HPIPM:Mexican Bean Beetle

From BugwoodWiki

Author: Gary L. Hein & Frank B. Peairs[1]

Photo by Frank Peairs, Colorado State University, Bugwood.org

Photo by Frank Peairs, Colorado State University, Bugwood.org

Photo by Eugene E. Nelson, , Bugwood.org

Mexican bean beetle, Epilancha varivestis Mulsant, is perhaps the most serious insect pest of dry beans in the High Plains region. Recent research has pointed to the usefulness of sampling egg masses in early July to make treatment decisions rather than waiting until damage has begun.

Identification (and life cycle/seasonal history)

Mexican bean beetle adults overwinter in debris in fields, along field margins and in fence rows. The beetles move out into the dry bean fields over an extended period of time in June and July. Females begin to lay egg masses on the beans after they have fed for 1 to 2 weeks. The overwintering adults are bronze in color and about 1/4 to 1/3 inch with 16 black spots on their wing covers. Bright yellow eggs are laid in masses of about 40 to 60 eggs each. Each female may lay up to 500 eggs over a 5-week period. Eggs will hatch in 5 to 10 days, and larvae are bright yellow with branched spines that give the larvae a fuzzy appearance. The spines of the later instars are black. Larvae will mature over the next 3 to 4 weeks, after which they pupate for about a week. Adults that emerge from the pupae will initially be bright yellow with black spots. Generally, one or two generations develop in this region. Eggs of the first generation will hatch in late June or July, and larvae develop through July into August. The next generation of adults will emerge in late July or early August and lay eggs in early August. Larvae of this second generation will mature through August and pupate in late August or early September.

Plant Response and Damage

Larvae and adults of the Mexican bean beetle feed on the underside of leaves, stripping the epidermis from the leaf and leaving a window-pane or skeletonizing damage. Leaf consumption increases as larvae increase in size. Two-thirds of the leaf area consumed by the larva will be consumed by the fourth or last instar larva. An adult beetle will also do considerable feeding and can consume over three times the daily leaf surface area as the fourth instar. If large numbers of larvae are still feeding when leaves are mostly yellowed, some larvae may begin to feed on the pods that remain green. This pod feeding does not commonly occur.

Management Approaches

Individual field history can be an important factor in determining beetle potential. Significant Mexican bean beetle populations will result when beetle populations the previous year were high and winter conditions were mild. Cold temperatures and minimal snowfall will result in increased winter mortality.

The following considerations are important in managing Mexican bean beetle in dry beans:

• Avoid early planting because overwintering adults are attracted to these fields and often concentrate in these early emerging fields.

• Scout fields for Mexican bean beetle adult damage after emergence, especially early planted fields. Adults may congregate in fields and cause significant early season defoliation on small beans.

• Avoid unnecessary insecticide applications to preserve biological control species. Many predator species are often present and these may impact egg mass survival.

• Preplant systemic insecticides can be used in fields that have a high risk of damage (e.g. planted early, and/or experienced very high populations previous year with mild winter).

The major defoliation from the Mexican bean beetle will begin in mid July and continue through August. Two sampling methods can be used to determine the potential for economic damage: egg mass sampling and defoliation/larval sampling.

Egg mass sampling is carried out just after the peak egg laying period (early to mid July). The number of egg masses found per meter (about 1 yard) of row is determined by sampling at several sites in a field. Egg mass densities are often more concentrated on field edges bordering overwintering sites (e.g. ditches, shelter belts, previous year’s cornfield). The economic threshold is 0.5 egg mass per meter (yard) of row. The threshold will increase or decrease depending on cost of insecticide, crop value, and beetle survival factors. Egg mass sampling has several advantages over sampling later when larvae are actively feeding. Egg mass sampling allows the grower to make a timely decision on the need to treat before damage has begun. In many cases, ground equipment can still be used to treat for beetles if row closure has not occurred. If aerial treatment is preferred, treatments can be scheduled for timely application before significant defoliation has begun.

Defoliation/larval sampling can be done as the insect is feeding in the field (Late July-early August). This is often difficult to do because feeding by the late instars often results in rapid defoliation. As a result, timely treatments are often complicated by delays in sampling and in applicator scheduling. This damage should be monitored as larvae are developing into the final instar and again as adults begin to emerge from the pupae. Treatment may be justified at this time if 1 larva per plant is present or average defoliation is approaching 5 to 15%. See the chapter on bean defoliation for additional guidance on assessing damage. When plants mature near the end of pod fill (about mid to late August), damage from Mexican bean beetle will become less important. Treatments in late July and early August may often be timed to also provide good control of western bean cutworms (see western bean cutworm chapter).

Products for Mexican Bean Beetle:

Insecticide	Product per Acre (fl oz. or oz. product)	Preharvest Interval, remarks
acephate2	See labels	PHI 14 days; REI 24 hrs
Baythroid XLR,1	0.8-1.6	PHI 7 days; REI 12 hrs.
carbaryl1,2	See labels	PHI 21 days; REI 12 hrs.
cyfluthrinR,1,2	See labels	PHI 7 days; REI 12 hrs.
dimethoate1,2	See labels	7 day PHI; REI 48 hrs. Do not feed treated vines.
esfenvalerateR,1,2	See labels	PHI 21 days; REI 12 hrs.
gamma-cyhalothrinR,1,2	See labels	PHI 21 days; REI 24 hrs.
lambda-cyhalothrinR,1,2	See labels	PHI 21 days; REI 24 hrs.
LannateR,1 SP, LV	4-16 (SP), 12-48 (LV)	PHI 14 days; REI 48 hrs.
Mustang MAX R,1	1.28-4.0	PHI 21 days; REI 12 hrs.
RRestricted use pesticide, 1Labeled for chemigation, 2Generic active ingredient, several formulations.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.