Dept. of Horticulture

Education and research for a green world

Monitoring Techniques
Monitoring plays an important role in successful integrated pest management programs. Effective implementation of a pest management program requires routine monitoring of pest activity. Currently, no economically viable monitoring strategy exists for cabbage maggot in Oregon. Over the last five years, we have been developing on-farm monitoring techniques for detecting spring emergence, seasonal flight, egg-laying events, and assessing mid-season injury and crop harvest damage. Our tools will provide growers and pest control advisors a way to project the time of emergence, pinpoint the location and levels of maggot infestations in a field, and allow timely and appropriate control measures to be applied. Documentation of fly activity is essential. The number of insects, the types of insects, and their stage of growth should all be recorded.

Adult flight, egg laying, larval injury, and harvest damage were monitored in the northern Willamette Valley beginning July 2000 throughout the entire growing season of 2005. We have developed a predictive model below, spring to estimated emergence and flight, in which thermal units are accumulated beginning January 1st with a 4.3 °C standard base threshold temperature. DD accumulations are calculated with weather data obtained from an "On-Line Insect Phenology" website (http://pnwpest.org/cgi-bin/ddmodel.pl). Our data indicates that there are three to five generations and five concerted egg-laying periods during the growing season and several substantial low-risk planting windows. Stay tuned in 2006 for a complete package maggot management tools.

Jump to: Cabbage Maggot Life Cycle | How does Degree-day modeling work? | How can we calculate the number of degree days? | Cabbage Maggot Degree Day Worksheet 2006 | Some References | Spring Emergence and Flight | Scouting Egg Levels | Crop Damage Assessment | Prevention and Exclusion Methods | Spatial Management and Mapping | Field Cultivation and Sanitation | Exclusion Fences and Row Covers

Degree–Day Modeling: When are cabbage maggots flying and laying eggs?
Half the battle in fighting pests is determining when pests arrive and when pests are most susceptible to treatment. Degree–day (DD) modeling is a tool to help predict when pests may become a problem. Using degree–days can be a wake up call or a pest alert system to help growers forecast cabbage maggot activity. Knowing the estimated Cabbage Maggot DD can help:

1. Time planting and harvesting dates (plant crop after high pest risk periods and harvest before fall flight);
2. Decide when to set up egg barriers in a field such as exclusion fences and row covers; and,
3. Predict the optimal time to treat for pests.

Studies conducted in the northern Willamette Valley (Clackamas and Marion counties–south of Portland Oregon), confirmed that timing early sprays to high egg-laying periods with the use of a DD model could be very effective. When damage levels were assessed in fields with two sprays timed according to 50% peak flight (DD model) were compared to plots treated with four scheduled un–timed sprays, there was no difference in amount of root damage caused from cabbage maggots.

Some people associate the term "modeling" with absolute reverence or some say it is hopeless numerical silliness; but neither is appropriate. We are developing a degree–day model for predicting cabbage maggot spring emergence from overwintering pupae and forecasting seasonal flight activity of the cabbage maggot in the Willamette Valley, Oregon. The prediction model, like much of agricultural decision–making, is our best educated guess based on of data 4 years of data (2001 through 2004) collected in Brassica fields in the northern Willamette Valley. The intent in providing a cabbage maggot DD model is NOT to have a computer attempting to make pest management decisions, but to improve the quality of information based upon which a grower can make decisions. In no way is the model intended to replace direct observations or experience or knowledge of particular field sites. The DD model and estimated times of maggot activity is just another tool and piece of the maggot puzzle. Our hope is that with experience in using the DD model and learning how it relates to the real world in a field, it will be a useful addition to the Integrated Pest Management (IPM) 'Toolbox'. Like any tool, the value depends on how well it is used.

