Food Preference of Wireworms Analyzed with Multinomial Logit Models

Many species of wireworms (larvae of click beetles, Elateridae) are poly- phagous root herbivores. In grasslands under restoration succession with various grass species, we aim to determine the role of wireworms in aboveground vegetation succession. Therefore, it is crucial to know whether wireworms prefer some food plants to others. We have investigated the root preference to different grass species for Agriotes obscurus and Athous haemorrhoidalis and whether these preferences can be explained by covariates. In Experiment 1, individual wireworms could choose between four different plants, one of each species (Anthoxanthum odoratum, Festuca rubra, Holcus lanatus, and Lolium perenne). In Experiment 2, groups of wireworms were released into the soil in the center of 16 plants (4 from each species). We used multinomial logit models (MLMs) to analze the data. In the appendix the use of multinomial response models is clarified with a fictitious example, using the SAS statistic software package. No preference was found in Experiment 1. In Experiment 2 we found differences in attractiveness of plant species depending on wireworm species: A. obscurus preferred grass species from nutrient-rich grasslands (L. perenne and H. lanatus). Both wireworm species disliked F. rubra. The distance from the release point influenced the probability of being found at a certain place at the end of the experiment: wireworms tended to stay in the proximity of the release point. A. haemorrhoidalis was more often found farther from the point of release than A. obscurus. Dispersal was farther from the release point in experiments with young plants (6 weeks) compared to older ones (9 weeks). Results are discussed in a broad ecological context.     

Effect of temperature on the morbidity and recovery of the Pacific Coast wireworm, Limonius canus, following contact with tefluthrin-treated wheat seeds

Late instars of the Pacific Coast wireworm, Limonius canus (LeConte) (Coleoptera: Elateridae), were exposed to wheat seeds treated with tefluthrin at 10 g active ingredient/100 kg wheat seed, for 2 min, at 10, 15, 20, and 25 °C. All wireworms were moribund within 20 min of first exposure and recovered fully within 7 h. Morbidity induction time (i) decreased considerably as the temperature (t) was increased from 10 to 15 °C, but did not decrease further when the temperature was increased from 15 to 25 °C. For each temperature, induction time decreased as wireworm weight (w) increased, so that . A strong negative temperature coefficient was observed in wireworm recovery, the time required for recovery (r) after exposure decreasing as both temperature (t) and wireworm weight (w) increased, so that r = 1102.47(w)^?0.1848 * e^?0.1012t. The temperature at which wireworms contact insecticides in the field may significantly affect the induction and duration of morbidity, and determine whether wireworms will become moribund before they are repelled by tefluthrin. The ability of wireworms to recover from tefluthrin‐induced morbidity may seriously limit the efficacy of this insecticide in reducing wireworm populations in the field.     

Varietal susceptibility of potatoes to wireworm herbivory

1 Wireworms, the soil dwelling larvae of click beetles, Agriotes spp., have recently become a more prevalent pest of potatoes. The present study investigated whether potato varieties showed variable susceptibility to wireworm herbivory, and also tested whether increased susceptibility was associated with lower concentrations of glycoalkaloids. Twelve varieties were originally screened across a range of experimental scales, including laboratory and tunnel experiments and a large‐scale field trial involving over 2000 tubers. 2 In laboratory no‐choice tests, Maris Peer, Marfona and Rooster varieties were significantly more susceptible to wireworm attack, with 63% of tubers showing damage, compared with just 15% of the less susceptible varieties of King Edward, Nadine and Maris Piper. There was also greater tissue consumption and weight gain when wireworms were reared on the most susceptible varieties. 3 In choice tests, wireworms showed a significant preference for those varieties previously identified as being the most susceptible to wireworm herbivory (4.2 holes per tuber) compared with the least susceptible (1.2 holes per tuber). Similar patterns of susceptibility were seen in the field trial, although there was generally more variation in susceptibility. 4 In a tunnel experiment, Marfona and Maris Peer were significantly more susceptibile to wireworm attack (47% of tubers showing damage) compared with Nadine, King Edward and Maris Piper (27% of tubers showing damage). Although Nadine, in particular, had the highest glycoalkaloid concentrations (309.33 mg/kg) and lowest amounts of wireworm herbivory, the relationship between susceptibility and glycoalkaloid concentrations was weak, suggesting that this is unlikely to be the sole mechanism underpinning varietal susceptibility.     

Environmental and behavioral constraints on the infection of wireworms by Metarhizium anisopliae

Environmental and behavioral factors that affect the infection of wireworms [Agriotes obscurus L. (Coleoptera: Elateridae)] by a unique isolate of Metarhizium anisopliae Sorokin (Hypocreales: Clavicipitaceae) were studied. After wireworms were placed in soil containing 10(6) M. anisopliae conidia/g and incubated at 6, 12, or 18 degrees C, significant disease development and wireworm mortality occurred only in those wireworms incubated at 18 degrees C. At this temperature, mortality was found to be dependant on the time exposed to the contaminated soil, and a minimum exposure time of 48 h was required to cause significant levels of mortality. Despite the restrictive effect of cooler temperatures on disease development and mortality, infected wireworms did not choose temperatures that inhibited disease development when given the opportunity to do so in a separate experiment. Finally, wireworms were repelled by M. anisopliae-contaminated soil at a rate that increased with the soil conidia concentration, but the rate of emigration was reduced when a food source was present. The results of this study indicate that factors including temperature, time exposed to M. anisopliae, conidia soil concentration, and food availability will affect mortality rates of wireworms and are likely to affect field performance of M. anisopliae as a biological control.     

Wireworm populations in relation to crop production: I. A large-scale flotation method for extracting wireworms from soil samples and results from a survey of 600 fields

The wireworm survey in the Eastern Counties revealed many cases where the observed wireworm damage failed to correspond with the estimated field population. A possible explanation for this was the inaccuracy of counts made by picking wireworms out of the soil samples by hand. Tests showed that such methods recovered an extremely variable proportion of the wireworms in the- samples and, on the average, only two-fifths of the larvae were obtained. A modified form of the washing and flotation technique used by Salt & Hollick (1944) was introduced for large-scale work and is described. By this method, ten samples of soil (4 in. diam. and 6 in. deep) bulked together are examined at a time and can be dealt with at the rate of 13 samples per man per hour with an efficiency of 95-100% in the extraction of wireworms. The populations estimated on 600 fields sampled between December 1942 and May 1943 have thrown more light on the size and composition of the wireworm population in grass and arable fields. Inspection of the crop results on fields tested by the washing process showed a much closer relationship between wireworm population and wireworm damage than had been obtained by the hand-sorting method in the previous year.     

Studies of wireworm population; some effects of cultivation

The number of wireworms in successive collections from five Cambridgeshire fields declined during the first year of cultivation to 25 % of the number present before the fields were ploughed from old grass. The decline in numbers was accompanied by a change in the composition of the wireworm population; the marked inverse correlation between number and size of larvae, which held for the wireworm populations under grass, progressively altered towards uniform numbers of the different size-groups. The incidence of natural control on the different sizes of larvae was obscured by their growth, and more medium-sized and large larvae were lost than would appear from the population-size histograms.
Three groups of factors are considered as contributing to the decline in numbers after ploughing. Direct observation showed that many wireworms were destroyed in the actual process of cultivation. Evidence was insufficient to prove that the physical condition of cultivated soil adversely affects the number of wireworms. Comparison of the smaller size-groups in pasture and in the cultivated fields showed that the wireworm population failed to replenish itself under arable conditions.     

Specific impacts of two root herbivores and soil nutrients on plant performance and insect–insect interactions

Soil‐dwelling insects commonly co‐occur and feed simultaneously on belowground plant parts, yet patterns of damage and consequences for plant and insect performance remain poorly characterized. We tested how two species of root‐feeding insects affect the performance of a perennial plant and the mass and survival of both conspecific and heterospecific insects. Because root damage is expected to impair roots’ ability to take up nutrients, we also evaluated how soil fertility alters belowground plant–insect and insect–insect interactions. Specifically, we grew common milkweed Asclepias syriaca in low or high nutrient soil and added seven densities of milkweed beetles Tetraopes tetraophthalmus, wireworms (mainly Hypnoides abbreviatus), or both species. The location and severity of root damage was species‐specific: Tetraopes caused 59% more damage to main roots than wireworms, and wireworms caused almost seven times more damage to fine roots than Tetraopes. Tetraopes damage decreased shoot, main root and fine root biomass, however substantial damage by wireworms did not decrease any component of plant biomass. With the addition of soil nutrients, main root biomass increased three times more, and fine root biomass increased five times more when wireworms were present than when Tetraopes were present. We detected an interactive effect of insect identity and nutrient availability on insect mass. Under high nutrients, wireworm mass decreased 19% overall and was unaffected by the presence of Tetraopes. In contrast, Tetraopes mass increased 114% overall and was significantly higher when wireworms were also present. Survival of wireworms decreased in the presence of Tetraopes, and both species’ survival was negatively correlated with conspecific density. We conclude that insect identity, density and soil nutrients are important in mediating the patterns and consequences of root damage, and suggest that these factors may account for some of the contradictory plant responses to belowground herbivory reported in the literature.     

