By Jawwad A. Qureshi and Larry Duncan
Root weevils are a major pest group for many kinds of plants, with at least nine species that colonize citrus. Species commonly infesting citrus include Diaprepes root weevil (Diaprepes abbreviates), blue-green citrus root weevils (Pachnaeus litus and Pachnaeus opalus), little leaf notcher (Artipus floridanus), Fuller rose beetle (Asynonychus godmani) and Sri Lanka weevil (Myllocerus undecimpustulatus undatus).
All these species go through egg, larval, pupal and adult stages during their lives. Adults are found in tree canopies where they feed on foliage, mate and deposit eggs. Neonate larvae drop to the ground and enter the soil where they feed on the roots for several months, which causes significant damage. Pupal and young adult stages are also spent below ground. Adults emerge from the soil throughout the year.
Diaprepes root weevil (Figure 1) invaded Florida in the 1960s and is the most damaging species to citrus and other commercial crops including ornamentals and root crops. Summer rains, flush and soil temperature (particularly during May and June) promote adult emergence from the soil. First peak emergence is often between May and July with a possible second peak during September through October.
Adult feeding damage can be seen as notching of the leaf margins (Figure 2). The prolonged adult feeding could defoliate and stress young trees. Adults can live for three to four months, and a single female can lay up to 5,000 eggs. Allowing adults to feed and reproduce for long periods increases the progeny which enter the soil and damage the root system.
The neonate larvae feed on the fibrous roots. As the larvae mature, they feed on larger structural roots, which damage bark and the cambium layer, finally girdling the roots and crown with deep grooves. These wounds increase the infection and damage caused by Phytophthora nicotianae and P. palmivora. Severe damage by this pest-disease complex debilitates trees and can lead to tree death. Both aboveground and belowground measures are required to protect against weevil damage, particularly now when HLB is already damaging roots.
Once weevil adults start to emerge, they remain abundant for one to two months. Foliar sprays of insecticides known for good contact activity mixed with oil provide longer residual effect. Two foliar sprays of insecticides, four weeks apart during peak summer flush starting late May through June, have been shown to reduce root injury and improve tree health. Addition of Micromite 80WGS to the second application further reduces weevil populations for about six weeks by making eggs sterile or nonviable. Most insecticides used for psyllid control kill multiple pests including weevils. For example, Micromite 80WGS is effective against Asian citrus psyllid, citrus root weevils, citrus rust mites and citrus leafminers.
Three species of egg parasitoids from the Caribbean were released in Florida. Aprostocetus vaquitarum and Quadrastichus haitiensis established with parasitism rates ranging from 70 to 80 percent in southern Florida. Entomopathogenic fungi and predators have also shown some potential to reduce weevils. However, the increased use of insecticides for psyllid control compromises the effectiveness of these beneficial organisms.
Chemical or mulch treatments of the soil surface provide barriers to larval entry into soil. Applications made uniformly from the trunk to the dripline two weeks after adult emergence prevented most larvae from entering the soil, provided that soil disturbance was minimized. Brigade WSB, the only recommended barrier insecticide, provides residual effect for about three weeks. Mulches of metalized reflective materials or fabrics also reduce larvae from entering soil and adults from emerging. Their use also improves root and tree health while increasing yield.
Adult weevils were reduced by 32 percent in Ray Ruby grapefruit planted on metalized reflective mulch compared with those planted on bare ground at the Florida Research Center for Agricultural Sustainability in Vero Beach. Commercial woven landscape fabric prevents weevil larvae from entering soil (Citrus Industry, June 2014). A video (www.youtube.com/watch?v=A3-y-F4LwYw) illustrating fabric use and the results of several experiments was prepared by scientists at Texas A&M University.The history and proper use of entomopathogenic nematodes (EPNs) to manage weevil larvae in the soil were reviewed in the February 2017 issue of Citrus Industry. Steinernema riobrave, introduced by BASF in 2016, is the only nematode species recommended for root weevil control. Research to optimize the EPN dose efficacy and persistence demonstrated that application of S. riobrave resulted in significant mortality of weevil larvae (80 to 95 percent per week compared to 15 to 40 percent in untreated plots) for at least one month following application. In these trials, “sentinel” Diaprepes root weevil larvae were buried in cages beneath trees (Figure 3) immediately before the EPNs were sprayed on the soil surface while trees were irrigated with microjets. Seven days following the EPN application, the cages were recovered and insect mortality was recorded. The insect cadavers were incubated to determine if killed by S. riobrave or other organisms. Cages were also buried at intervals to determine EPN persistence.
Treatment was highly effective for at least four weeks with activity lasting as long as six weeks (Figure 3). Application rates between 20 and 100 EPNs per cm2 of soil surface at a site near Fort Pierce caused overall weevil larvae mortality rates of 80 percent and higher per week (Figure 4). Treatment efficacy remained very high through the third week post-EPN application, at which time Hurricane Irma saturated the soil for long enough to eliminate any subsequent beneficial effects of the treatment.
The net effect of the EPN treatments on the cohort of larvae in the soil is only seen by integrating the mortality rates during the time that the treatment remains effective. Figure 5 compares the very high mortality rate of the sentinel larvae measured during the first week after treatment to the overall mortality likely to occur to resident weevils at the time of treatment and weevils that hatch and fall to the soil during the four weeks following treatment. This example is based on the assumption that the original population of larvae in soil when treated increased at a rate of 10 percent per week as neonate larvae fell to the soil. Compared to weekly weevil survivorship of 12 to 36 percent relative to untreated larvae, the monthly survivorship is likely closer to 2 to 9 percent, depending on the treatment dose.
At a given treatment cost, do these experiments reflect the best control we can achieve using EPNs? The answer depends in part on results of ongoing and future experiments to evaluate new approaches to optimizing application coverage. Because EPNs are applied via irrigation, there can be no additional savings by modifying the application method. If the cost of producing EPN products does not decline, the only way to reduce cost is to use fewer EPNs.
Newly planted trees require fewer EPNs to achieve the same application dose (EPNs per soil surface area) as mature trees because the young tree irrigation pattern is smaller. Therefore, young trees can be treated more frequently than mature trees, for less annual cost, thereby reducing the intervals during which roots remain unprotected by S. riobrave.
Perhaps the most likely means to achieve cost savings in mature trees is to determine how much of the root system requires protection to achieve adequate weevil control without sacrificing tree health and productivity. Tree health suffers more from damage in the root crown area than from damage to more distant lateral roots. Moreover, fewer weevils are required to extensively damage the conductive tissues of the relatively small crown than the more extensive lateral root system. Ongoing experiments are evaluating the outward distance traveled by EPNs applied to the base of the tree via drippers. If a sufficient portion of the root system can be protected by such an approach, even monthly EPN applications would be far less costly than currently recommended application rates and scheduling.
Jawwad A. Qureshi (firstname.lastname@example.org) is an assistant professor in the Department of Entomology and Nematology, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Indian River Research and Education Center in Fort Pierce. Larry Duncan is a professor with the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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