See MagNet Newsletter: Winter 2006-Degree-Day Modeling (link not ready yet)

Cabbage Maggot Life Cycle Return to top of page
We estimate 3–5 generations of cabbage maggot each year in the northern Willamette Valley. Each cycle takes approximately 41-65 days depending on the time of year. The cabbage maggot, also known as cabbage root fly, feeds on flowers for nectar and pollen, protein and carbohydrate sources, before mating. Female flies lay their eggs at the base of cabbage family plants called Brassicas. Eggs hatch (eclose) within 3–7 days. Larvae (maggots) feed for approx. 2–4 weeks on the root. After feeding, maggots turn into brown pupae (4–6mm size) and remain in the soil for 1–2 weeks before emerging into the next generation of flies. In the fall, cabbage maggot form overwintering puparia under the soil which emerge as flies the following spring (March-April).

Fig. 1. Life cycle of the cabbage maggot (Delia radicum (L.)).

How does Degree–Day modeling work? Return to top of page
Unlike humans, an insect's growth and development depends on the temperature of their environment. In warm years, insects develop faster then they do in cold years. For example, in the Valley it was much colder (and drier) in the spring of 2001 and because of these conditions took much longer for the maggot to develop and emerge from the soil. Insects often will not develop below a certain temperature. A cabbage maggot does not develop and grow below a lower threshold temperature of 39.7°F (4.3°C) or over an upper threshold temperature of 86°F (30°C). This knowledge is used to create a DD model for the cabbage maggot in the northern Willamette Valley and will be included in 2006 on: OSU, Integrated Plant Protection Center Online Phenology and Degree–Day Models in the Pacific Northwest (http://pnwpest.org/cgi-bin/ddmodel.pl).

1.) You can select the cabbage maggot model or manually enter data at this site including: Degree–day calculator enter your own thresholds.
2.) The lower and upper thresholds;
3.) Centigrade or fahrenhight preference;
4.) Starting and ending dates;
5.) And, you can select a weather station location or upload your own weather data file that is closest to you.

Below is an image of the online phenology screen and data needed to obtain current degree–day.

Fig. 2. Manually enter data at phenology model site.

Lower temp- 39.7°F Upper temp- 86°F Choose Single sine Select closest weather station: (Auroranwes or Agrimet) Calculate degree-days
	Website: http://pnwpest.org/cgi-bin/ddmodel.pl

Here are the results of accumulated degree–day values that the modeling website generated:
Image of results

Fig. 3 Screen showing Degree-Day Results in 2001.

How can we calculate the number of degree days? Return to top of page

Degree–day modeling calculates the quantity of time the temperature of a day has been satisfactory for insect development. "One degree–day is one day (24 hours) with the temperature above the lower development threshold by one degree". Since the temperature changes constantly throughout the day, several approximation methods have been developed to determine what the degree–day is for a specific day. The simplest method is called the Simple Average (subtracts the min temp from the max temp divides by two and then subtracts the min threshold temp and assumes temperature increases linearly from minimum to maximum). A more realistic approximation method of calculating degree–days is called the Single Sine, which assumes the increase from minimum to maximum temperature is curvilinear. The cabbage maggot degree–day model developed for the Willamette Valley uses a Single Sine approximation with a horizontal cutoff for temperatures over the upper temperature threshold. The weather data used for the model was obtained from National Weather Service (AgriMet) in Aurora, Oregon at the OSU–NWREC station at www.usbr.gov/pn/agrimet/wxdata.html. The Aurora weather was chosen as the central location to provide a regional estimation of flight. The weather obtained may over or under estimate times of insect activity, and most likely will not match your field exactly. You can use a min–max thermometer or perhaps you have an on–farm weather station to obtain daily temperatures for predicting cabbage maggot activity closer to your field site.

