Potential for Conservation Biological Control of Stink bugs in North Carolina

Potential for Conservation Biological Control of Stink bugs in North Carolina

Summary

In order to realize the full potential of egg parasitoids as biological control agents of stink bugs, it is important to understand critical aspects of their life history. This study uses the native egg parasitoid, Telenomus podisi, as a model to improve understanding of overwintering and adult nutrition requirements of these parasitoids in the field. Laboratory experiments indicate that in conditions simulating those found in winter in the southeastern U.S., T. podisi adults prefer to hang upside down between two surfaces of natural materials at least 1.5 mm apart. This suggests leaf littler may be the preferred overwintering refuge as opposed to tree bark. Insects are currently being extracted and identified from samples of leaf litter and tree bark taken next to soybean fields with known stink bug populations. Although studies with other parasitoids indicate aphid honeydew (collected from aphids, offered in ‘pure’ form) is an inferior food source, laboratory studies of adult lifespan and reproductive capacity of T. podisi indicate that aphid honeydew is as good a food source as honey or nectar. Because honeydew was offered in situ (i.e. parasitoids were offered intact leaf on plant with aphids present) a new study has begun to examine possible influence of plant surfaces on the quality of honeydew as a food source.

Objectives/Performance Targets

1. Determine the presence of overwintering T. podisi in potential refuge sites like tree bark, dried fruiting and flowering bodies, insect cocoons, fallen pine cones etc. that might be utilized by T. podisi in the field.
2. Determine the lifespan and reproductive ability of T. podisi when offered carbohydrate sources in the form of various flowering plants, and homopteran honeydew.

Accomplishments/Milestones

Objective One

In 2012 three new sites were targeted for winter sampling near Clayton (Johnston County, NC) that had a south-facing woodland border next to a soybean field with stink bug populations. A total of 57 fine mesh cages with a funnel-bottle collection system at their tops were wrapped around 3 feet of tree boles near the ground in January 2013 (Fig.1 in Document link). Ten pairs of pine and sweet gum trees were caged at each of two locations, and 7 pines and 11 sweet gums were caged at the third location. Insects that emerge in the spring from tree bark will be collected and identified. In addition to tree caging, leaf litter samples were collected from 1 sq. ft. areas adjacent to trees on the north and south sides of caged trees, and between tree pairs, at the first two of the three locations. These samples were taken to a greenhouse for extraction using mesh bags with funnel-bottle collectors at the top (Fig. 2 in Document link). Insects were collected, preserved in 70% ethanol, and are currently being identified. To date, one scelionid has been found in the samples.

A series of laboratory studies were done to better understand the space and orientation requirements of T. podisi females under overwintering conditions. Refuges of varying dimensions were built out of paper or hatched stink bug eggs and presented to 2-day old mated T. podisi females in petri-dishes. These arenas were exposed to overwintering conditions over a 5 day period in a growth chamber as reported in Table 1 in Document link.

For each of the experiments, on day 5 the position of wasps was observed and reported. To ascertain that wasps were not being attracted to the glue used in making refuges, 2-day old mated females were presented with a 2 choice test wherein one refuge (dimensions of 1cm x 1cm x 1cm) was glued to parafilm, which in turn was glued to the base of the petri-dish, while the other simply rested on the base of the petri-dish, with no glue used on it. Results indicated that 29 of 50 wasps went inside either of the paper refuges. Of those 29, 15 went in the refuge with glue and 14 went in the refuge without glue, indicating no preference for refuges with glue. Additional experiments conducted on the wasps with different types of refuges was done with position of the refuges randomized within the petri-dish. Results are shown in Fig. 3, 4A, 5A, 6A and 7A in Document link.

Discussion: Statistical analysis of the data is currently being conducted. However, it appears that, when exposed to overwintering conditions, T. podisi prefer to hang upside down from tops of refuges made of natural materials that have at least 1.5 mm between surfaces, and appear to prefer refuges that offer more cover. This suggests that T. podisi may prefer leaf litter as a winter refuge. The field sampling component of this study (currently underway) should help clarify this.

