Kevin M. Heinz

Host-size-dependent sex ratio theory and improving mass-reared parasitoid sex ratios

Although an effective parasitoid of agromyzid leafminers, Diglyphus isaea (Walker) (Hymenoptera: Eulophidae) is an expensive biological control agent in terms of production costs. In part, these costs arise from the production of male-biased offspring sex ratios. Here, we present a mass-rearing technique that will increase the proportion of females produced and reduce the need for frequent releases in biocontrol programs. By presenting female D. isaea groups of sequentially larger leafminer hosts to attack, we are able to generate progressively more female-biased sex ratios. After three days of providing increasingly larger hosts, we were able to reduce the sex ratios produced by individual females from 57% male to 36% male; sex ratios produced by groups of females dropped from 64% male to 45% male. Several attributes of D. isaea sex allocation allow us to manipulate sex allocation behavior. First, D. isaea is a solitary idiobiont; resources available to each offspring are present at the time of attack allowing the ovipositing female to accurately assess host quality. Host size positively affects both male and female wasps. Females laid more daughters in larger hosts and more sons in smaller hosts. We show that the observed relationship between host size and offspring sex ratio is due to maternal sex allocation decisions rather than differential mortality. Furthermore, assessment of the size threshold was relative to prior host encounters rather than an absolute assessment. Our simple memory model suggests that while females are influenced most strongly by recent encounters, females also base their assessment of the host-size threshold on prior host encounters. 2002 Elsevier Science (USA). All rights reserved.

INTERSPECIFIC COMPETITION AMONG INSECT PARASITOIDS: FIELD EXPERIMENTS WITH WHITEFLIES AS HOSTS IN COTTON

Manipulative field experiments assessing the importance of interspecific competition on the dynamics of parasitoid populations and the impact of multiple parasitoids on host populations are virtually absent from the ecological literature. We report findings from such experiments assessing competitive interactions among three species of parasitoids (Hymenoptera: Aphelinidae) of the silverleaf whitefly Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae), a pestiferous species of worldwide importance. The parasitoids used in the study (Encarsia pergandiella Howard, Eretmocerus mundus Mercet, and Encarsia formosa Gahan) encompass the range of life histories found within Aphelinidae, and they exhibit a high degree of niche overlap. The objectives of our studies were (1) to investigate the occurrence of interspecific competition among the whitefly parasitoids, (2) to quantify shifts in resource utilization patterns as influenced by experimental manipulations of the parasitoid assemblage, and (3) to ...

Influences of fertilization on population abundance, distribution, and control of Frankliniella occidentalis on chrysanthemum

We examined the effects of fertilization on population abundance and within‐plant distribution of western flower thrips, Frankliniella occidentalis Pergande (Thysanoptera: Thripidae), on potted chrysanthemum, Dendranthema grandiflora (Tzvelev). We also investigated the effects of fertilization on the number of insecticide applications needed to control F. occidentalis on potted chrysanthemum. Under greenhouse conditions, rate of change in population abundance of F. occidentalis increased with fertilization levels from 0 to 100% of the standard fertilization level (375 ppm N) and was four times higher on plants fertilized with the standard level (rate of change = 0.14) than on plants fertilized with 0% during the first 4 weeks after thrips inoculation. Within‐plant distribution of F. occidentalis was influenced by the phenology of the plants rather than total nitrogen content of plant tissues. Prior to flower opening, more F. occidentalis were found in the middle region of the plants. When the flowers began to open, more thrips were found feeding inside the flowers than on the leaves. We further showed that production time, the time from transplantation to flower opening, shortened considerably with increased fertilization level. Production time was shortest, 12 weeks, for plants fertilized with 100% of the standard fertilization level. When the fertilization level was reduced to 20%, production time lengthened to 13 weeks. When fertilization level was reduced to 0%, production time lengthened to 14 weeks. Increased fertilization from 0 to 100% of the standard level did not result in higher numbers of insecticide applications. All three insecticides (acephate, bifenthrin, and spinosad) were effective in keeping the thrips infestation below a predetermined level, five thrips per plant, but bifenthrin required the most number of applications to do so. For chrysanthemum, a fast‐growing crop and heavy utilizer of fertilizer, fertilization influenced not only the population growth of pest insects but also plant production time. As a result, optimizing fertilization level to reduce pest population growth may be a useful tactic in an Integrated Pest Management program for managing F. occidentalis on potted chrysanthemum. However, the effect of fertilization on production time and plant quality should also be considered when implementing this tactic.