By counting the number of degree–days beginning January 1st you can approximate when adult flies will with emerge from cabbage maggot puparia in the soil. To make things a little more interesting there are actually two different emergence times for the same overwintering population. The initial fly emergence (10%) of first emergence (early emergers consist of 70% of total spring population) begins at approx. 360° F DD (~200°C DD) and peaks (50%) at 454°F DD (242°C DD) (an average calendar date of March 23). The second emergence (30% of total population) peaks at 1235°F DD (714°C DD) (on average May 24). Flight into a field is delayed after emergence is detected by 5 days to 3 weeks, so close monitoring field activity is important. The end of spring flight (monitored with yellow water buckets) occurs after an accumulation of 1521°F DD (839°C DD) (on average June 8). Spring and fall cabbage maggot populations generally produce higher damage levels than summer populations because of favorable cool wet weather. If spring populations are not controlled and the environmental conditions are right, high levels of damage can extend into summer months. The beginning of fall flight starts at approx. 3849°F DD (2138°C DD) (average calendar date of 8/28) and ends around 5160°F DD (2860°C DD) (average calendar date of 11/4; first frost). Figure 2 graphically represents spring flight, the generations of cabbage maggot, and fall flight. Table 1 is provided as an easy–to–read reference showing degree–day totals and expected cabbage maggot activity. Less damage has been reported if a crop is planted after the spring flight (1521°F DD = 839°C DD) and harvested before the fall flight (3849°F DD = 2138°C DD).

Table 1. Important cabbage maggot activities in Brassica root crops associated with degree–day values (°F)in the Willamette Valley, OR

Cabbage Maggot Acitivity	*Degree-Days (°F)**	Approx. Calendar Date
Initial spring emergence (10%)	360 DD	Mar 8
Peak (50%) emergence	601 DD	April 4
End of spring flight (95%)	1521 DD	June 8
Egg-laying	~7 days after peak flight *Prefer developed plants (~30 days after planting)	Females lay eggs for 5 weeks, concentrating egg-laying over 2-week period
Fall flight begins	3849 DD	Aug 28
Fall flight ends (first frost)	5160 DD	Nov 4, first frost

There are countless factors that can influence insect development such as soil moisture (rainfall, irrigation), soil type (clay vs. sand), humidity, health of plant (nutrition, location in field: borders vs. middle) and day length; but temperature appears to be the primary factor. If you keep track of how warm the weather is beginning January 1st, you can predict CM activity (spring emergence and flight).

Figure 4. Early- and late-spring emerging population flight patterns. Two line patterns represent two different hatch dates from same overwintering generation and their suspected subsequent generations.

Cabbage Maggot Degree Day Worksheet 2006 Return to top of page
A worksheet is provided to record daily air temperatures on your farm, if you do not choose to use the automated online DD website (http://pnwpest.org/cgi-bin/ddmodel.pl).

Record the minimum and maximum temperature each day starting January 1st (scientists call this the BIOFIX Date). Using temperature readings at your field or a neighboring field makes a better calculation of degree–days for the local population of cabbage maggot. If you don't have an accurate thermometer in your area you can find the regional temperatures at: http://pnwpest.org/cgi-bin/ddmodel.pl. In fact, the best temperature is the soil temperature where the cabbage maggot is living, however the DD model is based on air temperature. Use the degree–day look–up table provided to determine the quantity of DD earned that day and add to worksheet.
Cumulatively sum the total DD daily.
Place yellow water traps on NE and SW borders of fields (depending on prevailing wind in the area), at a height of approximately 30cm and free from interfering vegetation, when 100 DDs have accumulated. Adjust the height of the trap as the plants grow. Make sure weeds do not obscure the trap. Watch for increases in flight. Flies move into the wind cueing into the chemical smells coming off Brassica plants. They also key into the visual sight of the plant.
Monitor for eggs in your fields following increased numbers flight to determine if flies actually chose your field. Ask yourself these questions to understand the potential risk of your field: Are your plants at a vulnerable stage of plant attractiveness (5–9 leaves, ~30 days after planting a root crop; ¼–½" roots) compared to other fields around you? Is your field next to an infestation source that occurred over the last 6 months? Does the planted field have a history of damage? Are you planting during a high–risk period (spring)? Is the field going to harvested before fall flight?
Record the actual dates for high fly numbers and percent egg levels. This will help you refine the DD model for your local area. You may also want to note local flowering plants that are flowering or setting leaf the same time as cabbage maggot emergence, high flight or egg–laying to give you additional warning signs next year.