Rearing Costelytra zealandica (Coleoptera: Scarabaeidae)

Interaction between larvae can be a cause of mortality when scarabaeid larvae are concentrated in a confined volume of soil. Larvae of Costelytra zealandica (White), the New Zealand grass grub, were held for 30 days at population densities between 1 and 50 larvae per 200 ml of medium, and the effect of crowding on their survival and weight change was assessed. Larval density had no effect on survival in soil plus chopped sheep dung (3 :1), with up to 10 2nd‐instar larvae or 5 3rd‐instar larvae per 200 ml of medium. Larval survival and weight gain were increased by adding germinating ryegrass seed (Grasslands Ruanui) to the medium base, but the effect of density did not diminish. Weight gain of 3rd‐instar larvae decreased as their density increased. When the amount of germinating ryegrass seed in the medium was varied, there was no significant difference between the survival of larvae held in treatments with 5, 25, and 50 g of seed per litre of medium. Survival was significantly lower with 0 and 100 g of seed per litre of medium. It is concluded that, in rearing conditions, the final density of fully grown 3rd‐instar larvae should be a maximum of about 20 per litre of medium to prevent mortality caused by larval interaction—provided that adequate food is available.     

Morbidity and recovery of the Pacific Coast wireworm, Limonius canus, following contact with tefluthrin-treated wheat seeds

Late instars of the Pacific Coast wireworm, Limonius canus (LeConte) (Coleoptera: Elateridae), were exposed to wheat seeds treated with tefluthrin at 5, 10, 15, 20, or 30 g active ingredient (a.i.) per 100 kg wheat seed for 1, 2, 4, 8, or 16 min. All wireworms were moribund within 20 min of first exposure and recovered fully within 12 h. The time required for recovery (t_r) after a single exposure increased with duration of exposure (e) and concentration (c), but decreased with wireworm weight (w), expressed as (t_r)^0.5 = 5.2812 + 0.9407e – 0.0259e² + 0.1569c + 0.0254ec – 0.0174w – 0.0057ew. For wireworms exposed to treated seeds for 2 min, the time required for induction of morbidity decreased as concentration of tefluthrin increased and as wireworm weight decreased, expressed as (t_i)^0.5 = 2.613 – 0.039c + 0.018w, where t_i is the induction time of morbidity, and c and w are as above. Wireworms re‐exposed to tefluthrin‐treated seeds after recovery from previous exposure were again moribund within 20 min of exposure, but recovery was significantly more rapid if the second exposure was 2, 4, 6, 8, 12, 18, 24, and 48 h after recovery from first exposure. Recovery from a second exposure was not significantly faster when the second exposure was 96 h after recovery from the first exposure. The ability of wireworms to recover from tefluthrin‐induced morbidity may seriously limit the efficacy of this insecticide in actually reducing wireworm populations in the field.     

Aggregation and mortality of Agriotes obscurus (Coleoptera: Elateridae) at insecticide-treated trap crops of wheat

Agriotes obscurus (L.) wireworms assembled in increasing numbers at rows of treated (Agrox DL Plus seed treatment) and untreated wheat, Triticum aestivum L., planted at increasing densities (0, 0.15, 0.30, and 0.60 seeds/cm). In treated wheat plots at all planting densities, no wireworm damage to seedlings was apparent, and total wireworms taken in core samples in wheat rows increased according to the asymptotic equation y = B0(1 -e(-Blx)), where B0 is the asymptote, B1 is the slope of the initial rise, and x is the seeding density. The number of dead wireworms in treated plots increased linearly and intercepted the asymptotic models (theoretical point at which 100% mortality of assembled population occurs) at 0.95 seeds/cm on 11 June and 1.14 seeds/cm on 18 June 1996. Untreated wheat at all densities planted had severe wireworm damage and significantly reduced stand. Populations that had assembled at the surviving untreated wheat were fewer than in the treated wheat plots, and although increasing with seeding density, did not follow the asymptotic model. The data suggest that A. obscurus populations can be assembled and killed in fallowed fields in large numbers at treated trap crops of wheat over a 19-d period when planted in rows spaced 1 m apart at a linear seeding density of 1.5 seeds/cm.     