Influences of fertilization on Aphis gossypii and insecticide usage

Abstract:  Fertilization levels for ornamental crops may influence pest population dynamics, crop quality, and pest management strategy. We examined the effect of fertilization on population growth and within‐plant distribution of melon or cotton aphid, Aphis gossypii Glover, on potted chrysanthemum, Dendranthema grandiflora (Tzvelev). In terms of pest management implications, we also investigated the effect of fertilization on the number of insecticide applications needed to control A. gossypii on potted chrysanthemum. Population growth rate of A. gossypii increased with fertilization levels from 0 to 38 ppm N and reached a plateau from 38 to 488 ppm N. Increased fertilization beyond 38 ppm N, 10% of the commercial standard, did not result in higher aphid number. Aphids responded to nutrient availability of plants by distributing themselves in areas with higher level of nitrogen. More aphids were found in the apical and middle strata of the plants than the basal stratum, which had the lowest nitrogen content. Leaf nitrogen content increased with increased fertilization level and was consistently higher in the apical and middle strata than the basal stratum. Increased fertilization from 0 to 375 ppm N did not result in higher number of insecticide applications. All three insecticides (bifenthrin, kinoprene or pymetrozine) were effective in keeping the aphid infestation below a pre‐determined level, five aphids per plant, but pymetrozine required the least number of applications. For chrysanthemum, a fast‐growing crop and heavy utilizer of nitrogen, increased fertilization shortened the time to flowering, which would allow growers to harvest their crop sooner and reduce the time for aphid population growth. Reduction in time to harvest could result in significant reduction of insecticide usage by reducing the time for aphid population growth. As a result, high fertilization together with minimal runoff may be a useful tactic to an integrated pest management (IPM) programme for managing A. gossypii on potted chrysanthemums.

Performance of the predator Delphastus pusillus on Bemisia resistant and susceptible tomato lines

Host plant resistance and biological control are often assumed to act additively to suppress populations of agricultural pests. Using tomato trichome based resistance to the whitefly Bemisia argentifolii, we tested this additivity assumption with Delphastus pusillus, a coccinelid predator of Bemisia. Various life history traits of D. pusillus were measured on the tomato cultivar ‘Alta,’ which possessed foliage with 3‐fold greater trichome densities than the second cultivar ‘VF145B7879.’ Beetles housed on VF145B7879 exhibited significantly greater lifetime fecundities and walking speeds than beetles housed on Alta. No cultivar‐specific differences were observed in D. pusillus longevities or handling times. Combining these observations with previously published reports of reduced B. argentifolii population growth rates on Alta compared to VF145B7879, we tested the hypothesis that increased levels of whitefly biological control could be obtained on VF145B7879 by comparison to Alta through releases of D. pusillus. Analyses of results obtained from replicated population trials detected significant reductions in whitefly populations due to D. pusillus releases, but they did not detect a significant influence of tomato cultivar on the ability of D. pusillus to suppress whitefly populations. Significantly longer beetle residence times on Alta than on VF145B7879 may have compensated for the significantly slower walking speeds and reduced lifetime fecundities observed on Alta and produced a neutral effect of foliar trichome densities on B. argentifolii biological control.