References: Return to top of page
[1] UC IPM: How to manage pests: Degree–days
http://axp.ipm.ucdavis.edu/WEATHER/ddconcepts.html

How To: Forecast pest problems
http://grounds-mag.com/mag/grounds_maintenance_forecast_pest_problems/

WSU Degree Day Modeling
http://entomology.tfrec.wsu.edu/modelling.html

Spring Emergence and Flight Return to top of page

	Emergent Cages During the fall seasons of 2000 - 2004, overwintering pupae were collected from infested rutabaga and turnip fields in the northern Willamette Valley. Collected pupae were buried under the soil at a depth of 5 -7.5 cm and an emergence cage (11 cm diameter) was placed over them (Figs. 1 and 2). The following spring, the cages were monitored for fly emergence until July 1st. Overwintering flies emerged from puparia and they moved upward into the collection container above. Number of flies caught was documented on a weekly basis. Fig. 2: Large emergent cages also were used for monitoring emergence of flies.
Fig. 1: Emergent cage used for monitoring spring emergence of D. radicum
	Yellow Water Traps A yellow water trap (Finch, 1991; Bracken 1988) was placed in the north-east corner of many growers' fields grown in root crops (Fig. 3). The trap was set above canopy level, free from surrounding vegetation. Flies were collected weekly from the water traps and serviced with new water and soap (to reduce surface tension). The traps were adequate for monitoring seasonal activity of adult flies, but the captures were unreliable as quantitative indicators of potential egg-laying and damage.
Fig. 3: A yellow water trap used to detect flight in the middle of a field and compared to fly catch of traps located on the perimeter of a field. More flies were caught in traps located on north-east sides of fields, the area of prevailing winds.

Scouting Egg Levels Return to top of page
egg scrape photo

Crop Damage Assessment Return to top of page
mid-season damage assessment photo

Prevention and Exclusion Methods Return to top of page
An Integrated Pest Management (IPM) 'toolbox' for cabbage maggots is being designed using a combination of relevant practices that prevent, maintain, and/or suppress cabbage maggots below economic thresholds, with the least impact on human health, the environment, and non-target organisms; and provide economic savings to the grower. Reliance on a single management tool like Lorsban, has shown to have undesirable consequences (e.g. resistance, secondary pest outbreaks, environmental hazards). By demonstrating the strengths and weaknesses of using a multiple array of tools on the farm, the 'PEST' Plan will help growers develop an economic plan with a sound IPM strategy for their personal farming system.

MagNet goals are to: 1) encourage use of farming practices that prevent or avoid pest build up, 2) expand the use of monitoring for better decision-making, 3) implement multiple, pest management tools and, 4) provide economical, practical, reliable, and safe practices to reduce pest loads.

Described below are 4 key principles of IPM, represented by the acronym "P.E.S.T."- Prevention, Exclusion, Suppression, and Thoughtfulness (Dreves 1996). Through the integrated use all of these principles, the grower is provided with a framework that supports long-term pest management rather than responding to pest problems with reactive, short-term solutions. Growing healthy plants under healthy soil conditions are baseline defenses against pests. It is important to monitor for pest presence on a regular basis to help ensure sound decision-making and avoid unnecessary treatment costs.

P = Prevention
Focus on preventing pest problems by implementing good farming practices, understanding the pest's life cycle and their vulnerabilities. Learn about factors that contribute to pest population increases and work to reduce or work with those conditions in your field. For example use of fall cultivation and sanitation practices to prevent maggot build-up; rotate new plantings away from maggot sources.

E = Exclusion
Design strategies to aid in repelling or deterring a pest from a crop. Monitor for pests to eliminate unnecessary treatment costs. For example, the use of row covers and exclusion fences.

S = Suppression
Select least harmful treatments and use effective application equipment. Apply treatments when pest levels have exceeded established tolerances and economic threshold levels. Rotate chemicals, be aware of potential resistance, and understand impact and risk of treatments.

T = Thoughtfulness
Educate yourself and farm staff, and learn more about pest biology, life cycles and pest behaviors. Determine how serious a pest problem is and research viable options before actions are taken. Acquire knowledge of landscape and surrounding ecosystem's effect on pest levels, which is important for an IPM program to be most effective.

Spatial Management and Mapping Return to top of page
photo of potential sources of cabbage maggots

photo of potential sources of cabbage maggots

GIS mapping of Brassica fields Return to top of page
In collaboration with Dr. Tim Righetti, from Oregon State University, we are developing an Excel-based geographic information system (GIS) that can be used to conduct most routine analytical procedures for many farm management purposes (Righetti and Halbleib, 2000; Halbleib 2001). Field attributes and the Universal Transverse Mercator (UTM) coordinates for a field are entered into Excel. The database can then be queried using standard Excel features and procedures. Spatial relationships are visualized by presenting the results of a query in an Excel scatter plot. The x and y axes of scatter plots are automatically scaled such that any geo-referenced image or map can be used as the scatter plot background. More complex GIS procedures, evaluations, and data analysis can also be accomplished using Excel. We have been working with Righetti to design a system suitable for the cole crop growers.

An example of a query that uses this Excel-based GIS approach is presented in Fig. 5a. In this example, we used data collected in 2001 - areas within 1/4 mile of a potential cabbage maggot-source (red stars represent the source). The sites with cabbage maggot damage assessments greater than 15% are represented with yellow dots. Most of these locations are clustered around identified sources. Locations with low cabbage maggot damage assessments (<15%) are presented as green dots. Low cabbage maggot sites are generally found at greater distances from potential cabbage maggot sources than sites with more damage. In Fig. 5b, the same query has been conducted using Arc View, the most widely used currently available GIS software package. Results are visually similar. This figure clearly suggests that the combination of degree-day predictions, monitoring, and spatial rotation of fields has potential to significantly reduce cabbage maggot infestation levels and the use of chlorpyrifos.

A spreadsheet environment has many advantages over currently available precision agriculture or other spatial management packages. Microsoft's Excel is inexpensive and available on almost every PC. Most growers are familiar with it. Furthermore, the advanced spreadsheet skills that one acquires while learning to process spatial information have broad applications to other farm management issues. A final advantage is that it is relatively simple to link the Excel files of individual growers to a web-based data system that combines information from many sources into a single usable spatial database.

figure 5a photo of computer print screen

Fig. 5a. Cabbage maggot infestation levels and cabbage maggot source fields: Excel-based. Lines represent 0.25 miles from source field. Yellow dots = >15% egg-infested plants; green dots = <15% egg infested plants.

figure 5b photo of computer print screen

Fig. 5b. cabbage maggot infestation levels and cabbage maggot source fields: ArcView. Dotted lines represent 0.25 miles from source field. Yellow dots = >15% egg-infested plants; green dots = <15% egg infested plants.

We believe that this inexpensive and grower friendly Excel-based GIS system has the potential to be successfully used by extension agents, growers and grower groups for spatial pest management in commercial production environments.

We envision a system where growers share an Excel-based, web-linked regional database that contains past planting records for potential cabbage maggot hosts. Field boundaries would be included in an easy to use system that provides a simple visual assessment of the status of each field. Planting dates, harvest dates, and records of pest incidence would all be entered into a spatial database. The locations of naturally occurring wild potential hosts and proposed future plantings would be entered into the system. Field information would be updated as monitoring of insect incidence proceeds throughout the growing season.

As a grower plans for the future he could quickly learn the planting and disease incidence history, or future plans of fields nearby any of his fields. One could also determine if potential natural hosts are known to exist at nearby locations. A grower's fields that have the least chance of becoming severely infested could be identified. By simply scheduling a planting at a later time, flies in adjacent fields may mature and die or eggs may dry up later in the season. Monitoring information and growth models can be used to predict when potential infestation from an adjacent field will decline. As growers become more familiar with the database, increasingly complex queries could be made.

Field Cultivation and Sanitation     Return to top of page

View Entire PDF Report: Fall Cultivation Trial 2003

Objective: To test the efficacy of fall cultivation practices to reduce the spring population of cabbage maggots (Delia radicum L., Diptera: Anthomyiidae)

Introduction:
Spring emergence of cabbage maggot populations (Diptera: Anthomyiidae, Delia radicum (L.)) can lead to severe maggot infestations throughout the year. Many growers disk their fields in the Fall in order to chop up the roots housing the pupae. Some growers plow the roots under, hoping that the flies can't emerge if they're buried deep enough. Others leave the disked roots on the surface, hoping that the larvae and pupae will dry out or be exposed to extreme weather, or exposed to natural enemies.

To test the efficacy of fall cultural practices for reducing fly emergence of overwintering pupae in the spring, we set up a three-treatment trial in a heavily-infested field of turnips and rutabagas (>50% damage). The dry fall weather allowed us to test the cultivation practice in the fall.

Exclusion Fences and Row Covers     Return to top of page

View Entire PDF Report: On-Farm Preliminary Evaluation of Exclusion Fence Usage for the Purpose of Managing Cabbage Maggot, Delia radicum (L.) in Turnips. Canby, Oregon Summer 2005, Summary Report 10/20/05

Objective: The objective of this study was to investigate the effect of an exclusion fence on the movement of D. radicum into fences and the reduction of CM damage in turnips.

Introduction:
Due to another mild winter with adequate spring precipitation, cabbage maggot (CM; Delia radicum L.) pressure was high in the northern Willamette Valley. Little to no control of spring & summer CM populations was reported after using scheduled sprayings of Lorsban, an organophosphate insecticide. The cabbage maggot is proving to be a major and devastating pest problem in the northern Willamette Valley and is spreading to other Brassica fields quite rapidly.

Several exclusion mechanisms have been suggested as a means to reduce CM damage in Brassica fields. One method is to construct a fence that will reduce the flight of the CM to a newly planted field. It has been observed that CM flies are low elevation and weak fliers If they meet another obstruction above them they will drop back down. A fence that has a lip on the top or an overhang will deter most flies from crossing in that direction. This offers possibility for cabbage maggot by using a suitable physical insect-exclusion barrier to reduce, or if possible prevent migration of adults into newly planted fields.

Benefiticial Habitat Enhancement Study    View Entire PDF Report   Return to top of page
Introduction:
The cabbage maggot (Diptera: Anthiomyiidae, Delia radicum (L.)) has many natural enemies. Maggot predators include rove beetles (Coleoptera: Staphylinidae) and ground beetles (Carabidae) (Bromley 2003). Big-eyed bugs, ladybugs, minute pirate bugs, damsel bugs, spiders, and centipedes are also seen roaming the soil surface and may play a role in feeding on maggot eggs found in the Brassica fields in western Oregon.

Placing straw in the irrigation furrows of onion fields in eastern Oregon successfully reduced the level of onion thrips, and increased the number of beneficial insects caught in field traps (Jenson 2001). The straw mulch most likely attracted beneficials and created a habitat and/or refuge for the beneficial insects.

We explored the idea of using straw mulch in a conventionally-managed Brassica field, to increase the number of beneficial insects in the field, thereby reducing cabbage maggot damage.

Objective: To reduce damage by cabbage maggots on turnips by creating an environment that attracts predatory insects.