Preventive application of an entomopathogenic fungus in cover crops for wireworm control

Efficacy of the Metarhizium brunneum Petch (Hypocreales: Clavicipitaceae) strain ART2825 for control of wireworms (Agriotes obscurus (L.), Coleoptera: Elateridae) was examined in a semi-field pot experiment. Pots were treated in late summer during sowing of spring oat as a cover crop. Survival of wireworms was assessed four weeks after their release in October 2013, and 30 weeks after release in April 2014. Viability and persistence of the fungus was determined by counting colony forming units from substrate samples and microsatellite analyses of recovered Metarhizium isolates. The number of colonies detected in the substrate in October 2013 increased with increasing concentrations of applied conidia, and no significant reduction was observed at the second evaluation date in April 2014. Increasing conidia application rates significantly increased mycosis and reduced wireworm survival, to a level comparable to that of treatment using insecticide-coated oat seeds. The preventive application of M. brunneum conidia to reduce wireworm populations in cover crops, preceding a damage-sensitive crop like potatoes, may be a promising biocontrol strategy.     

Technical scale production of encapsulated Saccharomyces cerevisiae and Metarhizium brunneum attractive to wireworms

Wireworms (Coleoptera: Elateridae) have recently become an increasing problem as agricultural insect pests due to the phasing out of effective control options. Entomopathogenic fungi such as Metarhizium brunneum have proven to be a promising microbial antagonist for wireworm control. Here, we tested whether the efficacy of M. brunneum can be increased through a combination with CO₂, emitted by Saccharomyces cerevisiae, as an attractant (=attract-and-kill). We aimed at a technical scale production of a formulated biological control agent offering a practical and economically feasible application for wireworm control. Therefore, a novel technical formulation process for encapsulated S. cerevisiae (Attract beads) and M. brunneum (Kill beads) was investigated. For the bead production by jet cutting, the parameters nozzle diameter, pump speed, cutting device speed and collecting distance were evaluated. In order to dry the beads in a short time while maintaining a high cell viability, different drying temperatures during fluidised-bed drying were tested and the best results were obtained with an inlet air temperature profile between 50°C and 40°C. CO₂ production of the beads in the soil was highest for co-applied Attract and Kill beads. The potential of beads to modify wireworm behaviour (Agriotes sputator) was tested in a rhizotron experiment. The Attract-and-Kill treatment (co-applied beads) significantly attracted wireworms, whereas Attract beads and Kill beads alone showed a weak, but non-significant attraction. Wireworm mortality could not be enhanced due to a low rate of mycosis from M. brunneum infection.     

Toxicity of 1,3-dichloropropene and fosthiazate to wireworms (Agriotes spp.)

Wireworms (Agriotes spp.) are sporadic but increasingly important pests of potatoes, sugar beet and cereals. Whilst effective chemical control is possible, the granular organophosphates normally require high rates of application and the seed dressings containing lindane (gamma‐HCH) have been withdrawn from use. The soil fumigant 1,3‐dichloropropene (1,3‐D as Telone II) and the granular nematicide fosthiazate (Nemathorin 10G) are currently used for the control of potato cyst nematodes. We investigated the effects of both of these chemicals on wireworms. Air‐vapour phase toxicities for 1,3‐D against wireworm were LD₅₀ 2.74 mg.litre.day and LD₉₉ 5.05 mg.litre.day. The in vitro soil phase toxicity was LD₉₉ 8.15 mg.litre.day. 1,3‐D soil phase activity against wireworm may be associated more with air‐vapour phase than a soil‐water phase activity. In glasshouse experiments 16.0 mg.litre.day of 1,3‐D gave 75% control. Fosthiazate, which is applied at approximately 2 μg g^?1 of soil for potato cyst nematode control, achieved an LC₅₀ at 3.20 μg g^?1. In both in vitro and glasshouse studies 1,3‐dichloropropene showed high toxicity to wireworm at dosages below the current commercial application rate for potato cyst nematode control and fosthiazate also showed useful efficacy. These chemicals may therefore prove to be valuable additional tools for limiting initial wireworm plant damage or reducing wireworm populations.     