Relationships of landscape, prey and agronomic variables to the abundance of generalist predators in cotton (Gossypium hirsutum) fields

A two-year field study investigated the possible effects of grain sorghum (Sorghum bicolor [L.] Moench) and uncultivated areas on the abundance of generalist predators in commercially-managed cotton (Gossypium hirsutum L.) fields in Texas, USA. From 63 to 70 fields were sampled for pests and predators over nine consecutive weeks during early stages of cotton development. Additional data on agronomic practices and landscape composition at three spatial scales were also collected for each field. Stepwise regression analyses were used to determine the relationships of landscape, agronomic and prey variables to the abundance of generalist predators. Because the variables most closely linked to predator levels could vary over time, separate regressions were conducted for three time periods corresponding to stages of grain sorghum growth (half-bloom, hard-dough, maturity) in each year. Significant relationships between predator abundance and agricultural landscape composition appear in both years and in all three time periods, but the specific relationships of landscape variables to cotton predator levels differed between and within years. At maturity in 2001, predator levels rose as the amount of uncultivated land from 1.6 to 3.2 km distant and the perimeter shared with grain sorghum increased. During 2002, the area of grain sorghum (half-bloom) and uncultivated land (hard-dough) within 1.6 km of cotton fields were both positively related to predator numbers. Cotton planting dates and the abundance of cotton fleahoppers (Pseudatomoscelis seriatus [Reuter]) were also strongly linked to predator numbers during both years. Results suggest that the total amount of grain sorghum or uncultivated land in an area is more important than the presence of these habitats adjacent to cotton fields, and that landscape composition may sometimes be the most important factor in determining predator abundance.

Evaluation of the nematodes Steinernema feltiae and Thripinema nicklewoodi as biological control agents of western flower thrips Frankliniella occidentalis infesting chrysanthemum

Abstract In greenhouse studies, we evaluated a commercial formulation of the entomopathogenic nematode Steinernema feltiae and the inoculative release of the thrips-parasitic nematode Thripinema nicklewoodi against western flower thrips (WFT), Frankliniella occidentalis Pergande infesting potted chrysanthemums. Foliar sprays of S. feltiae applied at 1.25–2.5×103 IJ mL−1 and 1000 – 2000 L ha−1 at 3-day intervals alone (targeting feeding stages) or in combination with soil applications (simultaneously treating non-feeding stages in the soil at the same rates) decreased but did not provide adequate control of thrips in flowering plants artificially infested with a dense population. Similar nematode treatments applied for four to five applications at 6-day intervals in two batches of initially clean chrysanthemums failed to prevent unacceptable damage to flowers and leaves from a dense natural infestation within the greenhouse. Although some IJ survived up to 48 h within flowers and flower buds, few nematode-infected thrips (larvae and adults) were recovered. In studies with T. nicklewoodi (which is not amenable for mass production), the inoculative releases of two parasitized hosts per plant enabled the nematode to become established within existing WFT populations under greenhouse conditions. However, relatively poor transmission and slow speed of kill (nematode primarily suppresses populations through host sterilization) prevented low level inoculations being effective over a single crop cycle. Further studies showed that transmission of T. nicklewoodi persisted for nine host generations, infected up to 83% of adult thrips and provided long-term suppression of discrete caged populations, but only after uneconomically high thrips densities had been reached.

Crop colonisation, feeding, and reproduction by the predatory beetle, Hippodamia convergens, as indicated by stable carbon isotope analysis

Abstract.  1. Habitat management to enhance natural enemy populations in agricultural systems may help regulate levels of crop pests, but little research addresses the behaviour of immigrating beneficial insects.

Field Testing Rubidium Marking for Quantifying Intercrop Movement of Predatory Arthropods

Abstract Applications of two doses of an internal physiological marker, rubidium, were applied as foliar solutions of rubidium chloride (RbCl) to field plots of cotton, Gossypium hirsutum L., and grain sorghum, Sorghum bicolor (L.) Moench, to determine the potential of this technique for detecting intercrop movement of generalist predators. Because artificially elevated rubidium levels of marked plants and insects decline with time, three applications were made at distinct phenological stages of grain sorghum to test if multiple mark-recapture experiments could be conducted at the same sites. Samples of predators and plant tissues were made at the onset of each sampling period and every 2 d through 6 d after RbCl application. Results showed that both crops, as well as all arthropod predators tested, exhibited significant elevation of rubidium levels over field-collected controls. Further, the effectiveness of the mark did not differ between crops, indicating that one set of protocols is sufficient for marking in both crops. No differences in mark effectiveness were detected between doses until the last sampling period, 6 d after mark application, and then only in one of three taxa. Using this method, we detected predator movement between crops, and the percentage of positive recaptures (>6%) was higher than those in similar studies using different marking systems. These results collectively suggest that this method can be used to elucidate the relationship between predator populations in these crops, and that it is potentially applicable to other multiple-crop agroecosystems. RESUMEN Aplicaciones foliares de rubidio, un marcador fisiológico interno, en soluciones de cloruro de rubidio (RbCl) a dos concentraciones fueron hechas en parcelas de campo de algodón, Gossypium hirtusum L., y sorgo, Sorghum bicolor (L.) Moench, para determinar el potencial de esta técnica en el estudio del movimiento entre cultivos, de depredadores generalistas comunes a ambos. Tres aplicaciones fueron hechas a distintas etapas fisiológicas del sorgo para determinar si varias pruebas de marca-recaptura pueden ser conducidas en sitios similares. Muestras de plantas y de artrópodos depredadores fueron tomadas al comienzo de cada prueba y cada dos dias hasta seis dias despues de cada aplicación de RbCl. Resultados mostraron que en ambos cultivos y en todos los artrópodos depredadores que se analizaron, los niveles de rubidio fueron significativamente elevados en comparación a controles de campo y que la efectividad de la marca no varió entre cultivos, indicando que un solo procedimiento es suficiente para marcar en ambos cultivos. No se detectaron diferencias en la efectividad de la marca entre las dos concentraciones usadas hasta el ultimo muestreo, seis dias despues de la aplicación, y solo en uno de los tres grupos taxónomicos examinados. La recolección de depredadores con marca positiva en parcelas cercanas no tratadas, indicó que la marca fue suficiente para detectar el movimiento de depredadores de un cultivo a otro. El porcentage de recapturas positivas fue mas alto (>6%) que en estudios similares previos. Estos resultados colectivamente sugieren que este método ayudará a aclarar la relación entre poblaciones de depredadores en estos cultivos y que es potencialmente aplicable a otros agro-ecosistemas de cultivos multiples.

Using hosts of mixed sizes to reduce male-biased sex ratio in the parasitoid wasp, Diglyphus isaea

Overproduction of males in mass rearing of parasitic Hymenoptera contributes to higher costs for biological control because only females directly kill pests. We present a technique, based on manipulating host composition, to generate less male‐biased sex ratios in parasitoid species that adjust their sex allocation in response to relative host size. Our system consisted of chrysanthemum, Dendranthema grandiflora Tzvelev var. ‘Miramar’; a leafminer, Liriomyza langei Frick (Diptera: Agromyzidae); and a commercially available parasitoid, Diglyphus isaea (Walker) (Hymenoptera: Eulophidae). We compared the offspring sex ratios of D. isaea females presented with different compositions of L. langei larvae on chrysanthemum. Presenting individual females with only large hosts increased mean sex ratio from 32 to 67% male over 2 days. However, presenting individual females with progressively larger hosts over 1 or 2 days reduced mean sex ratio from 90 to 100% male to less than 30% male. Groups of females produced sex ratios around 58% male if presented with both plants infested by only small hosts and plants infested by only large hosts. In comparison, groups of females produced sex ratios around 48% male if presented with plants infested by both small hosts and large hosts. We compared the use of both small hosts and large hosts to only large hosts for simulated mass rearing of wasps over 8 weeks. Using both small hosts and large hosts produced similar numbers of wasps as using only large hosts, but reduced mean sex ratio of weekly cohorts from 66% male to 56% male. The two techniques produced females of similar size, but using both large hosts and small hosts produced slightly smaller males than using only large hosts. The use of both small hosts and large hosts for mass rearing of D. isaea could reduce actual costs of females by 23%.