to attack by the complex than is 'Cleopatra' mandarin. See also Phytophthora Foot Rot and Root Rot, PP-156.

Pest Management

Sampling Methods of Root Weevil Larvae and Adults

The population abundance and distribution of endemic citrus root weevils, regardless of species, vary from grove to grove, within a grove and within a season. The seasonal abundance of adults within a citrus grove can be monitored using ground traps to capture emerging adults or via visual sighting of adults in the tree. No methods exist for monitoring larvae in the soil. By monitoring adult emergence, using traps, the approximate time and intensity of adult emergence can be estimated for each infestation. By knowing the species of weevil and their seasonal emergence pattern from soil, a grower can apply adult control measures when weevil populations are highest. Current research suggests that adult emergence coincides with the onset of summer rains in late-May through June, soil temperature, and the summer flush in central Florida groves.

Cultural Considerations

Citrus root weevil management begins with the selection of a Phytophthora-resistant rootstock that is certified weevil-free. Optimal soil drainage is fundamental to citrus root weevil management, particularly in heavier soils common to the coastal and interior flatwoods where insect and pathogen are high. Tree decline associated with Diaprepes distribution is often patchy within groves and most obvious in low lands. Stressed trees frequently harbor higher populations of adults because these stressed trees frequently generate more leaf flushes as food for adults. Spot treating these locations with a chemical or biological agent should be helpful. Regular fertilization and irrigation are crucial to new root growth in weevil-infested groves. Fertigation at monthly intervals has been used by growers to promote the growth of fibrous roots after Diaprepes has destroyed the taproot and inner crown of the tree. Skirt pruning and trunk banding can be effective in controlling flightless weevil species. Weed control is also needed to prevent movement into trees from stems of grasses and/or broadleaf weeds. Weed control is probably beneficial in reducing populations of alternate host plants. Current research suggests the use of sound cultural practices by the grower should be adequate for managing all citrus root weevils on mature trees except for the Diaprepes root weevil.

A wide range of parasites, predators and pathogens attack citrus root weevils at one or more developmental stages within the tree canopy or in the soil. Most of these natural enemies are widely distributed and are general feeders. By focusing on cultural tactics favoring tree health and not using chemical methods, growers are augmenting the natural enemies of citrus root weevils.

Pest Control Considerations

Pest management of Diaprepes and to a lesser extent, other citrus root weevils, must begin with control of different life stages, particularly adult weevils using the following options: 1) foliar sprays for egg and adult suppression, 2) chemical barrier for larval control, and 3) biological control of all subterranean stages with nematodes. The application of these control tactics is timed according to monitoring of adult emergence and the onset of leaf flushing in the spring/summer period. Any of these tactics should reduce root injury and sustain root health from grove to grove. For many groves, however, pest management might differ according to: 1) rootstock susceptibility to soil-borne diseases (i.e. Phytophthora spp.) and 2) root stress caused by excessive flooding and poor drainage of sandy loam soils. In certain grove situations, a soil fungicide for control of Phytophthora spp. should be advised.

Newly-planted resets and groves, less than 5 years old, with an established Diaprepes infestation on a susceptible rootstock can decline within 2 years without adult and/or larval control. A similar grove situation involving a resistant rootstock will have lesser tree decline, but will require adult suppression. Remember, groves planted on deep, sandy soils will often require no supplemental control and can rely on biological control agents.

Foliar sprays of different contact (knockdown) insecticides that include petroleum oil, to improve residual effect, are used to target adult weevils in the tree canopy. Although foliar sprays have been used by growers to suppress adults anytime of the year, recent research in central Florida has shown conclusively, that root injury is less and overall tree health improved when two foliar sprays are used 4 weeks apart during peak summer flush in late-May through June along with an egg-sterilant in the last application. The purpose of adult suppression with foliar sprays is to limit the number of gravid females and egg deposition, thereby reducing the number of larvae entering the soil. An egg-sterilant such as Micromite 80WGS has a 6-week residual effect where females lay sterile eggs and eggs contacting the leaf surface are nonviable. Note that the addition of petroleum oil to the spray mixture affects the bonding characteristics of the substance bonding the egg mass to the leaf.

Multiple applications of most foliar sprays within a season can incite abnormal increase in spider mite populations; and any pesticide, when used frequently, might cause secondary pest outbreaks or lead to resistance.

A chemical barrier applied as a band to the soil surface beneath the tree using a herbicide applicator provides a treated surface that will kill newly hatched invasive larvae before they reach the root system. Chemical must be uniformly applied from the trunk to the dripline of the tree to a moistened soil surface devoid of litter. Greater spray volume (~40 gal/A) should insure greater uniformity of coverage. Disturbance of the soil beneath the trees should be minimized to protect the soil barrier. Since neonates are killed upon exposure to treated soil as they pass through the barrier, it would apear that this control tactic should be best used for resets and young plantings infested with Phytophthora and where root injury by larvae must be minimal.

Timing chemical application to the time of the year when larval entry into the soil is highest, requires monitoring of adult weevils in the tree. Since highest larva recruitment occurs just after peak adult emergence, growers should apply soil treatment in early July, about 2 weeks after peak adult emergence in central Florida. Peak adult emergence is generally 2 to 3 weeks earlier in coastal groves.

Currently, Brigade WSB, a synthetic pyrethroid, is the only chemical registered for neonatal larvae control and applied as a soil barrier. Brigade has about 3 weeks residual in the soil and will suppress ants foraging on the soil surface. Generally, ant predators will recover after 30 days.

Parasitic nematodes that specifically attack insects are infectious to all larval stages of citrus root weevils. They are found in citrus soils naturally where they inflict mortality to all weevil life stages that are contacted. Nematodes are also sold to growers as biopesticides to control citrus root weevil larvae. They should be applied during months when soil surface temperatures are expected to exceed 70°F. Weevil larvae are generally highest in the soil during the summer (mid-July through September); therefore, one or more nematode applications are recommended at this time of the year if soil moisture via natural causes and/or irrigation is adequate. Nematodes should not be applied within 4 weeks of nematicide use. Properly modified herbicide applicators or microsprinkler irrigation systems are used to deliver nematodes into pre-moistened soil. Application of approximately one acre inch of water should also be applied to the irrigated acre immediately following application. Application late in the day or on cloudy days is encouraged to reduce nematode desiccation and exposure to lethal UV radiation.

Current nematode products are most effective when applied in sandy ridge type soils with more coarse soil texture, but are less effective in fine-textured soils at recommended rates. Higher rates can be applied to fine-textured soils.

A fungicide for control of Phytophthora spp. may be recommended under the following conditions as a supplemental strategy to larval and adult weevil control: 1) the soils are fine textured, poorly-drained or high in pH and calcium carbonate, 2) the trees are on rootstocks susceptible to Phytophthora spp., and 3) populations are above the damaging levels (20 and 40 propagules per cm³ soil) for P. nicotianae and P. palmivora, respectively. Remember, larval and/or adult weevil control must be effective before fungicide treatment is justified.

Table 1. Recommended Chemical and Biological Controls for Citrus Root Weevils

Pesticide	IRAC MOA¹	Mature Tree Rate²	Comments	Other Pests Controlled
Parasitic Nematodes
Bio Vector	NR	20,000 - 40,000 nematodes or greater per treated square foot	Bio Vector and Grubstake contain live nematodes to reduce Diaprepes and Pachnaeus sp. subterranean stages. Make one or more applications per year during the rainy season through fall. Apply through microirrigation or through herbicide boom to moist soil; one acre inch irrigation needed after application.

Soil Barrier
Brigade WSB	3	0.25 - 0.5 lb ai per acre	Restricted use pesticides. Soil barrier. Apply uniformly to moist, weed-free soil. Do not apply via irrigation. Do not exceed 32 oz per season.	Fire ants Asian cockroach
Capture 2EC	3	0.25 - 0.5 lb ai per acre		Fire ants Asian cockroach
Foliar Sprays
Danitol 2.4 EC	3	16 - 21 oz per acre	Restricted use pesticide. Contact foliar spray. Do not apply when temperature exceeds 94°F.	Thrips Citrus psyllid
Imidan 70 WP	1B	1 - 2 lb per acre	Contact foliar spray.
Kryocide 96 WP	9	8 - 20 lb per acre¹	Residual foliar spray. Controls Fuller rose and blue-green root weevils only. Minimum of 30 days between applications. Do not apply within 15 days of harvest. Do not exceed 90 lb per acre per season. Do not tank mix with products containing lime.	Orangedog Katydids
Micromite 80WGS + Petroleum Oil 97+% (FC 435-66, FC 455-88, or 470 oil)	15	6.25 oz + 1 gal oil per acre¹	Restricted use pesticide. Residual foliar spray. Maximum of 3 applications per season. Do not apply when temperature exceeds 94°F. 470 weight oil has not been evaluated for effects on fruit coloring or ripening. Heavier oils are more likely to be phytotoxic than lighter oils. Do not combine with Boron within 21 days to harvest.	Citrus leafminer Citrus rust mites
Orthene 97	1B	0.5 - 0.75 lb per acre¹	Contact/residual foliar spray. Nonbearing only. Apply at 100 gal water/acre or less.
Sevin 80 S + Petroleum Oil 97+% (FC 435-66, FC 455-88, or 470 oil)	1A	5 - 10 lb + 1 gal per acre¹	Contact/residual foliar spray. Lower rates will result in reduced residual activity. Do not exceed 20 lb a.i./acre/year for all uses. Not to exceed 2 applications per season. May increase spider mite populations. Do not apply when temperature exceeds 94°F. 470 weight oil has not been evaluated for effects on fruit coloring or ripening. Heavier oils are more likely to be phytotoxic than lighter oils.	Orangedog Katydids Grasshoppers Crickets Scale
Sevin 4 F + Petroleum Oil 97+% (FC 435-66, FC 455-88, or 470 oil)		1 - 2 gal + 1 gal per acre¹		Orangedog Katydids Grasshoppers Crickets Scale
Sevin XLR + Petroleum Oil 97+% (FC 435-66, FC 455-88, or 470 oil)		1 - 2 gal + 1 gal per acre¹
¹Mode of action class for citrus pesticides from the Insecticide Resistance Action Committee (IRAC) Mode of Action Classification V4.2.1 (2005). Refer to ENY624, Pesticide Resistance and Resistance Management, in the 2008 Florida Citrus Pest Management Guide for more details. ²Lower rates may be used on smaller trees. Do not use less than minimum label rate. FC = Florida Citrus.

Recommended Chemical Controls

READ THE LABEL.

See Table 1.

Rates for pesticides are given as the maximum amount required to treat mature citrus trees unless otherwise noted. To treat smaller trees with commercial application equipment including handguns, mix the per acre rate for mature trees in 250 gallons of water. Calibrate and arrange nozzles to deliver thorough distribution and treat as many acres as this volume of spray allows.

Footnotes

1. This document is ENY-611, one of a series of the Entomology and Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date printed: December 1995. Date revised: November 2007. This publication is included in SP-43, 2008 Florida Citrus Pest Management Guide. A copy of this publication may be found at http://edis.ifas.ufl.edu/CG006. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. For a copy of this handbook, request information on its purchase at your county Cooperative Extension Service office.

2. C.W. McCoy, professor emeritus, M.E. Rogers, assistant professor, Entomology and Nematology Department; S.H. Futch, extension agent IV; J.H. Graham, professor, Soil and Water Science Department; L.W. Duncan, professor, and H.N. Nigg, professor emeritus, Entomology and Nematology Department; Citrus REC, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition. Use pesticides safely. Read and follow directions on the manufacturer's label.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.

2008 Florida Citrus Pest Management Guide: Citrus Root Weevils1