The biology and management of wireworms (Agriotes spp.) on potato with particular reference to the U.K.

1 This paper reviews and interprets relevant work on the biology and management of wireworms (Agriotes spp.) within the context of potato production in Europe, with particular reference to the U.K. Although the review concentrates on Agriotes spp., the extensive world literature on other Elateridae of economic importance is also drawn upon. 2 Possible reasons for the apparent increase in the importance of wireworms on the potato crop are discussed, followed by a review of wireworm biology, risk assessment techniques (soil sampling, bait trapping and adult pheromone trapping), crop damage, and cultural, biological and chemical control methods. 3 It is clear that the process of site risk assessment followed by appropriate control measures (usually insecticide use) will remain the mainstay of wireworm management programmes. However, there is considerable scope for adopting new risk assessment techniques, such as pheromone trapping of adult beetles. 4 These control measures will need to be underpinned by a greater understanding of wireworm biology, particularly adult dispersal. Factors affecting the initiation and maintenance of wireworm populations in individual fields also require further study. The current use of insecticides could also be optimized by a better appreciation of the interactions between insecticide use, potato variety choice and harvest dates.     

Predation on Australian soldier fly,Inopus rubriceps (Diptera: Stratiomyidae), in pasture

Abstract

Invertebrate predation on the soil-living stages of Australian soldier fly may be a significant factor in the regulation of this pest of North Island grasslands. Laboratory and field experiments on larvae of Conoderus exsul and Agrypnus variabilis (Elateridae), adults and larvae of Thyreocephalus orthodoxus (Staphylinidae), and adults of Rhytisternus miser (Carabidae) suggest that all may influence the densities of soldier fly through predation on larvae and/or eggs. The functional responses of C. exsul, A. variabilis, and T. orthodoxus to soldier fly larval density are discussed. Selective removal of predators from pasture with insecticide showed that predation may account for up to 75% loss of the early instars of soldier fly.     

Predation of Thereva nobilitata (Fabricius) (Diptera: Therevidae) on Agriotes obscurus L. (Coleoptera: Elateridae)

Little is known about the natural enemies of wireworms (Coleoptera: Elateridae), but there are frequent anecdotal reports of (usually unnamed) stiletto fly larvae (Diptera: Therevidae) preying on various species. We observed larvae of Thereva nobilitata (Fabricius) feeding on larvae of the dusky wireworm, Agriotes obscurus L., during the summer of 2011, in Agassiz, British Columbia. This finding is of interest as: both the predator and the wireworm are introduced species to this area from Europe; T. nobilitata is uncommon in North America; and this predator has not been associated with any wireworm species previously. We observed that larvae of male and female T. nobilitata will feed on various sizes of A. obscurus larvae, most feeding being carried out by the smallest T. nobilitata larvae. These findings suggest future work should assess the potential for therevid larvae as top‐down regulators of Agriotes larvae under field conditions.     

Laboratory and field tests in 1966-67 on chemical control of wireworms (Agriotes spp.)

Of sixteen compounds applied to soil in laboratory tests, azinphos-ethyl, P2188 (O,O-diethyl S-chloromethyl phosphorothiolothionate), ‘Dursban’ (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate), P1973 (S-(N-methoxycarbonyl-N-methylcarbamoylmethyl) dimethyl phosphorothiolothionate), B77488 (O,O-diethylphosphorothioate O-esterwith phenylglyoxylonitrile oxime) and R42211 (O,O-diethyl O-(2-diethylamino-6-methyl-pyrimidin-4-yl) phosphorothioate) killed wireworms when first tested, but in second tests with the same soils only ‘Dursban’, P2188 and B77488 did so. Treating seeds with ‘Dyfonate’ (O-ethyl S-phenyl ethyl phosphonodithioate) or with ethion/γ-BHC mixtures killed few wireworms. Three field trials compared the organophosphorus insecticides ‘Dursban’, ‘Dyfonate’ and phorate with organochlorine standards. In trials with barley and potatoes the standard was 3 lb a.i./acre (3·36 kg/ha) of aldrin. The organophosphorus compounds increased plant stands of barley almost as much as aldrin, although they killed fewer wireworms; and they protected fewer potato tubers from wireworm damage. The third trial compared the organophosphorus compounds with 0·5 lb a.i./acre (0·56 kg/ha) γ-BHC sprayed on a site drilled with sugar beet seed dressed with dieldrin. The γ-BHC increased plant stands almost as much as did 3 lb a.i./acre of the organophosphorus insecticides, and killed as many wireworms.     

Evaluation of entomopathogenic nematodes for suppression of carrot weevil

The potential of entomopathogenic nematodes as biologicalcontrol agents for carrot weevil (Listronotus oregonensis) was evaluated throughboth laboratory and field experiments. In thelaboratory, Steinernema carpocapsae, S. riobrave, S. feltiae, Heterorhabditis megidis, H. bacteriophora, and a control (water only) werecompared in sand and muck soil against adults,and in sand against larvae. All nematodespecies produced high levels of larvalmortality. S. carpocapsae producedsignificantly greater adult mortality in sandthan other species or the untreated control. H. bacteriophora caused low adultmortality in sand, but the greatest adultmortality among treatments in a similar testthat used muck soil; S. carpocapsae wasranked second on muck soil. Other speciesconsistently produced intermediate (H.megidis and S. riobrave) or low (S.feltiae) levels of mortality on bothsubstrates. In the field, we compared theeffect of early season vs. late seasonapplications of H. bacteriophora or S. carpocapsae on carrot weevil mortality andparsley survival and yield. Significantdifferences among treatments in plant survivaland yield were not found; however treatmentsinvolving H. bacteriophora had higherplant survival than other treatments. Earlierapplication of this species was associated withhigher plant survival. S. carpocapsaetreatments had similar plant survival to thecontrol. Mortality of larvae and combinedstages of carrot weevil was significantlygreater at 1 week following H.bacteriophora application than for othertreatments. H. bacteriophora also showedgreater persistence than S. carpocapsaein treated plots. We conclude that H.bacteriophora is a good candidate for furtherevaluation as a biological control agentagainst carrot weevil on muck soils in theGreat Lakes region.     

Resporulation of Metarhizium anisopliae granules on soil and mortality of Tenebrio molitor: Implications for wireworm management in sweetpotato

In Australia, sweetpotato (Ipomoea batatas L.) is vulnerable to root feeding insect pests such as wireworms (e.g., Agrypnus spp.). The number of registered insecticides to control these insect pests is limited and often pest pressure, for example by wireworms, is severe close to harvest, further limiting what insecticides can be applied. Incorporating biological control agents such as entomopathogenic fungi (e.g., Metarhizium anisopliae) into integrated pest management programmes may be feasible in sweetpotato. M. anisopliae has been shown to be effective in controlling more than 200 insects and it is able to reside and grow in the rhizosphere and rhizoplane, suggesting that M. anisopliae could be a promising candidate against soil insect pests. In the study presented here, M. anisopliae was formulated into calcium alginate granules fortified with nutrients. The resporulation of the fungal granules was tested on four different soil types in the laboratory. The biocontrol efficacy of the resulting fungal growth was also examined using larval mealworms, Tenebrio molitor as a model insect in the laboratory and the glasshouse. Our results indicated that sterilised soil favoured optimal fungal resporulation, although different soil types did not have a significant effect on fungal resporulation. The resulting fungal resporulation and growth on sterilised soil caused high mortality (up to 76%) of larval mealworms in the glasshouse, whereas the fungal granules applied to non-sterile soil demonstrated poor resporulation that led to low mortality (13%) of larval mealworms. The result of this study indicates that the manipulation of microbial populations in field soil is required to enhance the fungal growth and potential insect control against wireworms in the field.     

An iridescent virus and a rickettsia from the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae)

Natural iridescent virus and rickettsia infections of Costelytra zealandica (White) and Odontria sp. indet. larvae were studied at a site in the upper Pareora Gorge scenic reserve, S. Canterbury. By sequentially sampling the site, it was found that neither the iridescent virus nor the rickettsiae appear to give rise to host mortalities that significantly alter the population density. Many larvae were found that appeared healthy, but carried inapparent iridescent virus infections. These diseases are not suitable for biological control of the grass grub.