Objective Two

Colonies of the following species of insects and plants were reared throughout 2012:
Insects-
Podisus maculiventris (Spined soldier bug)
Telenomus podisi (Egg parasitoid)
Tenebrio molitor (Mealworm/ Darkling Bettle) (Food source for the P. maculiventris colony)
Aphid species (Aphis craccivora)

Plants –
Fagopyrum esculentum (Buckwheat) (for flowers to be used in Objective 2 studies)
Glycine max (Soybean) (for P. maculiventris colony)
Vicia fava (Fava Beans) (for aphids to be used in Objective 2 studies)

Methods:

Longevity and fecundity of T. podisi was estimated in cages (Fig. 8 in Document link) using a randomized complete block design, with position of cages on shelves in a rearing-room acting as blocks. The treatments were as follows: Treatment 1 – Exposure of a single 2-day old mated female T. podisi to flower of buckwheat still attached to the plant. Treatment 2 – Exposure of a single 2-day old mated female T. podisi to a single fava bean leaf infested with cowpea aphids. The leaf remained attached to the plant. Leaves selected were not too old (near the base of the plant) or too young (at the apex of the plant), but rather in the middle, so that they did not wilt easily and were large enough to support a respectable number of aphids. The leaf was allowed to have 5 aphids at the start of the experiment. Control 1 – Clover honey in single mated 2-days old female T. podisi trials was provided as a single streak of pure honey approx. 50 mm long applied with an insect pin to the internal wall of the cage on the first day of each replication. Control 2 – Organic clover honey in single mated 2-days old female T. podisi trials was provided as a single streak of pure honey approx. 50 mm long applied with an insect pin to the internal wall of the cage on the first day of each replication. All the above mentioned treatments and control cages were misted with tap water everyday. Control 3 – Only tap water was misted on the cages regularly. Experimental cages were fashioned according to Witting-Bissinger et al. (2008). All experiments using T. podisi were carried out in a rearing room at 25 degree C; 70 % RH, and a 14 h photophase. Potted plants were placed on metal shelves so that flowers can be inserted between slots in the shelf above. Flower heads were positioned so that the head touches the inside of the lower screen and side of cages. Plants and their flowers were replaced when approx. 50% of florets had senesced. A single leaf, infested with cowpea aphids were also positioned in the treatment cage similarly. The leaf was replaced when it started showing signs of wilting or drying. Longevity of the wasps were recorded.

Fecundity was assessed for T. podisi during the same experiment used to monitor longevity. Each female T. podisi was provided with one Podisus maculiventris egg mass per day. The number of eggs per mass was rationed with 20 eggs/mass on the first day of the experiment. In the next 2 days, 10 eggs/mass were provided daily. From the fourth day of the experiment onwards, 5 eggs/mass were provided as long as the wasp survived. After every 24 hours, each egg mass was collected in vials and held for offspring emergence in the same rearing room. Only those females that produced both female and male progeny (i.e. mated) were included in data analysis.

Results and Discussion:
Although statistical analysis of the data is currently being conducted, it appears from the data (Fig. 9. A-E in Document link) that aphid honeydew may be as beneficial as nectar or honey in terms of enhancing the fecundity and longevity of Telenomus podisi. This is a deviation from other studies dealing with honeydew and parasitoids which concluded that honeydew is typically an inferior food source compared with artificial sugar-rich foods and some floral nectars because it contains sugars that do not readily elicit a feeding response or are nutritionally less suitable (Lee et al., 2004; Wackers, 2001). This may be explained by our presenting honeydew in situ (i.e. on living leaves with aphids present) as opposed to collected directly from aphids. Additional studies are currently underway to explore the possibility of phyllosphere yeasts enhancing nutritional value of honeydew.

• Photos, Tables and Graphs supporting project accomplishments

Impacts and Contributions/Outcomes

The results from the above mentioned experiments suggest that in the field, T. podisi may be able to benefit equally from nectar as well as honeydew which is ubiquitously present in agro-ecosystems. Field and lab overwintering studies to date suggest the importance of woodland field borders with leaf litter from hardwood trees as a refuge area during winter for scelionid parasitoids. This study, when complete should help to guide future conservation biological control efforts with both native and imported scelionid egg parasitoids of stink bugs and their relatives.

• Lee, J.C., Heimpel, G.E., Leibee, G.L., 2004. Comparing floral nectar and aphid honeydew diets on the longevity and nutrient levels of a parasitoid wasp. Entomol. Exp. Appl. 111, 189–199. Wackers, F., 2001. A comparison of nectar and honeydew sugars with respect to their utilization by the hymopteran parasitoid Cotesia glomerata. J. Insect Physiol. 47, 1077–1084. Witting-Bissinger, B. E.; Orr, D. B. and Linker, H. M. 2008. Effects of floral resources on fitness of the parasitoids Trichogramma exiguum (Hymenoptera: Trichogrammatidae) and Cotesia congregata (Hymenoptera: Braconidae). Biological Control 47: 180–186.

Collaborators: